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Progestin Contraceptives

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 due to concerns about side effects. Reportedly in some countries, 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. ACC&D has received the most inquiries about, and therefore has focused the most attention 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 difficult to evaluate FeralStat as its dose levels were 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. In dogs, 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. 

Safety

Complications of MA use can include pyometra, mammary hyperplasia and cancer, diabetes mellitus, and adrenal gland suppression. 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 MA 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:

Journal articles and reports:

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