Megestrol Acetate

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What is megestrol acetate?image 12
Megestrol acetate (MA) (brand names: Ovaban®, Megace®) is a synthetic progestin, a hormone used to postpone estrus (heat cycle) and to relieve the effects of false pregnancy. It can also be used to treat benign prostatic hypertrophy in male dogs and to treat skin and behavior conditions in cats.

Its use in cats and male dogs to treat various conditions is ‘off label’ or ‘extra label’. Many drugs are commonly prescribed for off label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their direction may be significantly different from those on the label.

How is megestrol acetate given?
Prior to giving this medication, your pet should be evaluated carefully with a thorough reproductive/heat cycle history, a complete physical exam, a check for breast tumors, and a vaginal smear.

This medication is given by mouth in the form of a tablet or liquid. It may be given with or without food, but if vomiting occurs after giving on an empty stomach, give future doses with a meal or a treat. Pregnant women administering this medication to pets should use caution, wear gloves while handling it, and wash hands thoroughly after administration.

This medication can take up to a few weeks before the full effects are noted, however, improvement in clinical signs may not be visibly noticeable and therefore laboratory tests may be needed to evaluate this medication’s effectiveness.

What if I miss giving my pet the medication?
If you miss a dose, give it when you remember, but if it is close to the time for the next dose, skip the dose you missed and give it at the next scheduled time, and return to the regular dosing schedule. Never give your pet two doses at once or give extra doses.

Are there any potential side effects?
Side effects in dogs may include increased appetite, weight gain, lethargy, behavior changes, and hair coat changes. More serious side effects include inflammation or infection of the uterine tissues which may cause vaginal discharge, breast tissue enlargement, breast tumors, acromegaly (increases in soft tissue mass, skin folds, abdominal enlargement), adrenal gland suppression, increased drinking and/or urination, or lactation (milk production).

Side effects in cats may include increased appetite, weight gain, lethargy, and behavior changes. More serious side effects include severe lethargy, vomiting, diarrhea, shaking, collapse, seizures, increased drinking and increased urination, changes in personality, weight loss, vaginal discharge, breast tissue enlargement, and breast tumors. In cats that have chronic viral infections, this medication can cause a recurrence of the viral disease. Rarely, liver toxicity can occur, and may cause yellowing of the skin, eyes, and gums, as well as decreased or lack of appetite.

This long-acting medication lasts for at least 4 – 6 weeks, and may last longer in pets with kidney or liver disease.

Are there any risk factors for this medication?
Megestrol acetate should not be used in pets that are allergic to it or pets that have uterine disease, diabetes mellitus, breast tumors, uterine bleeding, blood clots, or in pets that are pregnant. Do not use this medication to induce a heat cycle or in dogs prior to their first heat cycle. Avoid mating in dogs that have a heat cycle within 30 days of the completion of megestrol acetate therapy. Adrenal gland suppression may occur and steroid supplementation may be needed in times of stress.

This medication should be used with extreme caution in pets that are pregnant, especially during the second half of pregnancy. Use caution in lactating pets, as this medication does enter the milk and the effects are unknown. It should also be used cautiously in cats with known viral infections.

Are there any drug interactions I should be aware of?
The following medications should be used with caution when given with megestrol acetate: corticosteroids and rifampin.

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

Is there any monitoring that needs to be done with this medication?
Blood glucose levels, weight, development of extra breast tissue or tumors, vaginal discharge, adrenocortical function, and liver enzymes should be monitored while your pet is using megestrol acetate. Your veterinarian will also monitor your pet to be sure that the medication is working. Monitor your pet at home for severe side effects and contact your veterinarian if they occur.

How do I store megestrol acetate?
The tablets should be stored at temperatures less than 40°C (104°F), preferably between 2°C and 30°C (36°F and 86°F), and in tight containers away from moisture. The liquid should be stored between 15°C to 25°C (59°F to 77°F) in a tight container and protected from heat.

What should I do in case of emergency?
If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.

Contributors: Rania Gollakner, BS, DVM, MPH