Possible interaction between imipramine and butalbital.

A 44-year-old woman was admitted to the psychiatric unit for exacerbation of her depressive disorder. Blood concentrations of her antidepressant, imipramine, were within normal range and consistent with past concentrations. Her medical history was significant for a chronic headache disorder for which she was given a prescription containing butalbital on admission. The patient's depressive disorder was quickly controlled but relapsed 2 weeks later. Concentrations of imipramine showed a decrease of approximately 50%. Imipramine is metabolized in the liver by the cytochrome P-450 (CYP 1A2) system, and barbiturates are known inducers of this enzyme subset. To our knowledge, an interaction specifically between butalbital and imipramine has not been documented; however, these drugs are extensively prescribed and occasions may arise where they are given concurrently. We recommend repeat measurement of imipramine concentrations 1 week after the start of any butalbital-containing product or barbiturate, and dosage adjustments based on the results and on the patient's response to the change.