Synthetic sex hormones can be administered to suppress fertility. Contraceptives consisting of progestin (synthetic progesterone) are among the synthetic options. They are available in multiple countries under several brand names for short term contraception, including injections with up to six months of fertility control and oral medications with ongoing dosing needed for contraceptive effect. While these products have some effect in males, they have typically been used to suppress estrus in female dogs and cats. The products have also been prescribed for dermatologic and behavior issues.
These products are not marketed in the U.S. but can be accessed from compounding agencies with a veterinarian’s prescription. In the U.S they are typically not recommended at all, or not for more than one cycle, given concern about side effects. Reportedly, in some countries at least megestrol acetate can be purchased over the counter without veterinary prescription.
Synthetic progestins for this purpose, and some related brand names, include megestrol acetate (MA or MGA; Megecat, Ovaban®), medroxyprogesterone acetate (MPA; Depo-Provera), and proligestone (Covinan®). Progestins are not the only sex steroids used as contraceptives in cats and dogs. Androgens have also been studied and to a lesser extent used on a short term basis. There is more about each of these in ACC&D’s e-book (see below) ACC&D has received most inquiries about, and therefore focused most on MA.
MA has been used for dogs and cats over several decades in numerous countries under different brand names and at varying recommended doses. One result of this has been that experiences with and perspectives on the drug vary significantly. MA retains a poor reputation in the U.S. that it may not deserve. Used correctly under specific circumstances (by attentive owners over short time periods on pets intended for breeding), MA can provide relatively safe postponement of estrus and temporary prevention of pregnancy.
In the US, a very low-dose oral application was marketed briefly under the name FeralStat for short term use in feral cats; some individuals are now using a compounded equivalent based on their experience It was hard to evaluate given the dose levels are far lower than doses of MA studied for efficacy and safety.
Efficacy and duration
While varying by brand, MA is typically given in pill or powder form.
In cats, reports of effective doses ranging from 2.5 mg per week to induction dosing of 5 mg daily for five days, followed by weekly doses of 5 mg. Cats require ongoing treatment with MA during their heat season (January/February to mid-summer in northern temperate regions).
Early experimental reports found MA to be 92% effective at postponing estrus (heat) in female dogs when it was given at 2.2 mg/kg for 8 days starting at the very beginning of the heat cycle (proestrus). If not started early enough or if started at an insufficient dose, MA does not work. MA postpones estrus for what is approximately an entire reproductive cycle, usually 4-6 months (with a range of 1-7 months).
In studies, MA has also been used in conjunction with Suprelorin to suppress the flare estrus that can happen prior to onset of contraceptive impact from that implant.
Complications of MA use can include pyometra, mammary hyperplasia and cancer, diabetes mellitus, adrenal gland. Weight gain and lethargy, as well as other behavioral changes, are commonly reported. Typically extended use is not recommended.
A pharmacovigilence report published in 2003 cited that during the 1994- 2002 period studied, 132 million pills of megestrol acetate were sold for veterinary use in France. Incidence of complications reported were very low given this distribution.
To learn more about progestin and other sex steroid contraceptives for cats and female dogs, check out the resources below:
To learn more about progestin contraceptives for cats and dogs, check out the resources below:
ACC&D Megestrol Acetate Product Profile and Position Paper
ACC&D FeralStat Product Profile and Position Paper (in process of being updated)
Contraception & Fertility Control in Dogs & Cats (ACC&D e-book, available for free download)
Journal articles and reports:
Greenberg, M., D. Lawler, S. Zawistowski, and W. Jochle. "Low-dose megestrol acetate revisited: A viable adjunct to surgical sterilization in free roaming cats?" Veterinary Journal (2013) 196:304-308. (Dr. Mike Greenberg collaborated with ACC&D Directors and Scientific Advisors on this article.)
National Commission on Veterinary Pharmacovigilance. "Evaluation des effets indésirables à court et moyen terme des progestatifs oraux à base d’acétate de mégestrol utilisés pour la prévention et l’interruption des chaleurs chez les carnivores domestiques" ("Report on the undesirable short and medium-term effects of Megestrol Acetate based oral progestatives in the interruption and prevention of heat in domestic carnivores.")
Note that ACC&D contracted for a translation of this French study into English and can provide that document on request. Please e-mail us.