Finasteride Drug Interactions: What to Avoid and What to Tell Your Doctor

One of the more reassuring things about finasteride is that it has a relatively clean drug interaction profile. Unlike many medications that are heavily metabolized by the liver's CYP450 enzyme system, finasteride does not significantly affect these enzymes — which are responsible for the majority of drug-drug interactions. That said, there are still important things to discuss with your prescriber.

Does Finasteride Have Drug Interactions?

According to the FDA prescribing information, no clinically significant drug-drug interactions with finasteride have been identified. Finasteride does not appear to affect the cytochrome P450-linked drug metabolizing enzyme system. Clinical studies tested it in combination with many common drug classes including:

Antipyrine, digoxin, propranolol, theophylline, and warfarin — no clinically meaningful interactions found

ACE inhibitors, beta-blockers, diuretics, calcium channel blockers — no interactions

NSAIDs, benzodiazepines, H2 antagonists, quinolone antibiotics — no interactions

HMG-CoA reductase inhibitors (statins), acetaminophen, aspirin — no interactions

Finasteride + Alpha-Blockers: The Approved Combination

For BPH patients, finasteride is sometimes prescribed intentionally combined with an alpha-blocker (such as doxazosin). This combination is FDA-approved and recommended by AUA guidelines for moderate-to-severe BPH. However, the combination is associated with more side effects than either drug alone, including:

Asthenia (weakness/fatigue)

Postural hypotension (dizziness when standing up)

Peripheral edema (leg swelling)

Abnormal ejaculation (higher incidence than with either drug alone)

These are additive effects — both drugs have these side effects independently, and the rates increase when combined. This is expected and documented, not a dangerous interaction.

Important: Finasteride's Effect on PSA Is a Lab Interaction

Finasteride dramatically lowers PSA (prostate-specific antigen) levels by approximately 50% within 6 months. While not a drug interaction in the pharmacological sense, this has major clinical implications:

Any doctor ordering PSA screening must know you're on finasteride — otherwise a "normal" PSA result could mask prostate cancer development

Urologists use "PSA doubling" logic — even small increases in PSA while on finasteride warrant investigation

Establish a new baseline PSA at least 6 months after starting finasteride therapy

Finasteride and Liver Disease

Finasteride is extensively metabolized in the liver, primarily via CYP3A4 enzymes. In patients with hepatic impairment (liver disease), clearance may be reduced and drug levels may be higher than expected. The FDA recommends using finasteride with caution in patients with liver disease. No specific dose adjustment is recommended by the manufacturer, but close monitoring is advisable.

Finasteride and Blood Donation

The FDA advises that blood or plasma donation be deferred for at least one month after taking the last dose of finasteride. This is because finasteride could be present in donated blood and pose a risk to a pregnant recipient if the blood were transfused. The UK has the same one-month deferral period.

Finasteride and Pregnancy: A Critical Warning

Finasteride is absolutely contraindicated in pregnancy. It causes abnormal development of male fetal genitalia. Even handling crushed or broken tablets can expose a pregnant woman through skin absorption. If a woman who is pregnant touches broken finasteride tablets, she should wash the area immediately with soap and water. Men taking finasteride whose partners are pregnant should also be aware that traces of finasteride have been detected in semen — this is noted in the prescribing information, though evidence of risk via semen exposure is not fully established.

What to Tell Your Doctor and Pharmacist

Even though finasteride has few true drug interactions, always tell your healthcare providers:

All medications, supplements, and herbal products you take

Any history of liver disease

That you take finasteride whenever you have a PSA test ordered

If you or your partner is pregnant or planning to become pregnant

If you plan to donate blood (defer for at least 1 month after stopping finasteride)

For a full overview of finasteride side effects, see: Finasteride Side Effects: What to Expect and When to Call Your Doctor.

For a full patient guide to finasteride, see: What Is Finasteride? Uses, Dosage, and What You Need to Know in 2026.