Finasteride

Finasteride is an orally administered drug belonging to the family of 5-alpha-reductase inhibitors. It is mainly used to treat male androgenetic alopecia and benign prostatic hyperplasia (BPH). It works by reducing levels of dihydrotestosterone (DHT), the hormone directly involved in the progressive miniaturisation of the hair follicle.

• Androgenetic alopecia in adult men (dose of 1 mg/day).
• Benign prostatic hyperplasia (dose of 5 mg/day).
• Off-label use in postmenopausal women with female-pattern hair loss, always under strict medical assessment.

Testosterone is converted into dihydrotestosterone (DHT) by the enzyme 5-alpha-reductase. DHT is a much more potent androgen that, in genetically predisposed individuals, shortens the hair cycle and causes follicle miniaturisation.

Finasteride selectively inhibits type II 5-alpha-reductase, reducing the concentration of DHT in the blood and scalp by around 60-70%. By lowering DHT, hair loss is slowed and partial hair recovery is promoted.

In male androgenetic alopecia, the usual dose is 1 mg per day orally, at any time and with or without food. For prostatic hyperplasia, 5 mg/day is used.

The first results are usually seen from 3 to 6 months, with the maximum effect around 12 months. The benefit is maintained only for as long as treatment lasts: on stopping, hair loss usually resumes within 6-12 months.

Finasteride is well tolerated in most cases. The most commonly reported adverse effects are:

• Sexual disturbances: decreased libido, erectile dysfunction or ejaculation disorders. Usually uncommon and reversible, but they may persist after stopping the drug.
• Gynaecomastia (increase in breast tissue).
• Mood changes, depression and — with unknown frequency — suicidal ideation. The European authorities (EMA/CMDh, 2024-2025) have strengthened these warnings. If depressed mood or suicidal thoughts occur, the drug should be stopped and medical advice sought immediately.
• The 'post-finasteride syndrome' — persistence of symptoms after stopping the drug — has been described and remains under study.

It is important to remember that finasteride reduces PSA values by around 50%, which should be reported to the physician for any prostate screening.

• Pregnant women or women of childbearing age without contraception (teratogenic risk).
• Hypersensitivity to the active ingredient.
• Paediatric population.
• Caution in people with a history of depression or mood disorders: inform them of the psychiatric risk before starting treatment.

Finasteride is formally contraindicated during pregnancy. By inhibiting the formation of DHT, it can affect the development of the external genitalia of a male foetus. Pregnant women or those who may be pregnant must not handle broken or crushed tablets. There are insufficient data during breastfeeding, so it is also not advised.

Finasteride has few clinically relevant interactions. It is metabolised through the hepatic enzyme system CYP3A4. Its main practical implication is the reduction of PSA, which must be taken into account when interpreting prostate tests.

Level of evidence: high for male androgenetic alopecia. Numerous randomised controlled clinical trials, as well as the main international guidelines, support its efficacy and safety at 1 mg/day. The evidence in women is more limited and its use is considered off-label.