Interaction of antiretroviral medications with finasteride

Antiretroviral medications are known inhibitors and inducers of cytochrome p450 enzymes and can affect levels of non‐HIV medications. Finasteride 1 mg (Propecia), which prevents the conversion of testosterone (T) to dihydrotestosterone (DHT) is commonly prescribed for prevention of hair loss. This medication is a substrate of p450 3A4. Its efficacy may therefore be affected by HIV medications which induce or inhibit this enzyme. Levels of DHT to prevent hair loss are not well established, but likely need to be<15–20 ng/dl, or a DHT/T ratio of<0.02. Observational analysis in a private practice, measuring DHT and T levels in patients on finasteride and various antiretrovirals 21 patients were identified. 7 patients were taking protease inhibitors and had DHT levels<12 ng/dL; DHT/T<0.20. Three of these patients decreased their finasteride dose to 1 mg every‐other‐day and still have DHT <10. 8 patients were taking potent p450 inducers (efavirenz or etravirine) and had DHT levels between >20; DHT/T>0.025. Two of these patients increased the dose of finasteride to 2 mg/day and subsequently decreased DHT to 14 and 17. Two additional patients on efavirenz, however, had DHT levels of<15 without dose adjustment. Four patients taking nevirapine, a less potent inducer of p450 had DHT levels of<15, as did one patient on raltegravir, which does not affect CYP450. Antiretrovirals that affect CYP 3A4 may interact with finasteride. While it is unlikely that this interaction is dangerous, it may affect its efficacy of the finasteride. Evaluation of DHT/T levels, and/or dose adjustment of finasteride may be appropriate in men being treated for HIV.