Pharmacogenetic insights: CYP2D6 variability in Kurdish post-cesarean women on tramadol

Pharmacogenetic insights: CYP2D6 variability in Kurdish post-cesarean women on tramadol Shaymaa Faruq Hamadameen1, Kawa Fareq Dizaye1 1 Hawler Medical University, Erbil, Iraq Corresponding authors: Shaymaa Faruq Hamadameen (shaymaa.ha[email protected]u.edu.krd); Kawa Fareq Dizaye ([email protected]) Received 18 June 2025♦ Accepted 19 September 2025♦ Published 15 October 2025 Citation: Hamadameen SF, Dizaye KF (2025) Pharmacogenetic insights: CYP2D6 variability in Kurdish post-cesarean women on tramadol. Pharmacia 72: 1–8. https://doi.org/10.3897/pharmacia.72.e162100 Abstract CYP2D6 is a hepatic enzyme that metabolizes various medications: antidepressants, β-blockers, tamoxifen, and opioids such as tramadol and codeine. There is significant inter-individual variability in drug response due to high polymorphism. This study focuses on analyzing CYP2D6 polymorphism among Kurdish post-cesarean women on tramadol. Forty Kurdish women at Maternity Teaching Hospital were enrolled. DNA was extracted for CYP2D6 genotyping using a polymerase chain reaction. Patients were grouped into different phenotype groups based on their CYP2D6 activity scores. At 1 and 6 hours following tramadol injection, the participants were assessed for both analgesic efficacy and side effects. The CYP2D6*41 allele was the most frequent (26.25%). Thirteen genotypes were discovered, and none of the studied samples were identified as ultrarapid metabolizers. The poor metabolizer group exhibited the highest visual analog scale score mean at 1 hour (5.33 ± 1.70) and at the 6-hour mark (5.53 ± 1.05). In contrast, the prevalence of side effects did not significantly differ across the three phenotype groups. This study identified CYP2D6*41 as the most common allele among Kurdish post-cesarean women on tramadol, with poor metabolizers experiencing the highest pain scores. CYP2D6 polymorphism affected analgesic efficacy, while the prevalence of side effects remained unaffected. Keywords CYP2D6, pharmacogenomics, side effects, SNP, tramadol analgesia Introduction The International Association for the Study of Pain (IASP) defines pain as an unpleasant sense and emotion. Both the World Health Organization (WHO) and IASP recognize treating postoperative pain as a basic human right, crucial for recovery and reducing healthcare costs in developing countries (Raja et al. 2020). Patients commonly suffer from pain during their hospital admissions despite receiving treatments. Many people may hesitate to consult a healthcare provider, whether due to personal undervaluation opinion or avoiding the high medical burdens (Becerra-Bolaños et al. 2023). On the other hand, pain is sometimes not strictly considered by the medical caregiver, whether due to poor experience or unavailability of an adequate therapeutic plan (Carvalho et al. 2022). Pregnant women who have a cesarean section (CS) typically experience higher levels of postpartum pain than those with uncomplicated vaginal deliveries. Employing a step-by-step multimodal analgesia approach to reduce opioid use is considered the most effective way to manage postpartum pain (Shatil and Landau 2020). Research and clinical trials indicate that tramadol has a lower toxicity profile and is better tolerated compared to traditional oral nonsteroidal anti-inflammatory drugs (NSAIDs) (Sweileh Copyright Hamadameen SF & Dizaye KF. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Pharmacia 72: 1–8 DOI 10.3897/pharmacia.72.e162100 Research Article Hamadameen SF & Dizaye KF: ShortTitle2 et al. 2016). Tramadol is a centrally acting pain-relieving medication that is used for treating moderate to severe pain, often in conjunction with NSAIDs. In the liver, tramadol undergoes metabolism through O-demethylation by the cytochrome P450 (CYP) 2D6 enzyme, producing O-desmethyltramadol (M1) as the primary active metabolite of tramadol (Casajús et al. 2024). The active metabolite has 200–400 times greater affinity for the µ-opioid receptor (MOR) than tramadol (Smith et al. 2019). One of the limitations in achieving adequate pain relief is genetic polymorphism (Iversen et al. 2022; Matic et al. 2022). Studies have concluded that treating poor metabolizer (PM) or ultrarapid metabolizer (UM) phenotype patients resulted in higher costs than intermediate metabolizer (IM) and normal metabolizer (NM) patients, and they were at greater risk for adverse effects than IMs and NMs (Mohan et al. 2020). According to our knowledge, no study has been conducted on the impact of genetic polymorphism on tramadol analgesia for post-cesarean