Altrenogest
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: 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 โ inhibits secretion of **GnRH** โ decreases release of pituitary gonadotropins (**FSH** and **LH**), thereby suppressing estrus and ovulation in a dose-dependent manner.
Dosing by species
- To suppress estrus for synchronization ยท 1 mL per 110 pounds body weight (0.044 mg/kg) ยท PO ยท once daily ยท 15 consecutive days ยท May administer directly on tongue using a dose syringe or on the usual grain ration.
- For prevention of abortion/pregnancy loss ยท 0.088 mg/kg ยท PO ยท once daily
- To maintain pregnancy in mares with deficient progesterone levels ยท 0.044 mg/kg ยท PO ยท once daily ยท Until day 60, 120, or end of pregnancy ยท Three options: 1) until day 60+ AND endogenous progesterone >4 ng/mL; 2) until day 120; 3) until end of pregnancy.
- To maintain pregnancy in mares with placentitis ยท 22-44 mg (total dose; 10-20 mL) ยท PO ยท daily
- To suppress estrus (long-term) ยท 0.044 mg/kg ยท PO ยท daily
- For synchronization of estrous in sexually mature gilts that have had at least one estrous cycle ยท 6.8 mL (15 mg) per gilt ยท PO ยท once daily ยท 14 consecutive days ยท Apply as a top-dressing on a portion of gilt's daily feed allowance. Estrous should occur 4-9 days after completing treatment.
- Luteal insufficiency ยท 2 mL per 100 lbs of body weight ยท PO ยท once daily ยท Document luteal insufficiency and rule out infectious causes. Best to avoid during first trimester. Monitor pregnancy with ultrasound. Monitor carefully for pyometra.
- Luteal insufficiency, pre-term labor ยท 0.1 mL per 10 lb body weight ยท PO ยท once daily
Routes of administration
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)
Drug interactions
- Rifampin ยท 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
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.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.