PROPECIA
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PROPECIA (PROPECIA).
Finasteride is a competitive and specific inhibitor of type II 5α-reductase, an intracellular enzyme that converts testosterone to dihydrotestosterone (DHT). By inhibiting 5α-reductase, finasteride reduces serum and intraprostatic DHT levels, decreasing androgenic stimulation of the prostate. In hair follicles, reduction of DHT levels slows hair loss and promotes hair regrowth.
| Metabolism | Finasteride is extensively metabolized in the liver, primarily via the cytochrome P450 3A4 enzyme system. Two major metabolites, t-butyl side chain hydroxylation and ω-hydroxylation, have been identified; these metabolites possess less than 20% of the 5α-reductase inhibitory activity of finasteride. |
| Excretion | Primarily hepatic metabolism; 57% excreted in feces (as metabolites), 39% in urine (as metabolites, <0.1% as unchanged finasteride). |
| Half-life | Terminal elimination half-life is approximately 6-8 hours in young adults (range 4-12 hours), with clinical relevance for once-daily dosing; slightly prolonged in elderly (8-11 hours). |
| Protein binding | Approximately 93% bound to plasma proteins (mainly albumin). |
| Volume of Distribution | Approximately 1.1 L/kg (range 0.9-1.3 L/kg), indicating extensive tissue distribution with penetration into seminal fluid and scalp tissue. |
| Bioavailability | Oral bioavailability is approximately 65% (range 60-70%); not affected by food. |
| Onset of Action | Oral: Reduction in serum DHT is detectable within 8 hours; maximal suppression occurs within 24 hours; clinical improvement in hair growth typically begins after 3-6 months of daily dosing. |
| Duration of Action | After single dose, serum DHT levels return to baseline within 5-7 days; with chronic dosing, sustained suppression of DHT persists for up to 2 weeks after discontinuation; clinical benefit requires continuous therapy (hair regrowth lost 6-12 months after stopping). |
1 mg orally once daily
| Dosage form | TABLET |
| Renal impairment | No dose adjustment required for any degree of renal impairment |
| Liver impairment | No dose adjustment recommended; no studies in hepatic impairment |
| Pediatric use | Not indicated in pediatric patients; safety and efficacy not established |
| Geriatric use | No specific dose adjustment; limited data in elderly men with benign prostatic hyperplasia |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for PROPECIA (PROPECIA).
| Breastfeeding | Not recommended. M/P ratio unknown. Finasteride is excreted in rat milk; no human data. |
| Teratogenic Risk | Contraindicated in females of childbearing potential. Finasteride inhibits conversion of testosterone to DHT, and risk of hypospadias in male fetuses if exposure occurs during gestation. No adequate studies in pregnant women; animal studies show abnormal external genitalia in male offspring at doses 1-100 times human exposure. |
| Fetal Monitoring |
■ FDA Black Box Warning
PROPECIA is not approved for use in women or children. Finasteride is contraindicated in women who are or may become pregnant due to risk of abnormalities of the external genitalia of a male fetus. Women should not handle crushed or broken tablets when pregnant or may be pregnant.
| Serious Effects |
["Hypersensitivity to finasteride or any component of the formulation","Women who are or may become pregnant (due to risk of hypospadias in male fetuses)","Children (not indicated for use in pediatric patients)"]
| Precautions | ["Risk of prostate cancer: Finasteride may increase the risk of high-grade prostate cancer; digital rectal exam and PSA screening recommended before and during therapy.","Sexual dysfunction: Decreased libido, erectile dysfunction, ejaculation disorders, and decreased ejaculate volume have been reported; may persist after discontinuation.","Depression and suicidal ideation: Monitor for mood changes.","Breast cancer: Reported in men; evaluate any breast changes promptly.","Elevated PSA levels: Use caution interpreting PSA values in men on finasteride; adjust PSA levels by approximately 50% for clinical interpretation.","Hepatic impairment: Use with caution in patients with liver function abnormalities.","Pediatric use: Not indicated for use in children."] |
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| No monitoring required as drug is contraindicated in pregnancy. If inadvertent exposure occurs, perform fetal ultrasound for genital anomalies. |
| Fertility Effects | May decrease sperm count, motility, and morphology. Reversible upon discontinuation. Reported cases of male infertility or subfertility. |