pain. The objective of this current study is to study the impact of CYP2D6 polymorphism on tramadol pharmacodynamics using the visual analog scale (VAS) marker, with the recording of any adverse effects among postpartum patients after CS in Erbil city. Materials and methods Study design and patients This study enrolled 40 pregnant women who were admitted to Maternity Teaching Hospital for preoperative preparation in Erbil city, Kurdistan Region, Iraq. The age of the enrolled individuals ranged between 21 and 42 years, with the mean age ± standard deviation being 31.45 ± 5.61. Inclusion criteria The inclusion criteria included being between the ages of 18 and 50, having an indication for elective CS surgery, and receiving tramadol for postpartum pain. All of these patients were scheduled to receive postpartum care for 24 hours at the Maternity Teaching Hospital and belonged to the Kurdish ethnicity. Participants received a dose of 100 mg/2 ml of the intravenous tramadol drug (Trodon Hemofarm) after gaining back their pain sensation. Exclusion criteria Severe renal, liver, or respiratory diseases; seizure; psychiatric disorders or inability to consent; history of complicated surgeries or prolonged hospitalization; neonatal distress; inability to provide the VAS score; history of drug abuse; chronic alcohol or opioid consumption; or tramadol hypersensitivity. Ethical considerations Informed consent was obtained from each volunteer before enrollment into the study for genetic analysis, follow-up, and publishing. The study was approved specifically by the Medical Ethics Committee of Hawler Medical University/College of Medicine and performed in accordance with the Declaration of Helsink. This study was also registered at ClinicalTrials.gov, National Library of Medicine (Identifier: NCT06814652). Data collection and follow-up After enrollment of each patient, 2 ml of peripheral venous blood was collected into EDTA tubes after completing the admission requirements. A portion of the sample was sent to the hospital laboratory for hematologic analysis, while the remaining whole blood was stored for immediate DNA extraction after the clinical study part was complete. A structured questionnaire was prepared to be filled out for each patient before the CS surgery, and the data obtained included (1) patient demographics (age, weight, smoking status), (2) any comorbid conditions (e.g., hypertension, diabetes), (3) previous birth and/or abortion, and (4) current and past medication history within the past 3 months. The second interview was held for each patient after the operation, and information was gathered 1 hour after tramadol intake, including (1) VAS score recorded by the patient, (2) oxygen saturation (SpO₂), (3) blood pressure, (4) pulse rate, and (5) any side and/or toxic effects. Some information was obtained from patients’ hospital files (e.g., gestational age, neonatal weight, history of dexamethasone intake). The patients were interviewed for the third time 6 hours after tramadol administration, and the data recorded included (1) self-reported VAS score, (2) SpO₂, (3) blood pressure, (4) pulse rate, and (5) any new effects. After 24 hours from the administration of tramadol, the patients were interviewed in person or contacted via phone (if they had been discharged from the hospital) to assess their dietary state, whether they had bowel movements within the last 24 hours, and whether they had experienced any nocturnal pain. DNA extraction Genomic DNA was extracted using an Invitrogen DNA extraction kit (2024 Thermo Fisher Scientific, USA) from 2 ml of whole blood. The OneDrop TOUCH Pro spectrophotometer (Biometrics Technologies Co., UK) was used to determine both the quality and quantity of the isolated DNA template at Exogen Laboratory (Zheen International Hospital) in Erbil city, Kurdistan Region, Iraq. All the polymerase chain reaction (PCR) reagents and extracted DNA templates were refrigerated throughout. Pharmacia 72: 1–8 3 Genotyping and sequencing The PGX-CYP2D6 XL StripAssay (ViennaLab Diagnostics GmbH 2024) was used for in vitro amplification and hybridization for detection of CYP2D6 gene mutations. This PCR-based assay covers 19 polymorphic loci designed to detect both mutant and wild-type genes. Each allele is defined by a specific set of sequence variants. The PGX-CYP2D6 XL StripAssay works by amplifying target regions of the CYP2D6 gene, followed by hybridizing them to allele-specific probes on the test strips, as shown in Fig. 1. The type of hybridization signals is interpreted to assign specific alleles: *1, *2, *3, *4, *5, *6, *7, *8, *9, *10, *11, *12, *14, *15, *17, *29, *35, *39, *40, *58, *41, and *114. Once individual alleles were identified, they were combined into diplotypes, such as *2/*4. At the last step, the dried test strips were attached to the Collector sheet to assess the genotype of each sample, in which the purple-stained control line below the red marker indicated the correct function of the reagents. The extracted DNA samples were sent to Intergen Genetics and Rare Diseases Diagnosis Center in Ankara City, Turkey, for sequencing. The sequencing process was performed to validate ambiguous results (e.g., weak or overlapping) and ensure accurate allele assignment. Preparation of the libraries was conducted using the Nextera DNA Flex Library Prep (2025 Illumina, USA), and the MiSeq sequencing system was used to analyze the sequencing. In the final step of sequencing, the data analysis was performed with basecalling and demultiplexing by the MiSeq system, thereby generating raw data in BCL files. These files were later converted into a FASTQ format by the Illumina software. Quality control of the reads was conducted to assess sequencing quality. Low-quality bases and adapter sequences were trimmed, and the processed reads were aligned to a reference genome using alignment tools. Next, the aligned reads were converted into the BAM format. Thereafter, variant calling was performed using the software GATK HaplotypeCaller, followed by quality-based filtering. In the last step, variant annotation and interpretation were conducted. Later each patient was grouped to a phenotype based on the available genotypes with assigned CYP2D6 activity scores: a value of 0 refers to PM, 0.25< IM <1.25, 1.25< NM <2.25, and scoring above 2.25 indicates UM, according to the Clinical Pharmacogenetics Implementation Consortium (CPIC) guideline (Alali et al. 2022). Statistical analysis Data were analyzed using the Statistical Package for Social Sciences (SPSS, version 27). A Student’s t-test was applied to compare paired variables exhibiting normal distribution. For data not conforming to normality, the Kruskal–Wallis test was utilized, followed by Duncan’s multiple range test as a post hoc procedure to identify specific groups with statistically significant phenotypic differences. To assess the association between genotypes and side effects, a chi-square test was conducted. In addition, the Hardy–Weinberg equation (HWE) was used to calculate expected genotype frequencies from the allele frequencies (p2+2pq+q2 =1p^2 + 2pq + q^2 = 1p2+2pq+q2 =1). A chi-square test was used to determine consistency with Hardy–Weinberg equilibrium (p > 0.05), where: • p = frequency of the dominant allele (A) • q = frequency of the recessive allele (a) • p + q = 1 • p² = frequency of homozygous dominant genotype (AA) • 2pq = frequency of heterozygous genotype (Aa) • q² = frequency of homozygous recessive genotype (aa) Figure 1. Design of the test strips of PGX-CYP2D6 XL StripAssay (ViennaLab Diagnostics GmbH 2024, Austria). Hamadameen SF & Dizaye KF: ShortTitle4 Results In this study, 13 different CYP2D6 genotypes were found among the participants: (*1/*1), (*2/*2), (*2/*4), (*4/*4), (*4/*6), (*4/*41), (*7/*7), (*9/*9), (*10/*10), (15*/41*), (17*/41*), (39*/41*), and (41*/41*), as shown in Fig. 2. These genotypes were classified into three phenotype groups depending on the available CYP2D6 activity scoring system: PM, IM, and NM (Figs 3, 4). None of the studied patients had more than duplicate copies of the wild-type allele, resulting in an activity score above 2.25 to be grouped as UM. The main identified variant alleles in this study were 41 (26.25%) and 4 (18.75%), as shown in Fig. 5. The most commonly occurring variant was the CYP2D6*41 allele, with the majority of the participants (20.00%) being homozygous IMs (CYP2D6*41/41*) and 5.00% being heterozygous NMs (CYP2D6*39/*41). Furthermore, the remaining CYP2D6*41 allele carriers were heterozygous IMs, which had either a decreased function allele (CYP2D6*17/*41) or a non-functional allele (CYP2D6 *15/*41), with each occurring in 2.50% of the studied samples. The non-functional CYP2D6*4 allele was identified as the second most frequently occurring allele among the studied postpartum individuals, with 12.50% being homozygous PMs (CYP2D6 *4/*4), 2.50% heterozygous PMs (CYP2D6 *4/*6), and 7.50% of CYP2D6 *4 allele carriers being heterozygous IMs (CYP2D6 *2/*4), as shown in Figs 2, 5. There was no statistical significance between the observed genotype frequencies and the frequencies expected under Hardy–Weinberg equilibrium (p = 0.18), as shown in Table 1, meaning the population is in Hardy– Weinberg equilibrium. The