PROPECIA
Clinical safety rating
cautionComprehensive 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). |
| Molecular Weight | 372.55 |
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 | Contraindicated in women, especially during first trimester, due to risk of hypospadias in male fetuses. Finasteride inhibits conversion of testosterone to dihydrotestosterone, which is critical for male genital development. |
| 2nd trimester | Contraindicated in women. Avoid in pregnancy; potential for fetal harm, particularly male genital abnormalities. |
| 3rd trimester | Contraindicated in women. Avoid in pregnancy; potential for fetal harm, particularly male genital abnormalities. |
Clinical note
Comprehensive clinical and safety monograph for PROPECIA (PROPECIA).
| Placental transfer | Finasteride crosses the placenta in animal studies and is expected to do so in humans. It has been detected in umbilical cord blood at concentrations approximately 50% of maternal serum levels. |
| Breastfeeding | Not recommended during breastfeeding. Finasteride is excreted in human milk; however, no studies have assessed effects on the nursing infant. Risk-benefit assessment should consider the mother's clinical condition and the importance of the drug. |
| Lactation Rating | L5 |
| 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 | 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. |
■ 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 |
Pregnancy in womenFemale patients of childbearing potential (unless using effective contraception)Hypersensitivity to finasteride or any component of the formulationAdministration to children
| 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. |
| Food/Dietary | No clinically significant food interactions. May be taken with or without food. However, avoid excessive alcohol intake as it may exacerbate certain side effects (e.g., dizziness). |
| Clinical Pearls | Monitor patients for sexual dysfunction (e.g., decreased libido, erectile dysfunction) which may persist after discontinuation. Finasteride lowers serum PSA by approximately 50%; when interpreting PSA values in men taking Propecia, double the measured value for prostate cancer screening. Use with caution in patients with liver impairment; hepatic metabolism is primary clearance route. Avoid handling crushed or broken tablets in women who are or may become pregnant due to risk of teratogenicity (fetal genital abnormalities). Onset of hair regrowth typically takes 3-6 months; continue use for at least 12 months before assessing efficacy. |
| Patient Advice | Take exactly as prescribed, usually one tablet (1 mg) daily with or without food. · Do not stop or skip doses without consulting your doctor; continuous use is needed to maintain benefit. · It may take 3-6 months to see hair regrowth and up to 12 months for full effect. · Report any new or worsening sexual side effects (e.g., decreased libido, erectile dysfunction, ejaculation disorders) promptly. · Finasteride may increase the risk of high-grade prostate cancer; discuss screening risks with your doctor. · Do not donate blood while taking Propecia and for at least 1 month after stopping to prevent exposure to pregnant women. · Women who are pregnant or may become pregnant should not handle crushed or broken tablets due to risk of birth defects. · If a dose is missed, skip it and take the next dose at the usual time; do not double up. |
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