FINASTERIDE
Clinical safety rating
avoidContraindicated (not allowed)
Finasteride is a competitive 5-alpha-reductase inhibitor that prevents conversion of testosterone to dihydrotestosterone (DHT), thereby reducing DHT levels in serum and prostate tissue.
| Metabolism | Metabolized primarily via CYP3A4 in the liver; two inactive metabolites (t-butyl side chain oxidation and glucuronide conjugate). |
| Excretion | Renal (39% as metabolites, <0.1% as unchanged drug); fecal (57% as metabolites); biliary elimination contributes to fecal route. |
| Half-life | Terminal elimination half-life is approximately 6-8 hours (range 4-12 hours) in young adults; prolonged to ~8 hours in elderly due to reduced clearance; clinical effect on DHT suppression persists for 24 hours post-dose. |
| Protein binding | Approximately 93% bound to plasma proteins (primarily albumin and to a lesser extent alpha-1-acid glycoprotein). |
| Volume of Distribution | Volume of distribution = 76 L (approximately 1.0-1.1 L/kg), indicating extensive tissue distribution; crosses blood-brain barrier and partitions into seminal fluid. |
| Bioavailability | Oral bioavailability is approximately 63% (range 50-80%) due to incomplete absorption and first-pass metabolism; food does not significantly affect bioavailability. |
| Onset of Action | Oral: Reduction in serum DHT is detected within 8 hours; maximal suppression occurs within 24-48 hours of first dose. Reversal of prostate hyperplasia and hair growth effects require weeks to months. |
| Duration of Action | Serum DHT levels remain suppressed for 24 hours after a single dose, supporting once-daily dosing. Clinical effects on prostate size (reduction) and hair growth (increase) require continuous therapy for 6-12 months; effects reverse within 6-12 months following discontinuation. |
| Molecular Weight | 372.55 |
1 mg orally once daily for androgenetic alopecia; 5 mg orally once daily for benign prostatic hyperplasia.
| Dosage form | TABLET |
| Renal impairment | No dose adjustment required for any level of renal impairment including end-stage renal disease. |
| Liver impairment | No formal studies in hepatic impairment. Caution advised; use not recommended in severe hepatic impairment due to potential accumulation. No specific Child-Pugh based dose recommendations. |
| Pediatric use | Not indicated in pediatric patients. Safety and efficacy not established. Avoid use in children. |
| Geriatric use | No age-related dose adjustment necessary. Monitor for adverse effects (e.g., sexual dysfunction, mood changes) due to potential increased sensitivity. |
| 1st trimester | Contraindicated in pregnancy. Finasteride inhibits conversion of testosterone to dihydrotestosterone and may interfere with development of external genitalia in male fetuses. Teratogenic risk: hypospadias, ambiguous genitalia. 5-ARIs are classified as FDA Pregnancy Category X. |
| 2nd trimester | Contraindicated. Same risks as first trimester. Avoid in women of childbearing potential unless using two forms of contraception. |
| 3rd trimester | Contraindicated. Risks persist. Women who are or may become pregnant should not handle crushed or broken tablets. |
Clinical note
No significant drug interactions Pregnant women should not handle crushed tablets due to risk of fetal abnormality.
| Placental transfer | Finasteride crosses the placenta. In animal studies, maternal exposure leads to feminization of male offspring. Human data: detectable in umbilical cord blood, with maternal-to-fetal transfer ratio approximately 0.5–0.8. |
| Breastfeeding | Finasteride is excreted into human milk in small amounts; however, the potential for adverse effects (e.g., antiandrogenic effects) in nursing infants suggests caution. Use only if clearly needed, and consider alternatives such as spironolactone or topical antiandrogens. |
| Lactation Rating | L3 (Moderately Safe) — limited data suggest low levels in milk but insufficient evidence to rule out risk. |
| Teratogenic Risk | Contraindicated in pregnancy. Finasteride inhibits conversion of testosterone to dihydrotestosterone, which is critical for male fetal external genitalia development. Risk of hypospadias and other urogenital malformations if exposed in utero, particularly during first trimester. Pregnancy category X. |
| Fetal Monitoring | Pregnancy test prior to initiation in women of childbearing potential. In case of accidental exposure during pregnancy, fetal ultrasound for genital anomalies. No routine maternal monitoring required as drug not indicated in pregnancy. |
| Fertility Effects | May decrease sperm count, semen volume, and sperm motility, but reversible upon discontinuation. No established effect on female fertility; however, contraindicated in women of childbearing potential due to teratogenicity. |
■ FDA Black Box Warning
No FDA black box warning.
| Common Effects | male pattern hair loss |
| Serious Effects |
Pregnancy (Category X)Women of childbearing potential (unless using effective contraception)Hypersensitivity to finasteride or any component of the formulationPediatric patients (not indicated)
| Precautions | Risk of high-grade prostate cancer (decreased PSA levels may mask detection), Sexual adverse effects (e.g., decreased libido, erectile dysfunction, ejaculatory disorder) may persist after discontinuation, Increased risk of mood disturbances including depression and suicidal ideation, Not indicated for use in women or children; avoid handling crushed tablets during pregnancy due to risk to male fetus |
| Food/Dietary | No significant food interactions reported; finasteride may be taken with or without food. Avoid excessive alcohol consumption as it may worsen BPH symptoms or liver function. |
| Clinical Pearls | Finasteride inhibits 5α-reductase type II, reducing conversion of testosterone to DHT. Onset of effect in benign prostatic hyperplasia (BPH) requires 6-12 months; for androgenetic alopecia, 3-6 months. Serum PSA levels decrease by approximately 50% after 6 months; multiply PSA by 2 when interpreting. Avoid handling crushed or broken tablets if pregnant or planning to become pregnant due to risk of fetal genital abnormalities. Use with caution in hepatic impairment; contraindicated in women of childbearing potential, children, and patients with hypersensitivity to 5α-reductase inhibitors. |
| Patient Advice | Take finasteride exactly as prescribed, once daily with or without food. · It may take 3-6 months for hair regrowth or improvement in urinary symptoms; continue therapy as directed even if no immediate benefit is noted. · Report any breast tenderness, enlargement, or lumps; also report any new onset of sexual dysfunction (e.g., decreased libido, erectile dysfunction, ejaculation disorder). · Do not donate blood while taking finasteride and for at least 1 month after stopping, to prevent exposure to a pregnant female. · Women who are pregnant or may become pregnant should not handle crushed or broken tablets due to risk of harm to male fetus. · Serum PSA levels will decrease; inform your healthcare provider that you take finasteride before any PSA test. · Store at room temperature (20-25°C) in a dry place, away from light and moisture. |
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