VAS score was recorded for all three defined groups after receiving the same analgesic dose of tramadol intravenously, in which the mean VAS score was 4.60 ± 1.89 after 1 hour of tramadol administration (Fig. 6A). The highest recorded VAS score was 9 by a PM patient, while score 1 was the lowest that was marked by an IM participant. Pain intensity levels varied inter-individually, with the majority of enrolled NMs (75.0%) reporting moderate pain, whereas 47.60% and 33.30% of the studied subjects among the IM group experienced moderate and mild pain, respectively. In addition, most of the PM patients (70.10%) marked their pain intensity as moderate. The mean VAS score recorded 6 hours following tramadol intake was 4.20 ± 2.07 among postpartum women (Fig. 6B). The highest VAS value was 8, belonging to the PM group, while the lowest score was 1, which was asTable 1. Comparing genotype frequencies of this study and the expected frequencies by the Hardy–Weinberg equation. Genotype Observed frequency Expected frequency AA 12.0 (30.0%) 12.66 (31.64%) AB 21.0 (52.50%) 19.69 (49.22%) BB 7.0 (17.50%) 7.66 (19.14%) Data presented as frequency (percentage). Figure 2. Percentage of different CYP2D6 genotypes among post-cesarean participants. Figure 3. Prevalence of different CYP2D6 phenotypes among postpartum patients. Figure 4. Percentage of different activity scores of CYP2D6 among the participants. Figure 5. Percentage of different CYP2D6 allelic genotypes among postcesarean women. Pharmacia 72: 1–8 5 Table 2. Comparing the number of participants (percentage) of side effects among different groups of phenotype. Side effect Normal metabolizer Intermediate metabolizer Poor metabolizer p-value Yes No Yes No Yes No After 1 hour Drowsiness 5 (21.7%) 7 (41.2%) 14 (60.9%) 7 (41.2%) 4 (17.4%) 3 (17.6%) 0.42 Dry mouth 3 (20.0%) 9 (36.0%) 8 (53.3%) 13 (52.0%) 4 (26.7%) 3 (12.0%) 0.60 Sweating 3 (50.0%) 9 (26.5%) 2 (33.3%) 19 (55.9%) 1 (16.7%) 6 (17.6%) 0.53 After 6 hours Dizziness 2 (15.4%) 10 (37.0%) 7 (53.8%) 14 (51.9%) 4 (30.8%) 3 (11.1%) 0.50 Nausea 0.0% 12 (33.3%) 2 (50.0%) 19 (52.8%) 2 (50.0%) 5 (13.9%) 0.34 Table 3. Comparing the frequency (percentage) of adverse reactions among different allele carrier groups. Side effect CYP2D6*1 CYP2D6*2 CYP2D6*4 CYP2D6*10 CYP2D6*41 After 1 hour Drowsiness 2 (8.7%) 5 (21.7%) 7 (30.4%) 4 (17.4%) 7 (30.4%) Dry mouth 2 (13.3%) 3 (20.0%) 6 (40.0%) 2 (13.3%) 4 (26.7%) Sweating 2 (33.3%) 0.0% 1 (16.7%) 0.0% 3 (50.0%) After 6 hours Dizziness 2 (15.4%) 0.0% 4 (30.8%) 2 (15.4%) 4 (30.8%) Nausea 0.0% 0.0% 1 (25.0%) 0.0% 2 (50.0%) signed by 7.50% of individuals with either the PM or IM phenotype. Furthermore, at 6 hours, 35.0% of the individuals reported a higher VAS score than at the 1-hour mark, whereas 60.0% recorded a decrease. Mild pain was noted by 66.70% of NMs and 47.60% of IMs, while 57.10% of PMs experienced moderate pain. After 1 hour of receiving tramadol, drowsiness was the most common side effect among the participants, with the highest percentage (60.90%) reported among the IMs. Dry mouth was the second most frequent among the enrolled patients, including 52.0% of the IMs. However, sweating was the least reported side effect and was mostly observed in the NM group. Overall, PM patients were among the least to report the aforementioned side effects (Table 2). Meanwhile, CYP2D6*41 allele carriers showed the highest frequency of adverse reactions at both time intervals (Table 3). One hour after tramadol administration, drowsiness (60.90%) and dry mouth (53.30%) were the most common side effects among the participants, especially in the IM group. Sweating was the least reported side effect, mainly seen in NMs, whereas PMs reported the fewest side effects overall. Additionally, CYP2D6*41 allele carriers showed the highest frequency of adverse reactions at both time intervals. Following 6 hours of tramadol injection, dizziness emerged as the most common side effect with the highest incidence (53.80%) among the IM group, while nausea was the least noted effect among all three groups. Discussion This study analyzed single nucleotide polymorphisms (SNPs) of CYP2D6 genes in 40 Kurdish post-cesarean patients in Erbil city to determine inter-individual differences in analgesic efficacy and adverse reactions of tramadol. To our knowledge, this is the first study conducted to investigate the outcome of CYP2D6 polymorphism on post-cesarean pain in the Kurdish population specifically and in Iraq generally, who were taking tramadol. According to the combination of CYP2D6 alleles and their enzyme activity scores, CYP2D6 is grouped into four types: PM, IM, NM, and UM (Nahid and Johnson 2022). Figure 6. A. Comparing means of VAS scores after 1 hour among different phenotypes; B. Comparing means of VAS scores after 1 hour among different phenotypes. Data are presented as mean ± standard error. IM: intermediate metabolizer; NM: normal metabolizer; PM: poor metabolizer; VAS: visual analog scale. 