Finasteride
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: 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.
Dosing by species
- Adjunctive treatment of adrenal disease · 5 mg (total dose) · PO · once daily · tablet
- Benign prostatic hyperplasia · 0.1-0.5 mg/kg · PO · once daily · for a 10-50 kg dog, one 5 mg tablet daily
- 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
- Benign prostatic hypertrophy · 5 mg per dog · PO · q24h · Extended treatment course (<8 weeks) may be required for full efficacy · Alternative to castration. Use when delmadinone or osaterone are not appropriate.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
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)
Drug interactions
- Anticholinergic drugs · 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)
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.
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.