Finasteride
4-azasteroid synthetic compound
Also known as: Proscar · Propecia · finasteridum · MK-0906 · MK-906
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Benign prostatic hyperplasia | 0.1-0.5 mg/kg | PO | once daily | — | 🌎 NA |
| Benign prostatic hyperplasia | <15 kg: 1.5 mg (approx. 1/3 of a 5 mg tablet); 15-30 kg: 2.5 mg (1/2 tablet); >30 kg: 5 mg (one tablet) | PO | daily | — | 🌎 NA |
| Benign prostatic hypertrophy | 5 mg per dog | PO | q24h | Extended treatment course (<8 weeks) may be required for full efficacy | 🌍 EU |
- Benign prostatic hyperplasia: for a 10-50 kg dog, one 5 mg tablet daily
- Benign prostatic hypertrophy: Alternative to castration. Use when delmadinone or osaterone are not appropriate.
Ferrets
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive treatment of adrenal disease | 5 mg (total dose) | PO | once daily | — | 🌎 NA |
- Adjunctive treatment of adrenal disease: tablet
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Finasteride is a synthetic 4-azasteroid compound that acts as a competitive and specific inhibitor of 5-alpha-reductase.
In veterinary medicine, it is primarily utilized for the medical management of benign prostatic hyperplasia (BPH) in intact male dogs. It offers a highly effective alternative to surgical castration, particularly for valuable breeding studs, as it reduces prostate size without significantly impacting fertility or circulating testosterone levels.
Additionally, finasteride is used off-label as an adjunctive therapy for adrenal disease in ferrets, helping to manage androgen-driven clinical signs such as prostatic enlargement and urinary blockages in affected males.
Mechanism of action
Finasteride specifically and totally inhibits the intracellular enzyme 5-alpha-reductase (predominantly Type II).
- Pathway: Testosterone → (via 5-alpha-reductase) → Dihydrotestosterone (DHT)
DHT is a highly potent androgen and is the primary hormone responsible for the development and enlargement of the prostate gland. By blocking this conversion, finasteride significantly reduces DHT concentrations in the prostate, liver, and skin. This leads to prostatic involution and a reduction in clinical signs of BPH. Because it does not directly block testosterone or its receptors, normal testosterone-dependent functions (such as spermatogenesis and libido) are largely preserved.
Safety & warnings
Contraindications
- Hypersensitivity to finasteride
- Sexually developing animals
- Females (especially pregnant females)
- Sexually-developing dogs
- Breeding dogs (due to secretion in semen and teratogenic potential)
Adverse effects
- Decreased libido
- Decreased ejaculate volume
- Impotence (rare/mild)
- Secreted into semen
- Causes fetal anomalies (teratogenic)
Precautions
WARNING: Finasteride is highly teratogenic. Pregnant women or women who may become pregnant must exercise extreme caution.
- Hepatic Impairment: Use with caution in patients with significant hepatic impairment as metabolism of the drug may be reduced.
- Patient Selection: Finasteride should be used in adult males only; do not use in sexually developing animals.
- Females: Not indicated for use in females. FDA Category X for use during pregnancy in humans (demonstrates fetal abnormalities).
Drug interactions
May precipitate or aggravate urinary retention thereby negating the effects of the drug when used for BPH
Monitoring
- Prostate exam (to evaluate efficacy and reduction in size)
- Prostate size (via palpation or ultrasound)
- Resolution of clinical signs (e.g., tenesmus, dysuria, hematuria)
Pharmacokinetics
Half-life
Absorption
Absorbed after oral administration; in humans about 65% is bioavailable. The presence of food does not affect absorption.
Distribution
Distributed across the blood-brain barrier and is found in seminal fluid. In humans, about 90% is bound to plasma proteins.
Metabolism
Metabolized in the liver.
Elimination
Metabolites are excreted in the urine and feces. In humans, a single daily dose suppresses DHT concentrations for 24 hours.
Overdose
Limited information is available regarding acute toxicity. Mild gastrointestinal effects (such as vomiting or diarrhea) may be noted in the event of an overdose. Supportive care is recommended.
Available products
Formulations
- Oral tablets
- Oral: 5 mg tablet
Human-labeled
- Finasteride Oral Tablets: 1 mg & 5 mg; Proscar®, Propecia® (Merck); generic
- Proscar 5 mg tablets
Regulatory status
Not authorized for veterinary use. Should only be used under the cascade when delmadinone or osaterone are not appropriate.
POM (Prescription Only Medicine). Not authorized for veterinary use; cascade use only.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store tablets below 30°C in tight containers and protected from light.
Client information
CRITICAL WARNING FOR PREGNANT WOMEN: Women who are pregnant or may become pregnant must NOT handle crushed or broken tablets. The active ingredient can be absorbed through the skin and cause severe birth defects in male fetuses.
- Patience is Key: Therapy might be prolonged (several weeks to months) before noticeable efficacy can be determined. Regular dosing compliance is mandatory for success.
- Lifelong Commitment: Once the drug is stopped, the prostate will typically start growing again. Long-term therapy is usually required unless the dog is neutered.
- Breeding: This medication is often chosen because it generally preserves fertility and breeding ability in stud dogs, though minor decreases in ejaculate volume may occur.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.