FINASTERIDE
Clinical safety rating: avoid
Contraindicated (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. |
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 | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
No significant drug interactions Pregnant women should not handle crushed tablets due to risk of fetal abnormality.
| Breastfeeding | Not recommended. Finasteride is excreted in human milk; M/P ratio not reported. Risk to nursing infant unknown, but potential for adverse effects on male infant genitalia. Use contraindicated during breastfeeding. |
| 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. |
■ FDA Black Box Warning
No FDA black box warning.
| Common Effects | male pattern hair loss |
| Serious Effects |
["Pregnancy (category X; risk of hypospadias in male fetuses)","Known hypersensitivity to finasteride or any component of the formulation"]
| 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"] |
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| 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. |