Testosterone — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	IM	1-2 mg/kg (testosterone cypionate or enanthate)	Every 2-4 weeks	Duration: Variable; for replacement therapy, lifelong Notes: Adjust dose based on clinical response and serum testosterone levels.
Dog	PO	1-2 mg/kg (testosterone undecanoate)	Every 24 hours	Duration: Variable Notes: Oral absorption is erratic; not commonly used.
Cat	IM	1-2 mg/kg (testosterone cypionate)	Every 2-4 weeks	Duration: Variable Notes: Use with caution; may cause aggression.
Horse	IM	0.5-1 mg/kg (testosterone propionate or cypionate)	Every 2-4 weeks	Duration: Variable Notes: Not for use in performance horses; check regulations.

Clinical Indications & Species Uses

General Indications

Hormone replacement therapy in males with androgen deficiency
Anabolic effects in chronic debilitation (off-label)

Dog (Canine)

Replacement therapy in male dogs with hypogonadism
Management of benign prostatic hypertrophy (controversial)
Anabolic support in chronic wasting conditions (off-label)

Horse (Equine)

Replacement therapy in geldings or stallions with hypogonadism
Anabolic support in debilitated horses (off-label)

Pharmacology & Mechanism of Action

Drug Class: Androgenic Anabolic Steroid | Pharmacological Group: Hormone / Anabolic Agent

Mechanism of Action: Testosterone is an endogenous androgen that binds to androgen receptors in target tissues, leading to activation of gene transcription and subsequent protein synthesis. This results in the development and maintenance of male secondary sexual characteristics, increased muscle mass, bone growth, and erythropoiesis. In veterinary medicine, exogenous testosterone is used to replace deficient endogenous hormone or to produce anabolic effects.

Pharmacodynamics: Testosterone exerts its effects by binding to the androgen receptor, which then translocates to the nucleus and modulates gene expression. It promotes nitrogen retention, protein anabolism, and increases in muscle mass and strength. It also stimulates erythropoietin release, leading to increased red blood cell production. In males, it maintains spermatogenesis and libido. In females, it can cause virilizing effects. The drug also has some glucocorticoid-like effects and can affect lipid metabolism.

⚡ Pharmacokinetics Summary

Absorption: Testosterone is poorly absorbed orally due to first-pass metabolism. It is well absorbed after intramuscular or subcutaneous injection, with depot formulations providing sustained release. Transdermal and buccal routes are also available but less commonly used in veterinary medicine.

Distribution: Testosterone is extensively bound to plasma proteins, primarily sex hormone-binding globulin (SHBG) and albumin. It distributes widely throughout the body, with highest concentrations in the male reproductive organs, skeletal muscle, and bone. It crosses the blood-brain barrier and placenta.

Metabolism: Testosterone is metabolized primarily in the liver via reduction and conjugation to inactive metabolites, including androsterone and etiocholanolone. These are then conjugated with glucuronic acid or sulfate.

Excretion: Metabolites are excreted primarily in the urine (about 90%) and to a lesser extent in bile and feces. A small fraction is excreted unchanged.

Half-Life: The half-life of testosterone is short, approximately 10-100 minutes in plasma, but the duration of action of depot formulations is much longer (weeks).

Bioavailability: Oral bioavailability is very low (<10%) due to extensive first-pass metabolism. Intramuscular administration of depot formulations provides near-complete bioavailability.

Protein Binding: Approximately 98% of circulating testosterone is bound to plasma proteins (SHBG and albumin).

Available Formulations & Strengths

Injectable Solution (oil-based) 25 mg/mL, 50 mg/mL, 100 mg/mL, 200 mg/mL (IM)

Oral Capsule 10 mg, 25 mg, 40 mg (PO)

Transdermal Gel 1% (10 mg/g), 2% (20 mg/g) (Topical)

Implant (pellet) 75 mg, 150 mg (SC)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to testosterone or any component
Pregnancy (especially in females; virilization of fetus)
Lactating animals
Known or suspected prostate cancer (in males)
Breast cancer (in males)
Severe renal or cardiac disease (may exacerbate fluid retention)
Hypercalcemia (may worsen)
Use in food animals (prohibited in many countries)

Warnings & Clinical Precautions:

Use with caution in animals with hepatic dysfunction, cardiac disease, renal disease, or epilepsy (may lower seizure threshold).
May cause sodium and water retention, leading to edema.
In prepubertal animals, may cause premature epiphyseal closure.
May accelerate growth of prostatic tissue; monitor for prostatic disease.
In females, may cause virilization (clitoral enlargement, deepening of voice).
May suppress spermatogenesis at high doses.
Use in performance animals may be prohibited; check with regulatory bodies.
Avoid in animals with a history of aggression (may increase aggression).

Adverse Effects & Reactions

Common:

Injection site pain or inflammation
Increased aggression
Increased libido
Sodium and fluid retention
Mild increase in red blood cell count

Serious / Severe:

Hepatotoxicity (rare)
Prostatic hyperplasia or neoplasia
Premature epiphyseal closure in young animals
Virilization in females
Exacerbation of cardiac or renal disease due to fluid retention

Rare:

Cholestatic jaundice
Peliosis hepatis
Increased risk of thromboembolism
Hypercalcemia

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Warfarin and other anticoagulants	Testosterone may increase the anticoagulant effect, increasing bleeding risk.	Moderate
Corticosteroids	Additive fluid retention and potential for increased edema.	Moderate
Insulin or oral hypoglycemic agents	Testosterone may improve glucose tolerance, potentially requiring dose adjustment of hypoglycemic agents.	Mild
Hepatic enzyme inducers (e.g., phenobarbital)	May increase metabolism of testosterone, reducing its efficacy.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Aggression
Hyperexcitability
Priapism (in males)
Fluid retention and edema
Hepatotoxicity (with chronic overdose)
Hypercalcemia

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Discontinue the drug. Manage fluid retention with diuretics if necessary. Monitor liver function and serum calcium. In severe cases, consider administration of an aromatase inhibitor (e.g., anastrozole) to reduce estrogenic effects, but this is rarely needed. Provide a quiet environment to reduce aggression.

Food Animal Withdrawal Times

Testosterone is prohibited for use in food animals in many countries (e.g., US, EU). No withdrawal times are established because it is not approved for use in food animals.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F); protect from light.

Light Sensitivity: Light-sensitive — protect from direct exposure.

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

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Testosterone is FDA-approved for use in dogs and horses for certain indications (e.g., replacement therapy). However, many products are used extra-label.

Extra-Label (Off-Label) Use: In the US, extra-label use of testosterone in food animals is prohibited. In non-food animals, extra-label use is permitted under AMDUCA if a valid veterinarian-client-patient relationship exists and appropriate withdrawal times are considered (though not established).

Clinical Pearls & Practice Notes

💡 Clinical Insights: Testosterone is primarily used in veterinary medicine for androgen replacement therapy in male dogs and horses with hypogonadism. It is also used off-label for its anabolic effects in chronic wasting conditions, but this use is controversial due to potential adverse effects. In cats, its use is rare and not well-documented. Testosterone is a controlled substance in many jurisdictions, so proper record-keeping is essential. Because of its potential for abuse and adverse effects, it should be used only when clearly indicated and under veterinary supervision. In performance animals, its use is often prohibited, so check with the relevant regulatory body. For food animals, it is strictly prohibited in many countries. Always monitor animals for signs of fluid retention, aggression, and prostatic changes during therapy.

References & Literature

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