Altrenogest
17 alpha-Allyl-17beta-hydroxyestra-4,9,11-trien-3-one
Also known as: Regu-Mate · Matrix · allyl trenbolone · A-35957 · A-41300 · RH-2267 · RU-2267
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceSuppression of estrus in normally cycling mares (extra-label)
Verified clinician required
- q5-7d
NA
Governing clinical context (2)
- ■ Monitor pregnancy via ultrasonography and progesterone concentration 12
- ■ This medication is considered to be a hazardous drug as defined by the National Institute for Occupational Safety and Health (NIOSH). Talk with your veterinarian or pharmacist about the use of personal protective equipment when handling this medicine.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Altrenogest is a synthetic oral progestational agent (progestin) widely used in veterinary reproductive medicine.
- Horses: Indicated to suppress estrus in mares to allow predictable cycling, synchronize estrus, and maintain pregnancy in mares with luteal deficiency or placentitis.
- Swine: Used to synchronize estrus in sexually mature gilts.
- Dogs: Used off-label for luteal insufficiency and as a tocolytic treatment to prevent premature delivery.
Clinical Pearl: Altrenogest is highly lipophilic and readily absorbed through intact human skin. It poses significant health risks to handlers, particularly pregnant women and individuals with a history of thromboembolic or estrogen-dependent diseases.
Mechanism of action
Altrenogest acts as an agonist at progesterone receptors.
- Transforms proliferative endometrium into secretory endometrium.
- Enhances myometrial hypertrophy and inhibits spontaneous uterine contractions (tocolytic effect).
- Exerts negative feedback on the hypothalamus and pituitary gland → inhibits secretion of GnRH → decreases release of pituitary gonadotropins (FSH and LH), thereby suppressing estrus and ovulation in a dose-dependent manner.
Safety & warnings
Contraindications
- Horses intended for food purposes
- Mares with chronic uterine infections
- Manufacturer lists pregnancy as a contraindication (though clinically used extensively to maintain pregnancy)
Adverse effects
- Minimal at labeled dosages
- Slight, sporadic elevations in Ca++, K+, alkaline phosphatase, and AST
- May exacerbate chronic uterine infections
- Risk of pyometra in dogs (due to exogenous progesterone effects)
Precautions
Human Handling Warnings
Altrenogest can be absorbed through intact skin, and absorption is enhanced under occlusive materials (e.g., latex gloves). The following individuals MUST NOT handle this product:
- Women who are or suspect they are pregnant
- Anyone with a history of thrombophlebitis or thromboembolic disorders
- Anyone with cerebrovascular or coronary artery disease
- Women with known or suspected breast carcinoma
- People with known or suspected estrogen-dependent neoplasias
- Women with undiagnosed vaginal bleeding
- People with tumors that developed during the use of oral contraceptives
If skin exposure occurs, wash immediately with soap and water. If eye exposure occurs, flush with water for 15 minutes and seek medical attention.
Drug interactions
May decrease progestin activity if administered concomitantly due to microsomal enzyme induction and increased progestin metabolism.
Monitoring
- Pregnancy viability (via ultrasound)
- Endogenous progesterone levels (if treating luteal deficiency)
- Signs of pyometra in dogs
Pharmacokinetics
Half-life
Absorption
In horses, peak levels usually occur within 15-30 minutes post-dose (44 mg/kg PO).
Elimination
Primarily eliminated in the urine. Peak urine levels occur 3-6 hours after oral dosing. Detectable in urine up to 12 days post-administration.
Overdose
The LD50 of altrenogest is 175-177 mg/kg in rats. No specific information is available regarding the effects of an accidental acute overdose in horses or other veterinary species.
Available products
Formulations
- Oral solution: 0.22% (2.2 mg/mL) in oil
Veterinary
- Altrenogest 0.22% (2.2 mg/mL) in oil solution (Regu-Mate, Matrix)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store at room temperature. Altrenogest is extremely sensitive to light; dispense and store in light-resistant containers.
Client information
⚠️ CRITICAL HANDLING WARNINGS
This medication can be easily absorbed through intact human skin and poses severe health risks to certain individuals.
DO NOT HANDLE THIS MEDICATION IF YOU ARE:
- Pregnant or suspect you might be pregnant
- A person with a history of blood clots, stroke, or heart disease
- A person with a history of breast cancer or estrogen-dependent tumors
- Experiencing undiagnosed vaginal bleeding
Handling Instructions:
- Always wear protective, non-porous gloves when handling the medication or equipment used to administer it.
- If the medication accidentally contacts your skin, wash it off immediately with soap and water.
- If it gets in your eyes, flush with water for 15 minutes and seek medical attention.
- If accidentally swallowed, do not induce vomiting; contact a physician or poison control center immediately.
Pet Care Instructions:
- Administer exactly as prescribed by your veterinarian.
- Store the medication away from direct light.
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.