# Finasteride: veterinary dose and monograph

> Finasteride is a veterinary drug. A 5-alpha-reductase inhibitor used in male dogs to treat BENIGN PROSTATIC HYPERPLASIA (BPH) — it blocks conversion of testosterone to the more potent dihydrotestosterone (DHT) that drives prostatic enlargement, shrinking the prostate and… Indications include benign prostatic hyperplasia (BPH) in intact male dogs — medical management preserving fertility; reduction of prostatic size/discharge where castration is declined or breeding is to be maintained. This monograph lists 2 reference doses for dogs and ferrets, each with route, frequency and source.

Source: https://vetmasterji.com/drugs/finasteride · Last reviewed: 2026-10-01

## At a glance

- **Drug class:** Veterinary drug
- **Also known as:** Proscar, Fincar, Finast
- **Species with doses:** Dogs and Ferrets
- **General dose range:** 0.1–0.5 mg/kg PO q24h — check the species tables for the indication
- **Routes:** PO
- **Last reviewed:** 1 October 2026

## Indications

- Benign prostatic hyperplasia (BPH) in intact male dogs — medical management preserving fertility
- Reduction of prostatic size/discharge where castration is declined or breeding is to be maintained

## Finasteride dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

### Dogs

*Finasteride doses for dogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Benign prostatic hyperplasia (intact male) | 0.1–0.5 mg/kg | PO | q24h | — |

- Benign prostatic hyperplasia (intact male): Papich 5e: 0.1 mg/kg PO q24h; dogs 10–50 kg one 5 mg tablet daily. Plumb’s p.522: 0.1–0.5 mg/kg once daily (Sirinarumitr; Bartges); <15 kg 1.5 mg, 15–30 kg 2.5 mg, >30 kg 5 mg daily (Romagnoli). Prostate shrinks over weeks; effect reverses on stopping. Pregnant women should not handle crushed tablets.

### Ferrets

*Finasteride doses for ferrets*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Adrenal disease (adjunct — prostatic enlargement) | 5 mg per ferret PO once daily | PO | q24h | — |

- Adrenal disease (adjunct — prostatic enlargement): Plumb’s p.522 (Johnson 2006).

## Contraindications

Pregnant women handling the drug (teratogenic — male fetal genital malformation). Use in females / breeding males where DHT suppression is undesirable beyond intent. Hepatic impairment. Known hypersensitivity. (Castration is curative for BPH where breeding is not required.)

## Species-specific warnings

> **Caution:** Dogs: Rule out prostatitis/neoplasia first (finasteride only treats hyperplasia). Effect reverses on stopping; castration is curative if fertility is not required.

## Adverse effects

- Generally well tolerated; does not reduce libido/fertility at therapeutic doses
- Reversible effect (BPH recurs after stopping)
- Reduced semen volume (minor)
- Handler hazard (teratogenic via skin absorption)

## Drug interactions

- No major adverse drug interactions of clinical note
- Other anti-androgens (osaterone/delmadinone) target BPH differently (overlapping use)

## Pregnancy and lactation

Not for use in pregnancy; the major issue is HANDLER safety — pregnant women must not handle tablets (teratogenic to male fetus via skin absorption).

## Monitoring

- Resolution of BPH signs (discharge, dysuria/tenesmus) and prostate size (ultrasound) over weeks
- Recurrence if discontinued (BPH returns)
- Handler safety (gloves; pregnant women must not handle)
- Rule out other prostatic disease (prostatitis, neoplasia) before/with treatment

## Toxicity and overdose

Very well tolerated in treated dogs — overdose has a wide margin and is generally without significant toxicity. The principal hazard is to people (teratogenicity through skin absorption, especially of crushed tablets) rather than to the patient.

Management: No specific antidote required (wide animal margin). Supportive care. Human exposure (especially pregnant women): wash skin, seek medical advice. Reinforce gloved handling and intact tablets.

## Clinical pearls

- Medical BPH management that PRESERVES fertility/libido — useful for breeding dogs or when castration is declined; the prostate shrinks over a few weeks
- The effect is reversible — BPH recurs when you stop; castration remains the curative option if breeding isn't needed
- HANDLER WARNING: pregnant women must not handle the tablets (teratogenic via skin absorption) — gloves, intact tablets
- Confirm it is BPH (not prostatitis/neoplasia) — finasteride treats hyperplasia, not infection or cancer

## Mechanism of action

Competitively inhibits 5-alpha-reductase (type II), the enzyme that converts testosterone to dihydrotestosterone (DHT) — the androgen chiefly responsible for prostatic growth. Lowering intra-prostatic DHT causes the hyperplastic prostate to involute/shrink, resolving BPH signs, while circulating testosterone (and thus libido/fertility) is largely preserved. The effect depends on continued dosing (DHT and prostate size return after discontinuation).

## Formulations and products

- 5 mg tablet (Proscar)
- 1 mg tablet (Finpecia — lower strength)
- Fincar / Finast — 5 mg tablet, Tablet (human-label)
- Proscar — 5 mg tablet, Tablet (human-label)

## Storage

Tablets: 15–30 °C, dry, protect from light; handle with gloves and do not crush/split (teratogenic dust). Keep away from pregnant individuals.

## Before you use this dose

> **Caution:** Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

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## Frequently asked questions

### What is the dose of Finasteride for dogs?

Reference doses for Finasteride in dogs: 0.1–0.5 mg/kg PO q24h (Benign prostatic hyperplasia (intact male)). Confirm against the label and the patient before use.

### What is the dose of Finasteride for ferrets?

Reference doses for Finasteride in ferrets: 5 mg per ferret PO once daily PO q24h (Adrenal disease (adjunct — prostatic enlargement)). Confirm against the label and the patient before use.

### When should Finasteride not be used?

Pregnant women handling the drug (teratogenic — male fetal genital malformation). Use in females / breeding males where DHT suppression is undesirable beyond intent. Hepatic impairment. Known hypersensitivity. (Castration is curative for BPH where breeding is not required.)

### What are the side effects of Finasteride in animals?

Generally well tolerated; does not reduce libido/fertility at therapeutic doses; Reversible effect (BPH recurs after stopping); Reduced semen volume (minor); Handler hazard (teratogenic via skin absorption).

## Sources

- Plumb's Veterinary Drug Handbook
- Merck Veterinary Manual, 11th edition
- Papich, Papich Handbook of Veterinary Drugs, 5th edition

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.