Decreased citalopram concentration caused by enzyme induction effect of rifampin: a case report

Citalopram is a selective serotonin reuptake inhibitor used to treat depression and various anxiety disorders, which is mainly metabolized by the P450 (CYP) enzyme. Rifampin is a rifamycin with bactericidal activity against <i>Mycobacterium tuberculosis</i>. Rifampin significantly induces the P450 (CYP) enzyme system, which makes it susceptible to potential drug interactions with other medications. However, there have been few reports on the possible interaction between rifampin and citalopram. We report a 76-year-old patient who had been taking citalopram 20 mg daily for long-term treatment of depression. After 2 months of rifampin treatment for tuberculosis, the patient presented with intractable depressive symptoms, insomnia, and a profound sense of hopelessness. The trough plasma concentration of citalopram was monitored, which was 8.19 ng/mL, and failed to reach the guideline-recommended effective therapeutic range (50-110 ng/mL). Based on the therapeutic drug monitoring results of citalopram, the dosage of citalopram was adjusted to 40 mg daily, resulting in a significant improvement in depressive symptoms. This case provides further evidence of a clinically significant interaction that may occur between rifampin and citalopram.