Methyltestosterone — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	1-2 mg/kg	q48h or 2-3 times weekly	Duration: Variable; often 2-3 months for alopecia Notes: For alopecia, may take 2-3 months for hair regrowth. For urinary incontinence, often combined with estrogens.
Cat	PO	2.5-5 mg/cat	q48h	Duration: Variable Notes: Used off-label for appetite stimulation or estrus suppression.
Horse	IM	0.5-1 mg/kg	Weekly	Duration: Short-term Notes: Not recommended; potential for abuse and regulatory issues.

Clinical Indications & Species Uses

General Indications

Androgen replacement therapy in males
Anabolic effects in debilitated animals
Stimulation of erythropoiesis in certain anemias

Dog (Canine)

Treatment of androgen-responsive alopecia
Hormone-responsive urinary incontinence (in combination with estrogens)
Cryptorchidism (off-label)
To suppress estrus in females (off-label)

Cat (Feline)

To suppress estrus (off-label)

Pharmacology & Mechanism of Action

Drug Class: Androgen | Pharmacological Group: Anabolic steroid / Androgenic hormone

Mechanism of Action: Methyltestosterone is a synthetic androgen that binds to androgen receptors in target tissues, leading to changes in gene expression. It promotes protein synthesis, growth of male accessory organs, and secondary sexual characteristics. It also has anabolic effects, increasing muscle mass and bone density. In females, it can suppress gonadotropin release, reducing estrogen production.

Pharmacodynamics: Methyltestosterone exerts androgenic and anabolic effects. It increases nitrogen retention, protein synthesis, and erythropoietin production. It also stimulates sebaceous gland activity and may cause virilization. In males, it suppresses endogenous testosterone production via negative feedback on the hypothalamus and pituitary.

⚡ Pharmacokinetics Summary

Absorption: Methyltestosterone is well absorbed after oral administration, but undergoes significant first-pass metabolism in the liver, reducing bioavailability. It is also available as buccal tablets and topical formulations.

Distribution: It is widely distributed in the body, with high affinity for androgen receptors. It is bound to plasma proteins, primarily albumin and sex hormone-binding globulin (SHBG).

Metabolism: Methyltestosterone is extensively metabolized in the liver via reduction and conjugation, forming various metabolites including 17-keto steroids.

Excretion: Metabolites are excreted primarily in the urine, with some in bile. The elimination half-life is relatively short.

Half-Life: Approximately 2-4 hours in dogs; variable in other species.

Bioavailability: Oral bioavailability is low (approximately 20-30%) due to first-pass metabolism.

Protein Binding: Approximately 98% bound to plasma proteins.

Available Formulations & Strengths

Oral Tablet 10 mg, 25 mg (PO)

Buccal Tablet 10 mg (Buccal)

Capsule 10 mg, 25 mg (PO)

Contraindications & Clinical Warnings

Contraindications:

Pregnancy (may cause virilization of female fetus)
Lactation (may pass into milk)
Known hypersensitivity to androgens
Prostatic carcinoma or breast cancer in males
Severe hepatic or renal disease
Hypercalcemia
Cardiac, renal, or hepatic edema

Warnings & Clinical Precautions:

Use with caution in animals with cardiac, renal, or hepatic disease due to risk of fluid retention.
May cause premature epiphyseal closure in young animals; avoid in growing animals.
May suppress spermatogenesis and cause infertility in males.
May cause virilization in females (clitoral enlargement, deepening of voice).
Use with caution in animals with diabetes mellitus as it may alter glucose tolerance.
May exacerbate sleep apnea in humans; relevance in animals unknown.
Do not use in food animals due to lack of withdrawal times and regulatory restrictions.

Adverse Effects & Reactions

Common:

Increased aggression
Virilization in females
Increased libido
Fluid retention
Hepatotoxicity (with prolonged use)

Serious / Severe:

Hepatic dysfunction (cholestatic jaundice, peliosis hepatis)
Hypercalcemia
Premature epiphyseal closure
Prostatic hypertrophy
Behavioral changes (aggression)

Rare:

Erythrocytosis
Increased risk of thromboembolism
Hepatocellular carcinoma (with long-term use)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Anticoagulants (e.g., warfarin)	May increase anticoagulant effect; monitor PT/INR.	Moderate
Corticosteroids	May increase risk of edema and fluid retention.	Moderate
Insulin or oral hypoglycemics	May alter glucose tolerance; adjust dose.	Moderate
Hepatotoxic drugs (e.g., NSAIDs, azoles)	Increased risk of hepatotoxicity.	High

Overdose & Toxicity Management

Signs of Toxicity:

Nausea
Vomiting
Edema
Hypercalcemia
Hepatotoxicity
Aggression
Priapism in males

Emergency Treatment Protocol: Discontinue drug. Provide symptomatic and supportive care. Monitor liver function, serum calcium, and electrolytes. In severe cases, consider gastric lavage if recent ingestion. No specific antidote.

Food Animal Withdrawal Times

Not approved for use in food animals. Extra-label use prohibited in food animals in many countries.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25°C, excursions permitted to 15-30°C).

Handling & Special Conditions: Protect from moisture. Keep container tightly closed.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for human use; not FDA-approved for veterinary use, but may be used off-label in companion animals.

Extra-Label (Off-Label) Use: In the US, extra-label use in food animals is prohibited. In companion animals, extra-label use is permitted under AMDUCA with appropriate veterinary oversight.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Methyltestosterone is a potent androgen with limited use in veterinary medicine due to availability of safer alternatives. It is primarily used in dogs for androgen-responsive alopecia and occasionally for urinary incontinence in combination with estrogens. Its use is associated with significant adverse effects, including hepatotoxicity and behavioral changes. Due to its potential for abuse and regulatory restrictions, it should be used with caution and only when clearly indicated. Monitoring liver function, serum calcium, and behavior is recommended during therapy. In food animals, its use is strictly prohibited.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)