3,42 4,33 5,53 0 1 2 3 4 5 6 7 VAS Score after 6 hours Phenotype NM (a) IM (ab) PM (c) B 4,33 4,71 5,33 0 1 2 3 4 5 6 7 VAS Score after 1 hour Phenotype NM (a) IM (ab) PM (c) A Hamadameen SF & Dizaye KF: ShortTitle6 In the current study, the most profound alleles were the decreased-function CYP2D6*41 allele (26.25%) and the non-functional 4 allele (18.75%). On the other hand, the CYP2D6*1 variant was the most frequent wild-type allele (15%). Similar to the current study, many studies in various populations (Bosnian (Nefic 2018), Saudi Arabian (Hassen et al. 2023), Turkish (Ün et al. 2019), Sri Lankan (Ranasinghe et al. 2024), and Han and Uighur) have concluded that CYP2D6 allele frequencies follow Hardy-Weinberg equilibrium (Abudukeremu et al. 2024), suggesting genetic stability. However, the Japanese population had shown deviations that could be due to certain causes such as gene duplications and/or deletions (Ota et al. 2015). These findings in genetic profiles are important in pharmacogenetics and for individualizing drug therapies. The allele frequency findings in this study were compared with those of various Middle Eastern populations (Khalaj et al. 2019; Alali et al. 2022). The CYP2D6*1 allele has been the least frequently observed wild-type allele in most of the studies, with its prevalence among Kurdish post-cesarean patients being lower than among Persians (43.5–90.0%), Turks (32.0–87.8%), Saudi Arabians (78.4– 79.1%), and Egyptians (47.7–85.1%) (Khalaj et al. 2019). The frequency of the *2 allele in the Kurdish postpartum population was close to that of the UAE (12.2%), but this allele was more common and more similar across other Middle Eastern populations: Syrians (30.39%) (Khalaj et al. 2019; Alali et al. 2022), Persians (32.0%), Turkish (19.6%–35.3%) (Khalaj et al. 2019), Algerians (28.3%), Egyptians (28.3%), and Palestinians (27.5%) (Alali et al. 2022). Meanwhile, the percentage of the *4 alleles was comparable to Egyptians (9.6%–22.0%) (Alali et al. 2022). The frequency of the *10 allele was close to Iraqi Arabs (10.42%) (Sagban et al. 2023), Turkish people (6.0%–26.0%) (Khalaj et al. 2019), and Jordanian populations (14.8%) (Khalaj et al. 2019; Alali et al. 2022). In contrast, the CYP2D6*10 variant was less frequent among other Middle Eastern countries: Syrians (Khalaj et al. 2019), UAE, Saudi Arabians (Khalaj et al. 2019; Alali et al. 2022), Algerians, Moroccans, Egyptians, and Palestinians (Khalaj et al. 2019). This allele has been one of the most common variants in East Asian countries (China, Japan, and South Korea) and a major focus of pharmacogenetic research in that region during the last decade (Abudukeremu et al. 2024). In the current study, the frequency of the *39 allele was low, near the percentage of UAE (4.0%) (Khalaj et al. 2019), while this allele was more frequent among Iraqi Arabs (11.07%) (Sagban et al. 2023). The CYP2D6*41 allele frequency was close to that of Iraqi Arabs (27.04%) (Sagban et al. 2023), and it was observed to be less common in other Middle Eastern populations: Turkey (Sagban et al. 2023), Saudi Arabia, UAE (Khalaj et al. 2019; Alali et al. 2022), Syria, Algeria, Egypt, Lebanon, and Palestine (Alali et al. 2022). However, a higher number of patients may be required to confirm these results in the Kurdish population. These patterns indicate that the genetic profile of the Kurdish post-cesarean population is more comparable to Iraqi Arabs, Persians, Turks, and other Arabic population profiles, with significant differences suggesting ethnic and geographic gene diversity. Overall, Kurds are more genetically similar to Middle Eastern populations, with a lower frequency of functional CYP2D6 alleles (*2 and *39) and a higher proportion of reduced-function alleles (*4, *10, and *41) (Khalaj et al. 2019; Alali et al. 2022; Sagban et al. 2023). The greater prevalence of decreased-function alleles (*4, *10, and *41) among Kurdish post-cesarean women suggests that a significant percentage may have altered metabolism for drugs in general and tramadol activation in particular. Therefore, Kurdish patients are at greater risk of experiencing side effects and/or decreased therapeutic efficacy of CYP2D6-metabolized drugs, suggesting the importance of pharmacogenetic screening in the post-cesarean population. Furthermore, the