CAUTION: finasteride approximately halves measured serum PSA at the licensed 1 mg dose, so any PSA value taken during finasteride therapy must be DOUBLED for interpretation against population reference ranges in men over 40.
Mechanism
Finasteride reduces dihydrotestosterone (DHT) in prostate tissue and thereby reduces prostate volume and PSA secretion. The Propecia 1 mg SmPC and BNF entry both flag this effect. The practical consequence: an apparently "normal" PSA on finasteride may correspond to a prostate-cancer-suspicious value in an untreated baseline.
Practical advice
If a PSA test is being interpreted while on finasteride, ALWAYS tell the requesting clinician you are on finasteride 1 mg daily (or the dose) and ALWAYS multiply the result by 2 for comparison against age-related reference ranges. Some labs will do this calculation automatically if the medication is declared at the time of request. For men starting finasteride who may need PSA monitoring in future, a baseline PSA before starting is clinically useful.