Fluvoxamine interaction with warfarin

To report a case of an elevated international normalized ratio (INR) in a patient receiving fluvoxamine and warfarin.

A 79-year-old white woman was admitted for suicidal thoughts. Her medical history included depression, chronic obstructive pulmonary disorder, asthma, hypertension, atrial fibrillation with pacemaker placement, and breast cancer with lumpectomy and subsequent left mastectomy. Her medication list prior to admission was extensive, including warfarin 5 mg on Sunday, Monday, Wednesday, and Friday; warfarin 2.5 mg on Tuesday, Thursday, and Saturday, and citalopram 10 mg/d. Citalopram was changed to fluvoxamine during her hospital stay. This resulted in an elevated INR that persisted for approximately 7 days.

The metabolism of warfarin involves several cytochrome P450 isoenzymes, including CYP1A2, CYP2C9, CYP2C19, and CYP3A4. Fluvoxamine has the potential to inhibit CYP1A2, CYP2C9, CYP2C19, and CYP3A4 to a significant degree. It also has 11 inactive metabolites that may contribute to enzyme inhibition. Fluvoxamine has an extensive elimination half-life of 17-22 hours after a single dose, which increases with multiple dosing by 30-50%. It can take approximately 10 days to reach a steady-state concentration. In the elderly, the half-life is increased by 50%.

The coadministration of warfarin and fluvoxamine can result in an increase in the anticoagulant effect of warfarin. This anticoagulant effect can be seen for several days after the discontinuation of fluvoxamine. Our case emphasizes the need to closely monitor potential drug interactions in the elderly, especially those concerning fluvoxamine and warfarin.