Kurdish genetic profile is unique, supporting the importance of including this data in global pharmacogenomic databases. In addition to tramadol, these mutated variants can also affect the metabolism of several other CYP2D6 substrates, such as tamoxifen and antidepressants. Therefore, implementing pharmacogenetic testing could help optimize drug efficacy and safety across a broader range of drugs in Kurdish women undergoing CS. The results of the VAS records from the current study suggest a significant difference between the PM and NM genotype groups affecting the pain-relief efficacy of tramadol for postpartum pain. Moreover, the recorded VAS score and pain intensity level were lower in the IM than in the PM patients, whereas the recorded pain assessments observed in the NM phenotype were the lowest in value, both at the initial 1-hour and at the 6-hour mark. Larger cohorts with the presence of UMs will be very important to strengthen and confirm the available evidence and to support personalizing treatments and targeting broader clinical applications in the future. Overall, the PM group showed a higher VAS score mean following 1 hour (5.33 ± 1.70) and 6 hours (5.53 ± 1.05) of administering tramadol compared to the other groups, whereas the NM patients showed a lower VAS score mean after 1 hour (4.33 ± 1.44) and 6 hours (3.42 ± 2.15) of receiving tramadol. However, pain intensity level was the highest in the PM group, slightly lower in the IM group, and the lowest mean VAS score was recorded by the NM group at both 1 hour and 6 hours after tramadol administration. These VAS results showed phenotype correlation in post-cesarean analgesia, in which pain intensity decreased with increasing CYP2D6 metabolism capability, supporting CYP2D6 metabolic status as an important approach in guiding personalized analgesia, especially in surgical operations needing opioid analgesics (e.g., tramadol) metabolized by the CYP2D6 enzyme. Several previous clinical studies have demonstrated that CYP2D6 polymorphism impacts drug efficacy. For example, Wang et al. concluded that UM individuals convert the parent tramadol drug rapidly to its active O-desmethyltramadol metabolite; therefore, they are more likely to experience both a better analgesic effect and increased adverse reactions than the other groups. On the other hand, PMs achieve a lower plasma concentration of Pharmacia 72: 1–8 7 M1, which may result in resistance to its analgesic effect (Lassen et al. 2015). Overall, no significant difference in the incidence of side effects among CYP2D6 phenotype groups (NM, IM, and PM) was observed. To our knowledge, no previous investigations have studied CYP2D6 polymorphism in the Iraqi post-cesarean population. These findings of the current study regarding both analgesic efficacy and side effects aligned with the results of Lopes et al. (2020). Some guidelines recommend increasing the dose of tramadol or switching to alternative analgesic drugs in IM and PM patients, since a greater plasma level of the tramadol prodrug and a slower clearance were attained in PM and IM individuals than in NM and UM groups (Saiz-Rodríguez et al. 2021). CYP2D6 IMs and PM patients had smaller concentrations of the M1 metabolite in the blood at 30 minutes and 2 hours after tramadol administration than the UMs and NM patients; following 2 hours, these concentration differences decreased (Casajús et al. 2024). After 1 hour of tramadol administration, the mean VAS score of the patients with the CYP2D6*10/*10 gene variant was 3.83 ± 1.84, compared to 4.53 ± 1.77 in wild-type allele carriers. Following 6 hours of tramadol injection, the mean VAS score of the CYP2D6*10/*10 volunteers was 4.0 ± 1.67, whereas it was 3.8 ± 2.34 for the wild-type group. The findings of the current study showed no statistically significant difference in VAS scores, meaning that the CYP2D6*10/*10 genotype did not affect the response of the patients significantly at either reported time point. There have been many controversial results regarding previous studies about the effect of the CYP2D6*10/*10 genotype on responses to tramadol analgesia in East Asia. For example, Xu et al. reported that individuals with the *10/*10 genotype showed a higher incidence of adverse reactions such as nausea and vomiting (Wen et al. 2020). Meanwhile, Dong et al. found that the homozygous variant of CYP2D6*10 needed greater doses of tramadol to achieve effective postsurgical analgesia than wild-type allele carriers (Dong et al. 2015). In addition, there was no significant difference in the incidence of adverse effects between the CYP2D6*10/*10 patients and the other