Background & Objectives: Low serum folate level is often reported as an adverse drug sequela of long term phenytoin usage seen with prolonged duration of phenytoin therapy. There is no previous study to prospectively track the serum folate level with usage of phenytoin, which is the objective of this study. Methods: Twenty-fi ve patients between the ages of 18-50 years diagnosed to have epilepsy and planning to start phenytoin were recruited in this study. Assessment of serum folic acid was done by chemiluminiscent method prior to the start of phenytoin and after 6 months of treatment. The serum folate level of 10 age and sex matched healthy control was also taken. Results: The average serum folate level was 7.48 + 2.04 ng/mL prior to the start of phenytoin therapy, which fell to 3.9 + 1.95 ng/mL after 6-month of phenytoin therapy (p-value <0.001). The average serum folate level for the age and sex matched 10 control samples was 14.46 + 2.81 ng/mL. Conclusion: A signifi cant fall of serum folic acid levels is seen in epilepsy patients after 6 months treatment with phenytoin.