Altrenogest — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Horse (Mare)	PO	0.044 mg/kg (1 mL per 250 lb) once daily	Once daily	Duration: 10-15 days for estrus suppression; for synchronization, administer for 10-15 days, then withdraw to induce estrus. Notes: Administer on feed or directly into the mouth. Wear gloves when handling; avoid skin contact in women of childbearing age.
Pig (Gilt/Sow)	PO	15 mg per animal per day (typically 5 mL of 0.22% solution)	Once daily	Duration: 14 consecutive days for estrus synchronization. Notes: Administer top-dressed on feed. Ensure each animal receives the full dose. Withdraw after 14 days; estrus typically occurs 4-7 days later.

Clinical Indications & Species Uses

General Indications

Estrus suppression and synchronization in horses and pigs.

Horse (Equine)

Suppression of estrus in mares
Synchronization of estrus for timed breeding
Management of persistent estrus behavior

Pharmacology & Mechanism of Action

Drug Class: Progestin | Pharmacological Group: Synthetic progestogen

Mechanism of Action: Altrenogest is a synthetic progestogen that mimics the actions of endogenous progesterone. It binds to progesterone receptors in the hypothalamus and pituitary gland, suppressing the release of gonadotropins (LH and FSH) via negative feedback. This inhibits follicular development and ovulation, maintaining a state of anestrus or diestrus. In mares, it is used to suppress estrus and synchronize estrous cycles. In swine, it is used to synchronize estrus in gilts and sows by maintaining luteal phase-like conditions.

Pharmacodynamics: Altrenogest has potent progestational activity, with a higher affinity for progesterone receptors than endogenous progesterone. It suppresses the preovulatory LH surge, preventing ovulation. It also maintains pregnancy in some species. In horses, it effectively controls estrus behavior and allows for timed breeding. In pigs, it synchronizes estrus, allowing for batch farrowing. Its effects are reversible upon withdrawal, leading to a return to normal cyclicity.

⚡ Pharmacokinetics Summary

Absorption: Altrenogest is well absorbed after oral administration. In horses, peak plasma concentrations are reached within 1-2 hours. In pigs, absorption is also rapid, with peak levels within 1-3 hours.

Distribution: It is widely distributed in the body, with high concentrations in adipose tissue due to its lipophilic nature. It crosses the placenta and is excreted in milk.

Metabolism: Altrenogest is extensively metabolized in the liver, primarily by reduction and conjugation. The main metabolites are less active than the parent compound.

Excretion: Metabolites are excreted primarily in urine and feces. In horses, the elimination half-life is approximately 24 hours. In pigs, the half-life is shorter, around 14 hours.

Half-Life: Horse: ~24 hours; Pig: ~14 hours

Bioavailability: Oral bioavailability is high, estimated at 70-100% in horses and pigs.

Protein Binding: Approximately 90% bound to plasma proteins.

Available Formulations & Strengths

Oral Solution 0.22% (2.2 mg/mL) (PO)

Oral Solution 0.44% (4.4 mg/mL) (PO)

Contraindications & Clinical Warnings

Contraindications:

Pregnant animals (unless intended for pregnancy maintenance, but not approved for that use)
Animals with uterine disease or abnormal genital tract
Hypersensitivity to altrenogest or any component
Use in animals intended for breeding within a short period if not desired
Do not use in animals with liver dysfunction

Warnings & Clinical Precautions:

For use in horses and pigs only; not for use in other species.
Avoid skin contact; altrenogest is absorbed through human skin and can cause menstrual cycle disruption or adverse effects in pregnant women.
Wear impermeable gloves when handling the product.
Do not administer to mares with a history of uterine infection or endometritis.
Use with caution in animals with cardiovascular or renal disease.
In pigs, ensure accurate dosing to avoid ineffective synchronization.
Keep out of reach of children.

Adverse Effects & Reactions

Common:

Mild behavioral changes (e.g., reduced estrus behavior)
Slight decrease in appetite (transient)
Local irritation if spilled on skin (in humans)

Serious / Severe:

Uterine infections (if used in animals with existing uterine pathology)
Prolonged suppression of estrus if overdosed
Mammary gland enlargement or lactation (rare)

Rare:

Allergic reactions
Hepatotoxicity (with prolonged high doses)
Thromboembolic events (theoretical)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Corticosteroids	May enhance the immunosuppressive effects; monitor for increased susceptibility to infections.	Moderate
Other progestins or progestogens	Additive effects; increased risk of adverse effects.	Moderate
Gonadotropin-releasing hormone (GnRH) analogs	May counteract the suppressive effects on the pituitary; reduced efficacy of synchronization.	Moderate
Hepatic enzyme inducers (e.g., phenobarbital)	May increase metabolism of altrenogest, reducing efficacy.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Prolonged suppression of estrus
Lethargy
Decreased appetite
In mares: prolonged diestrus, potential cystic endometrial hyperplasia
In pigs: reduced fertility if bred too soon after withdrawal

Emergency Treatment Protocol: There is no specific antidote. Treatment is symptomatic and supportive. Discontinue the drug and monitor for return to normal cyclicity. In cases of accidental human exposure, wash skin thoroughly and seek medical advice if symptoms occur.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days

Altrenogest is not approved for use in food animals in many countries. In the US, it is not approved for use in cattle, sheep, goats, or poultry. For horses and pigs, withdrawal times are not established; however, it is recommended to follow local regulations. In the US, for horses, there is no withdrawal time for meat, but for pigs, a withdrawal time of 0 days is listed for the labeled indication. Check with local regulatory authorities.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20°C to 25°C (68°F to 77°F), excursions permitted between 15°C and 30°C (59°F and 86°F).

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

Handling & Special Conditions: Protect from light. Keep container tightly closed. Do not freeze.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved by FDA for use in horses (Regu-Mate) and pigs (Matrix).

Extra-Label (Off-Label) Use: In the US, altrenogest is approved for use in horses and pigs. Extra-label use in other species is prohibited under AMDUCA if an approved animal drug exists for that species. For food animals, extra-label use is not permitted if it results in violative residues. In the EU, altrenogest is approved for use in pigs and horses; use in other species is off-label and subject to cascade regulations.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Altrenogest is a valuable tool for estrus synchronization in horses and pigs. In mares, it is used to suppress estrus for management convenience and to synchronize cycles for artificial insemination. In pigs, it is used to synchronize estrus in gilts and sows, allowing for batch farrowing and improved management. It is important to administer the drug accurately and consistently, as missed doses can lead to breakthrough estrus. In mares, it is often used for 10-15 days, with estrus occurring 4-6 days after withdrawal. In pigs, a 14-day treatment is standard, with estrus occurring 4-7 days after withdrawal. Always wear protective gloves when handling the drug to prevent human exposure. Monitor animals for adverse effects and ensure proper withdrawal times if used in food animals, though it is not approved for use in most food animals.

References & Literature

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