groups in this study, similar to the results presented by Wen et al. (2020). None of the CYP2D6*10/*10 participants reported nausea 6 hours after taking tramadol, while 10% of patients belonging to the PM or IM groups did. All the admitted patients had received the antiemetic drug metoclopramide prior to tramadol injection, which made it hard to report two of the common adverse effects of tramadol after 1 hour: nausea and vomiting. A key strength of this study includes being among the first to investigate the effect of the CYP2D6 genotype on tramadol analgesia specifically in postpartum women. Conclusion This study identifies CYP2D6*41 as the most common allele among Kurdish post-cesarean women on tramadol, with the PM patients experiencing the highest pain scores. The CYP2D6 polymorphism affected analgesic efficacy, while the prevalence of side effects remained unaffected. Additional information Conflict of interest The authors have declared that no competing interests exist. Ethical statements Clinical trials: ClinicalTrials.gov: NCT06814652. The authors declared that no experiments on humans or human tissues were performed for the present study. Informed consent from the humans, donors or donors’ representatives: Hawler Medical University/ College of Medicine. The authors declared that no experiments on animals were performed for the present study. The authors declared that no commercially available immortalised human and animal cell lines were used in the present study. Use of AI No use of AI was reported. Funding No funding was reported. Author contributions Conceptualization: Shaymaa Faruq Hamadameen, Kawa Fareq Dizaye; Methodology: Shaymaa Faruq Hamadameen, Kawa Fareq Dizaye; Investigation: Shaymaa Faruq Hamadameen; Data curation: Shaymaa Faruq Hamadameen, Kawa Fareq Dizaye; Formal analysis: Kawa Fareq Dizaye; Writing (original draft): Shaymaa Faruq Hamadameen,Writing (review and editing): Shaymaa Faruq Hamadameen, Kawa Fareq Dizaye; Supervision: Kawa Fareq Dizaye. Author ORCIDs Shaymaa Faruq Hamadameen https://orcid.org/0000-00024415-7902 Kawa Fareq Dizaye https://orcid.org/0000-0003-1739-8273 Data availability All of the data that support the findings of this study are available in the main text. References Abudukeremu M, Ayoufu A, Tuerhong A, Paizula X, Ou J-H (2024) Distribution of CYP2D6 and CYP2C19 gene polymorphisms in Han and Uygur populations with breast cancer in Xinjiang, China. Open Life Sciences 19: e20220728. https://doi.org/10.1515/biol-2022-0728 Hamadameen SF & Dizaye KF: ShortTitle8 Alali M, Ismail Al-Khalil W, Rijjal S, Al-Salhi L, Saifo M, Youssef LA (2022) Frequencies of CYP2D6 genetic polymorphisms in Arab populations. Human Genomics 16: 1–6. https://doi.org/10.1186/s40246022-00378-z Becerra-Bolaños Á, Armas-Domínguez A, Valencia L, Jiménez-Marrero P, López-Ruiz S, Rodríguez-Pérez A (2023) Pain Prevalence and Satisfaction with Pain Management in Inpatients: A Cross-Sectional Study. Healthcare 11: e3191. https://doi.org/10.3390/healthcare11243191 Carvalho JA, Souza DM de, Domingues F, Amatuzzi E, Pinto MCM, Rossato LM (2022) Pain management in hospitalized children: A cross-sectional study. Revista Da Escola De Enfermagem Da USP 56: e20220008. https://doi.org/10.1590/1980-220x-reeusp-2022-0008en Casajús A, Zubiaur P, Alday E, Soria-Chacartegui P, Saiz-Rodríguez M, Gutierrez L, Aragonés C, Campodónico D, Gómez-Fernández A, Navares-Gómez M, Villapalos-García G, Mejía-Abril G, Ochoa D, Abad-Santos F (2024) Impact of CYP2D6 and CYP2B6 phenotypes on the response to tramadol in patients with acute post-surgical pain. Clinical and Translational Science 17: e13698. https://doi. org/10.1111/cts.13698 Dong H, Lu S-J, Zhang R, Liu D-D, Zhang Y-Z, Song C-Y (2015) Effect of the CYP2D6 gene polymorphism on postoperative analgesia of tramadol in Han nationality nephrectomy patients. European Journal of Clinical Pharmacology 71: 681–686. https://doi.org/10.1007/ s00228-015-1857-4 Hassen LM, Daghestani MH, Omair MA, Althomali AK, Almukaynizi FB, Almaghlouth IA (2023) CYP2D6 genetic polymorphisms in Saudi systemic lupus erythematosus patients: A cross-sectional study. Saudi Medical Journal 44: 237–245. https://doi.org/10.15537/ smj.2023.44.3.20220581 Iversen DB, Andersen NE, Dalgård Dunvald A-C, Pottegård A, Stage TB (2022) Drug metabolism and drug transport of the 100 most prescribed oral drugs. Basic & Clinical Pharmacology & Toxicology 131: 311–324. https://doi.org/10.1111/bcpt.13780 Khalaj Z, Baratieh Z, Nikpour P, Khanahmad H, Mokarian F, Salehi R, Salehi M (2019) Distribution of CYP2D6 polymorphism in the Middle Eastern region. Journal of Research in Medical