Elephant Formulary

© 2003-17 Susan K. Mikota DVM and Donald C. Plumb, Pharm.D. Published by
Elephant Care International

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Elephant specific information, if available, is in blue.

Chemistry – An orally administered synthetic progestational agent, altrenogest has a chemical name of 17 alpha-Allyl-17-hydroxyestra-4,9,11-trien-3-one. It may also be known as allyl trenbolone .


Storage/Stability/Compatibility – Altrenogest oral solution should be stored at room temperature.


Pharmacology – Progestins are primarily produced endogenously by the corpus luteum. They transform proliferative endometrium to secretory endometrium, enhance myometrium hypertrophy and inhibit spontaneous uterine contraction. Progestins have a dose-dependent inhibitory effect on the secretion of pituitary gonadotropins and also have some degree of estrogenic, anabolic and androgenic activity.


Uses/Indications – Altrenogest is indicated (labeled) to suppress estrus in mares to allow a more predictable occurrence of estrus following withdrawal of the drug. It is used clinically to assist mares to establish normal cycles during the transitional period from anestrus to the normal breeding season often in conjunction with an artificial photoperiod. It is more effective in assisting in pregnancy attainment later in the transition period. One group of authors (Squires et al. 1983) suggest selecting mares with considerable follicular activity (mares with one or more follicles 20 mm or greater in size) for treatment during the transitional phase. Mares that have been in estrus for 10 days or more and have active ovaries are also considered to be excellent candidates for progestin treatment.


Altrenogest is effective in normally cycling mares for minimizing the necessity for estrus detection, for the synchronization of estrus and permitting scheduled breeding. Estrus will ensue 2-5 days after treatment is completed and most mares ovulate between 8-15 days after withdrawal. Altrenogest is also effective in suppressing estrus expression in show mares or mares to be raced. Although the drug is labeled as contraindicated during pregnancy, it has been demonstrated to maintain pregnancy in oophorectomized mares and may be of benefit in mares who abort due to sub-therapeutic progestin levels.


Pharmacokinetics – The pharmacokinetic parameters of altrenogest were not found. Other progestin agents are rapidly metabolized by the liver.


Contraindications/Precautions – The manufacturer (Regu-Mate® — Hoechst) lists pregnancy as a contraindication to the use of altrenogest, however it has been used clinically to maintain pregnancy in certain mares (see Dosages below). Altrenogest should also not be used in horses intended for food purposes.


Adverse Effects/Warnings – Adverse effects of altrenogest appear to be minimal when used at labeled dosages. One study (Shideler et al. 1983) found negligible changes in hematologic and most “standard” laboratory tests after administering altrenogest to 4 groups of horses (3 dosages, 1 control) over 86 days. Occasionally, slight changes in Ca++, K+, alkaline phosphatase and AST were noted in the treatment group, but values were only slightly elevated and only noted sporadically. No pattern or definite changes could be attributed to altrenogest. No outward adverse effects were noted in the treatment group during the trial.


Use of progestational agents in mare’s with chronic uterine infections should be avoided as the infection process may be enhanced.


The manufacturer (Regu-Mate® — Hoechst) lists the following people as those who should not handle the product:

1.   Women who are or suspect that they are pregnant

2.   Anyone with thrombophlebitis or thromboembolic disorders or with a history of these events

3.   Anyone having cerebrovascular or coronary artery disease

4.   Women with known or suspected carcinoma of the breast

5.   People with known or suspected estrogen-dependent neoplasia

6.   Women with undiagnosed vaginal bleeding

7.   People with benign or malignant tumor which developed during the use of oral contraceptives or other estrogen containing products


Altrenogest can be absorbed after skin contact and absorption can be enhanced if the drug is covered by occlusive materials (e.g., under latex gloves, etc.). If exposed to the skin, wash off im­mediately with soap and water. If the eyes are exposed, flush with water for 15 minutes and get medical attention. If the product is swallowed, do not induce vomiting and contact a physician or poison control center.


Overdosage – The LD50 of altrenogest in is 175-177 mg/kg in rats. No information was located regarding the effects of an accidental acute overdose in horses.


Drug Interactions – Rifampin may decrease progestin activity if administered concomitantly. This is presumably due to microsomal enzyme induction with resultant increase in progestin metabolism. The clinical significance of this potential interaction is unknown.


Doses –


To suppress estrus for synchronization:

a)   Administer 1 ml per 110 pounds body weight (0.044 mg/kg) PO once daily for 15 consecutive days. May administer directly on tongue using a dose syringe or on the usual grain ration. (Package insert; Regu-Mate® — Hoechst)

b)   0.044 mg/kg PO for 8-12 days (Bristol 1987)


To maintain pregnancy in mares with deficient progesterone levels:

a)   22 – 44 mg daily PO (Squires et al. 1983)


To suppress estrus (long-term):

a)   0.044 mg/kg PO daily (Squires et al. 1983)


Client Information – See the Adverse Effects/Warnings section for specific recommendations on handling, etc.


Dosage Forms/Preparations/FDA Approval Status/Withholding Times –


Veterinary-Approved Products:

Altrenogest 0.22% (2.2 mg/ml) in oil solution in 150 ml & 1000 ml bottles; Regu-Mate®  (Hoechst); (Rx)  Approved for use in horses not intended for food.



Human-Approved Products: None