Phenytoin blood levels and their relationship with physiological and demographic factors: A retrospective analysis of gender, age, and drug interactions

Phenytoin, a first-generation antiepileptic drug, requires therapeutic blood levels between 10–20 μg/mL for efficacy. Maintaining this range poses challenges. This study investigated the effects of gender, age, chronic diseases, and concomitant medications on phenytoin blood levels in epilepsy patients. In this retrospective study, data from 142 epilepsy patients on phenytoin therapy were analyzed to evaluate the effects of physiological factors (gender, age, and chronic diseases) and pharmacological factors (drug interactions) on phenytoin blood levels. Blood concentrations were categorized as therapeutic (10–20 µg/mL), subtherapeutic (<10 µg/mL), or toxic (>30 µg/mL). Statistical analysis included descriptive statistics, Kruskal-Wallis, Mann-Whitney U, Chi-square, and Spearman’s correlation tests (SPSS 27.0). Ethical approval was obtained from the Biruni University Scientific Research Ethics Committee. Of the 142 patients, 66.9% were male, and 33.1% were female, with a mean age of 62.17 years. Subtherapeutic levels were found in 81% of patients, while 12.7% had therapeutic and 6.3% had above-therapeutic levels. Among those with above-therapeutic levels, 77.8% exhibited drug interactions. Mean phenytoin levels were significantly higher in females than males (p<0.05) but lower in patients with drug interactions (p<0.05). No significant correlation was found between chronic diseases and phenytoin levels. Patients in intensive care units had higher levels than other groups (p<0.05). Phenytoin blood levels are influenced by gender, age, chronic diseases, and drug interactions. Individualized therapy and regular monitoring are essential to optimize seizure control and minimize toxicity. Future studies with larger cohorts are needed to validate these findings and improve phenytoin management.