Short-term use of megestrol acetate for temporary estrus postponement in dogs

Updated October 16, 2023

Megestrol acetate (MA) is a progestin (a synthetic form of progesterone) that can be used to postpone estrus and prevent pregnancy in female dogs. It is an oral contraceptive approved in many countries, including the United States (by the U.S. Food & Drug Administration, or FDA) and Canada. In the U.S., MA can be acquired with a prescription from a veterinarian. Brand names in the U.S. include Megace®, marketed for humans but used off-label for other species, Ovaban®, which is not currently commercially available, and there are also generic (i.e., non-brand-name) options. In the United Kingdom, MA is available by the brand name Ovarid®.

Oral versions of megestrol acetate are effective at temporarily postponing estrus in female dogs, but there are multiple restrictions and risks around use of the product. Safety concerns are particularly pronounced with extended and/or improper use.

The FDA advises to administer oral MA at least 30 days prior to estrus (during anestrus), and to not continue treatment for more than 24 months. The FDA also suggests that MA not be used for breeding animals (FDA Register 2014). Side effects of the drug  include increased appetite, weight gain, hair coat changes and behavior changes (VCA). To postpone estrus, MA should be administered during the deep, or late, anestrus stage of the dog’s estrus cycle, when the dog’s endogenous progesterone is low. To determine that the dog is in anestrus, it’s necessary to work with a veterinarian to using vaginal cytology and/or serum progesterone assay (Romagnoli 2003). Utilizing MA for fertility control for long periods of time and/or administering MA while the hormone progesterone is high, such as during estrus or diestrus, increases the risk for serious, adverse reactions such as mammary tumors, cystic endometrial hyperplasia (a precursor to pyometra), diabetes and obesity. Young and healthy dogs without reproductive disease or mammary tumor diagnoses are the best candidates for temporarily utilizing MA. Depending on the breed of dog, estrus, or the time when female dogs are fertile, will return in 2-9 months (Romagnoli 2003, Romano, J. E. 2022).

Although there is generally low risk in using MA to delay estrus when used at the appropriate dose for the dog’s body weight, and administering during anestrus, ACC&D does not recommend utilizing MA in female dogs for an extended period of time. If administering MA to delay estrus, we strongly recommended working closely with a veterinarian to ensure proper dosing, timing, physical check-ups and hormonal testing, for the caregiver and veterinarian to be observant of potential side effects, and to discontinue use by 24 months.

References and recommended reading

Food and Drug Administration. “Federal Register, Oral Dosage Form New Animal Drugs; Change of Sponsor.” May 2014. Doi: www.federalregister.gov/documents/2014/05/20/2014-10415/oral-dosage-form-new-animal-drugs-change-of-sponsor#page-28825.

Gollakner, R. (n.d.). Megestrol Acetate VCA Animal Hospitals | Doi: https://vcahospitals.com/know-your-pet/megestrol-acetate#:~:text=What%20is%20megestrol%20acetate%3F.

Romagnoli, S., & Concannon, P.W. (2003). Clinical Use of Progestins in Bitches and Queens: A Review. In: P.W. Concannon, G. England, J. Verstegen, C. Linde-Forsberg (Eds.), Recent Advances in Small Animal Reproduction. International Veterinary Information Service, Ithaca, New York. https://www.ivis.org/library/recent-advances-small-animal-reproduction/clinical-use-of-progestins-bitches-and-queens-a-0.

Romano, J. E. 2022. Hormonal control of estrus in dogs - management and Nutrition. Merck Veterinary Manual. Doi: https://www.merckvetmanual.com/management-and-nutrition/hormonal-control-of-estrus/hormonal-control-of-estrus-in-dogs?query=megestrol+acetate+in+dogs.

More information, visit our MA webpage.

About our position statements

ACC&D’s mission is to advance non-surgical fertility control to effectively and humanely reduce the number of unwanted cats and dogs. We also have interest and expertise in subjects that complement this mission, including modeling of free-roaming cat and dog populations and methods of identifying non-surgically sterilized animals. When news, organizations, and research related to our field arise, ACC&D plays an important role in interpreting new science and claims. We are an independent organization with no financial ties to any company or product sales, and we have found that our role as a watchdog, and our commitment to data and sound science, are as essential as our efforts to advocate for and advance non-surgical options.