Finasteride

Key facts

• Finasteride (sold as Propecia for hair loss and Proscar for prostate enlargement) blocks the enzyme that converts testosterone to DHT, the hormone responsible for scalp hair thinning and prostate growth.
• For hair loss, effects build slowly over 3 to 6 months of daily use, and any gain reverses within about a year of stopping. For prostate symptoms, benefits also take months to appear.
• Women who are or may become pregnant must never handle broken or crushed finasteride tablets. Skin contact with the drug can harm a male fetus's genital development; intact tablets are safe to handle.
• A minority of men notice reduced libido, erectile difficulty, or reduced ejaculate volume; tell your prescriber if these persist.

What finasteride treats

Finasteride treats male pattern hair loss on the scalp crown and mid-scalp, and benign prostatic hyperplasia (BPH), a non-cancerous enlargement of the prostate that causes a weak stream, frequent urination and night-time trips to the toilet. It also lowers the risk of acute urinary retention and the need for prostate surgery in men with BPH. It is not used in women or children.

How finasteride works

Finasteride blocks 5-alpha-reductase, the enzyme that converts testosterone into dihydrotestosterone (DHT). Lower DHT levels let scalp hair follicles stay in their growth phase for longer and slow further miniaturisation, and they let an enlarged prostate gradually shrink, easing pressure on the urethra.

Before you take it

• Women who are pregnant or could become pregnant must never handle broken or crushed tablets; DHT is needed for normal male genital development, and even skin contact can pose a risk to a male fetus.
• Tell your prescriber about significant liver disease before starting finasteride.
• Finasteride can lower PSA blood test readings by about half; tell any doctor ordering a PSA test that you take it, so results are interpreted correctly.
• Tell your prescriber if you or a close relative has had breast cancer, since rare cases of male breast cancer have been reported with finasteride.

Side effects

Common effects include reduced sex drive, difficulty getting or keeping an erection, less semen at ejaculation, and mild breast tenderness or swelling.

Stop and seek urgent medical care for any of these:

• Lumps, pain or discharge from breast tissue.
• Signs of an allergic reaction: swelling of the face or throat, hives, or trouble breathing.
• Persistent testicular pain.

Safety essentials

• Keep tablets intact and away from women who are pregnant or may become pregnant. Broken or crushed tablets must never be handled by them, because of the risk to a male fetus.
• Give it time: hair regrowth or prostate symptom relief takes 3 to 6 months, and stopping the drug reverses any hair benefit within about a year.
• Report sexual side effects rather than stopping abruptly; your prescriber can discuss whether they are likely to resolve or whether an alternative suits you better.
• Mention finasteride use before any PSA prostate-cancer screening test, since it lowers the reading.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.