Sciences: The Official Journal of Isfahan University of Medical Sciences 24: e61. https:// doi.org/10.4103/jrms.JRMS_1076_18 Lassen D, Damkier P, Brøsen K (2015) The Pharmacogenetics of Tramadol. Clinical Pharmacokinetics 54: 825–836. https://doi.org/10.1007/ s40262-015-0268-0 Lopes GS, Bielinski SJ, Moyer AM, Black Iii JL, Jacobson DJ, Jiang R, Larson NB, St Sauver JL (2020) Sex Differences in Associations Between CYP2D6 Phenotypes and Response to Opioid Analgesics. Pharmacogenomics and Personalized Medicine 13: 71–79. https:// doi.org/10.2147/PGPM.S239222 Matic M, Nijenhuis M, Soree B, de Boer-Veger NJ, Buunk A-M, Houwink EJF, Mulder H, Rongen GAPJM, Weide J van der, Wilffert B, Swen JJ, Guchelaar H-J, Deneer VHM, van Schaik RHN (2022) Dutch Pharmacogenetics Working Group (DPWG) guideline for the gene-drug interaction between CYP2D6 and opioids (codeine, tramadol and oxycodone). European journal of human genetics: EJHG 30: 1105–1113. https://doi.org/10.1038/s41431-021-00920-y Mohan N, Edmonds KP, Ajayi TA, Atayee RS (2020) Clinical Tolerability and Safety of Tramadol in Hospitalized Patients. Journal of Pain & Palliative Care Pharmacotherapy 34: 211–218. https://doi.org/10.10 80/15360288.2020.1817227 Nahid NA, Johnson JA (2022) CYP2D6 pharmacogenetics and phenoconversion in personalized medicine. Expert Opinion on Drug Metabolism & Toxicology 18: 769–785. https://doi.org/10.1080/174252 55.2022.2160317 Nefic H (2018) The Genetic Variation of CYP2D6 Gene in the Bosnian Population. Medical Archives (Sarajevo, Bosnia and Herzegovina) 72: 396–400. https://doi.org/10.5455/medarh.2018.72.396-400 Ota T, Kamada Y, Hayashida M, Iwao-Koizumi K, Murata S, Kinoshita K (2015) Combination analysis in genetic polymorphisms of drug-metabolizing enzymes CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A5 in the Japanese population. International Journal of Medical Sciences 12: 78–82. https://doi.org/10.7150/ijms.10263 Raja SN, Carr DB, Cohen M, Finnerup NB, Flor H, Gibson S, Keefe FJ, Mogil JS, Ringkamp M, Sluka KA, Song X-J, Stevens B, Sullivan MD, Tutelman PR, Ushida T, Vader K (2020) The revised International Association for the Study of Pain definition of pain: concepts, challenges, and compromises. Pain 161: 1976–1982. https://doi.org/10.1097/j. pain.0000000000001939 Ranasinghe P, Sirisena N, Vishnukanthan T, Ariadurai JN, Thilakarathne S, Priyadarshani CDN, Bhagya Hendalage DP, Dissanayake VHW (2024) Frequency of pharmacogenomic variants affecting efficacy and safety of anti-cancer drugs in a south Asian population from Sri Lanka. BMC medical genomics 17: e143. https://doi.org/10.1186/ s12920-024-01919-2 Sagban S, Sahib AS, Abdulamir AS, Kadhim HM (2023) Impact of CYP2D6 polymorphisms on the efficacy of tamoxifen in Iraqi women with breast cancer. Journal of Contemporary Medical Sciences 9. https://doi.org/10.22317/jcms.v9i6.1396 Saiz-Rodríguez M, Valdez-Acosta S, Borobia AM, Burgueño M, Gálvez-Múgica MÁ, Acero J, Cabaleiro T, Muñoz-Guerra MF, Puerro M, Llanos L, Martínez-Pérez D, Ochoa D, Carcas AJ, Abad-Santos F (2021) Influence of genetic polymorphisms on the response to tramadol, ibuprofen, and the combination in patients with moderate to severe pain after dental surgery. Clinical Therapeutics 43: e86–e102. https://doi.org/10.1016/j.clinthera.2021.03.005 Shatil B, Landau R (2020) Opioid Use and Misuse in Pregnancy. Clinics in Perinatology 47: 769–777. https://doi.org/10.1016/j.clp.2020.08.004 Smith DM, Weitzel KW, Elsey AR, Langaee T, Gong Y, Wake DT, Duong BQ, Hagen M, Harle CA, Mercado E, Nagoshi Y, Newsom K, Wright A, Rosenberg EI, Starostik P, Clare-Salzler MJ, Schmidt SO, Fillingim RB, Johnson JA, Cavallari LH (2019) CYP2D6-guided opioid therapy improves pain control in CYP2D6 intermediate and poor metabolizers: a pragmatic clinical trial. Genetics in Medicine: Official Journal of the American College of Medical Genetics 21: 1842–1850. https:// doi.org/10.1038/s41436-018-0431-8 Sweileh WM, Shraim NY, Zyoud SH, Al-Jabi SW (2016) Worldwide research productivity on tramadol: a bibliometric analysis. SpringerPlus 5: e1108. https://doi.org/10.1186/s40064-016-2801-5 Ün İ, Barlas İÖ, Uyar N, Taşdelen B, Tiftik N (2019) Distribution of drug-metabolizing enzymes coding genes CYP2D6, CYP3A4, CYP3A5 alleles in a group of healthy Turkish population. Turkish Journal of Biochemistry 44: 142–146. https://doi.org/10.1515/tjb-20170226 Wen Q-H, Zhang Z, Cai W-K, Lin X-Q, He G-H (2020) The Associations Between CYP2D6*10 C188T Polymorphism and Pharmacokinetics and Clinical Outcomes of Tramadol: A Systematic Review and Meta-analysis. Pain Medicine 21: 3679–3690. https://doi.org/10.1093/ pm/pnaa140