finasteride

5-alpha-reductase inhibitor· Brands: Propecia, ProscarPO

Mechanism of action

Finasteride is a competitive inhibitor of type II 5-alpha-reductase, the enzyme that converts testosterone to dihydrotestosterone (DHT). By reducing DHT levels, it decreases prostate size and slows hair loss. It does not bind to the androgen receptor.

Pharmacokinetics

Oral bioavailability is approximately 65%. It is metabolised in the liver via CYP3A4. The half-life is 6 to 8 hours, but pharmacological effects persist longer due to slow enzyme dissociation. Excretion is mainly in faeces (about 57%) and urine (about 39%). It crosses the blood-brain barrier and is present in semen.

Indications & dosing

Androgenetic alopecia (male pattern hair loss)

For men only; may take 3-6 months for effect. Not indicated for women due to teratogenicity.

adult · PO: 1 mg once daily · For men only; may take 3-6 months for effect. Not indicated for women due to teratogenicity.

Benign prostatic hyperplasia (BPH)

For men with symptomatic BPH; reduces prostate size and may improve urinary flow. Used alone or with an alpha blocker.

adult · PO: 5 mg once daily · For men with symptomatic BPH; reduces prostate size and may improve urinary flow. Used alone or with an alpha blocker.

Contraindications & precautions

Pregnancy · Finasteride is teratogenic to male fetuses; do not use in women who are or may become pregnant.
Women of childbearing potential · Not indicated for women; handling crushed tablets may cause fetal abnormalities.
Hypersensitivity to finasteride · Avoid in patients with previous allergy to finasteride or any excipient.
Children under 18 years · Safety and efficacy not established; use not recommended.
Women who are or may become pregnant should not handle crushed or broken tablets, and should avoid contact with semen from a treated male.
Monitor PSA levels: finasteride reduces serum PSA concentration, so PSA values must be interpreted with caution and an adjusted baseline considered.
Consider prostate cancer screening before starting treatment, and advise patients to report any breast changes, such as lumps or pain.
Use with caution in patients with hepatic impairment, as finasteride is extensively metabolised in the liver.
Advise patients that sexual side effects may persist after discontinuation in some cases.
Inform patients that finasteride may affect PSA testing and that they should tell their healthcare provider they are taking this medicine.

Adverse effects

Decreased libido
Erectile dysfunction
Decreased ejaculate volume
Testicular pain
Gynecomastia
Breast tenderness
Depressionserious
High-grade prostate cancerserious
Male breast cancerserious
Angioedemaserious

Monitoring, pregnancy & toxicity

Prostate-specific antigen (PSA) level · At baseline and periodically during treatment; interpret with caution as finasteride reduces PSA by approximately 50%
Symptoms of benign prostatic hyperplasia (e.g. urinary flow, hesitancy) · At follow-up visits
Sexual function (e.g. erectile dysfunction, reduced libido) · Monitor for changes during treatment
Pregnancy: Finasteride is contraindicated in women who are or may become pregnant, as it can cause abnormalities in male fetuses. It is not indicated for use in women. Women of childbearing potential should avoid handling crushed or broken tablets. Men taking finasteride should inform their partner of the potential risk if pregnancy is planned.

Clinical notes

Used for male pattern hair loss and benign prostatic hyperplasia. In BPH, it reduces prostate volume and improves urinary flow. In hair loss, it increases hair count and slows thinning. Contraindicated in women of childbearing potential due to risk to a male fetus. Adverse effects include decreased libido, erectile dysfunction, and reduced ejaculate volume, which are generally reversible. Finasteride lowers PSA levels, so PSA results should be interpreted accordingly.

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BNF; local formulary