Finasteride

Overview

Finasteride (Proscar®, Propecia®) is a 5-alpha-reductase inhibitor used in dogs primarily for the management of benign prostatic hypertrophy (BPH). It provides a medical alternative to castration, particularly in breeding males where preservation of fertility and libido is desired.

The drug reduces prostatic size and associated clinical signs by decreasing androgenic stimulation. Clinical improvement may be observed within the first week of therapy, although maximal effect may take several weeks of continuous treatment.

Finasteride does not significantly affect spermatogenesis or libido, although it may reduce ejaculate volume. Its therapeutic effects are reversible, and prostate enlargement typically recurs after discontinuation.

Mechanism of Action (MOA): Finasteride competitively inhibits 5-alpha-reductase, the enzyme responsible for converting testosterone to dihydrotestosterone (DHT). The enzyme acts in the prostate, liver and skin; because dihydrotestosterone is the principal androgen driving prostatic development, lowering it reduces androgen-dependent prostatic growth.

Indications

Finasteride is used in dogs for the management of androgen-dependent conditions, particularly those involving prostatic enlargement, where preservation of reproductive function is desired.

• Benign prostatic hypertrophy (BPH): Used as a medical alternative to castration to reduce prostate size and improve urine and faecal elimination.
• Breeding males: It is the treatment of choice for breeding males: testosterone is preserved, so libido and semen quality do not decline, and fertility is retained during treatment.
• Prostatic cysts: Prostatic parenchymal and diffuse epithelial cysts involute during finasteride treatment.
• Paraprostatic cysts do not respond to finasteride.
• Prostatitis / prostatic abscess (adjunctive): In prostatitis and prostatic abscess, finasteride may be used to induce involution of prostatic parenchyma, epithelial cysts and abscesses when maintenance of breeding potential is desired; it supports, and does not replace, appropriate antimicrobial therapy.

Dosage (Reference)

Dog

In dogs, finasteride is used for long-term management of benign prostatic hypertrophy (BPH), with doses often rounded to the nearest half or full tablet.

Cat

Finasteride is not commonly used in cats, and no established dosing recommendations are available for feline patients.

Warnings & Precautions

Finasteride is generally well tolerated but requires appropriate patient selection and handling precautions due to its hormonal effects and potential occupational exposure risks.

• Contraindications: Do not use in sexually developing animals, sexually mature females, or in patients with known hypersensitivity to the drug.
• Hepatic impairment: Use cautiously in patients with significant liver disease due to hepatic metabolism of the drug.
• Reversibility of effect: Reduction in prostate size is maintained only during treatment; enlargement is expected to recur after discontinuation.
• Handling precautions: Finasteride is classified as a hazardous drug: use appropriate personal protective equipment and do not cut, crush or otherwise manipulate the tablets.
• Handling by at-risk people: Women, especially those who are pregnant or of childbearing potential, should avoid contact with broken tablets due to risk of transdermal absorption.
• Delayed response: Full clinical effect may take several weeks; therapy should not be prematurely discontinued if immediate improvement is not observed.
• Excretion in semen: Finasteride is secreted into semen and can cause fetal abnormalities; take this into account when treated dogs are used for breeding.
• Pregnancy: Do not use in pregnant animals.
• Extra-label use: No veterinary-labelled finasteride product is available, so canine use is extra-label; in the UK it is recommended only where delmadinone or osaterone are not appropriate.
• Finasteride is not indicated in female patients, and it is unknown whether it is excreted in milk.
• Clinical benefit begins to decline about one week after treatment is discontinued.

Drug Interactions

Clinically relevant interactions with finasteride are limited but may involve effects on urinary function when combined with certain medications.

• Anticholinergic drugs: May precipitate or worsen urinary retention when used concurrently.
• Tamsulosin (beneficial combination): May be combined deliberately with tamsulosin, an alpha1A-adrenoceptor antagonist that relaxes prostatic and urethral smooth muscle, to improve urine flow.
• Other interactions: No other drug interactions have been reported in animals.

Side Effects & Overdose

Side Effects

Finasteride is well tolerated in dogs; one study reported no adverse effects after 21 weeks of treatment at 1 mg/kg.

• Gastrointestinal signs: Vomiting or nonspecific gastrointestinal upset may occur.
• Reproductive effects: Decreased ejaculate volume may be observed without affecting semen quality.
• Sexual effects: In humans, limited, mild and transient sexual effects have been reported, including decreased libido, decreased ejaculate volume and impotence.

Overdose

The reported oral LD50 in dogs is greater than 1 g/kg; reported gastrointestinal effects included nausea and vomiting.

• Gastrointestinal signs: Nausea or vomiting may occur with large overdoses.
• Management: Treatment is supportive; there is no known antidote and finasteride is unlikely to be removed by dialysis. After a known or suspected overdose, consult a 24-hour poison-control service that provides veterinary-specific guidance.

Key Notes

Practical clinical insights to optimize the use of finasteride in dogs:

• Targeted hormonal therapy: Specifically reduces dihydrotestosterone (DHT), the main driver of prostatic growth.
• Non-surgical alternative: A non-surgical option when castration is not desired.
• Consistent daily suppression: In humans, a single daily dose suppresses DHT concentrations for 24 hours.
• Blood-brain barrier penetration: Human pharmacokinetic data show distribution across the blood-brain barrier and into seminal fluid.
• Protein binding: In human pharmacokinetic data, finasteride is about 90% protein bound.
• Oral pharmacokinetics: Human pharmacokinetic data report approximately 65% oral bioavailability, peak plasma concentrations 1-2 hours after dosing and an elimination half-life of 5-6 hours; food does not affect absorption, and metabolites are excreted in urine and faeces.
• Expected response: Reported prostate-size reduction with finasteride was 50-70% between 16 and 53 weeks; castration produced about an 80% reduction within 12 weeks.
• Client advice: May be given with or without food; if the dog vomits on an empty stomach, give it with food. The response takes several weeks to judge, so doses should not be skipped.
• Storage: Store tablets below 30°C in a tight container, protected from light.
• Stability of compounded finasteride formulations has not been evaluated.
• Monitoring: A prostate examination and baseline prostatic ultrasonography before treatment, followed by repeat ultrasonography 6–8 weeks after initiation, may be considered for monitoring during therapy.
• When treating prostatic cysts, reassess cyst size by imaging at 4-week intervals during treatment.
• Large prostatic cysts may be drained percutaneously under ultrasound guidance before finasteride therapy is started.
• After treatment stops, the prostate returns to its pretreatment size in about 8 weeks.