Finasteride Drug Interactions: Complete Guide
Medically reviewed by [Reviewer Name, MD] | Last reviewed: August 2026 | Editorial policy
Bottom line: Finasteride has a generally favorable drug interaction profile. It is primarily metabolized by CYP3A4, and potent CYP3A4 inhibitors or inducers can alter plasma levels, though the clinical significance at the 1 mg dose is modest. Always inform your prescribing physician of all medications before starting finasteride.
Key Drug Interactions
| Drug / Class | Interaction Type | Clinical Significance |
|---|---|---|
| Alpha-blockers (tamsulosin, terazosin) | Additive: both reduce prostate symptoms in BPH | Moderate — common combination for BPH; monitor BP |
| Strong CYP3A4 inhibitors (ketoconazole oral, ritonavir, clarithromycin) | May increase finasteride plasma levels | Low at 1 mg AGA dose; more relevant at 5 mg BPH dose |
| Strong CYP3A4 inducers (rifampicin, carbamazepine, St. John’s Wort) | May reduce finasteride plasma levels | Low–moderate; possibly reduced efficacy |
| Warfarin / anticoagulants | No known pharmacokinetic interaction; isolated case reports of elevated INR | Monitor INR if starting finasteride on warfarin |
| 5-alpha-reductase inhibitors (dutasteride) | Additive 5-AR inhibition if combined | Not recommended without specialist guidance |
| NSAIDs / anti-inflammatories | No known significant interaction | Minimal |
PSA Monitoring Note
Finasteride reduces PSA by approximately 50% after 6–12 months of use. Any clinician ordering PSA while a patient is on finasteride should double the measured PSA value before comparing to age-adjusted reference ranges. Failure to account for this effect can miss clinically significant PSA elevations.
See: Finasteride and alcohol | Finasteride and biotin | Dosage overview | Finasteride vs. dutasteride