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Manufacturer – Megestrol acetate – generic (e.g. Megace®: Bristol-Myers Squibb; Ovaban®: Schering-Plough)

Product information – Megestrol acetate is a synthetic derivative of progesterone with anti-estrogenic activity. Megestrol acetate and Ovaban® are approved for use in domestic dogs for no more than 2 cycles. Megace® is approved for treatment of estrogen-sensitive tumors and for anorexia in humans because it increases appetite. Both have been used occasionally for contraception in carnivores. Megestrol acetate can be used temporarily during the initial stimulation phase of the GnRH agonist products. It is provided in various dose tablets.

Dose for suppression of initial estrus/ovulation caused by Suprelorin - The domestic dog dose is 2mg/kg body weight, but must be extrapolated for other taxa.

Dose for contraception in carnivores - Dosage studies have not been conducted for exotic species. A dose of 0.5-1.0mg/kg is recommended (the higher dose for smaller species and the lower dose for larger ones).

Latency to effectiveness - As with implants and injections, separation or alternative contraception should be used for 1-2 weeks after initiation of treatment.

Estrous cycles during contraceptive treatment - Synthetic progestins may effect contraception by blocking ovulation, causing thickening of cervical mucus, slowing ovum transport, and/or interfering with fertilization or implantation. However, follicle growth may continue and sometimes be accompanied by estrogen production sufficient to cause estrous behavior. Ovulation may occur even though pregnancy does not ensue. Higher progestin doses may be preferred, so that estrous behavior is prevented, but may not be effective in completely suppressing follicle growth and some estradiol production.

Duration of efficacy and reversibility - Duration of efficacy may not be much more than one day, so must be administered daily. Following cessation of treatment, rapid clearance can result in ovulation within a few days, but actual latency to conception will vary by individual.

Use during pregnancy - Progestins are not recommended in pregnant animals because of the possibility of prolonged gestation, stillbirth, abortion, etc. in some species, although the effect may depend on dose. Progestins in late pregnancy seem not to interfere with parturition in primates, but this is a taxon-specific phenomenon.

Use during lactation - Progestins are sometimes prescribed for lactating women and are considered generally safe for nursing infants.

Use in pre-pubertals or juveniles - Future reproduction was not affected in calves of domestic cows on MGA-treated feed, but no studies of pre-pubertal treatment with MGA or other progestins have been conducted with other species, so possible long-term effects on fertility are not known.

Consideration for seasonal breeders - Treatment should begin at least one month before the anticipated onset of the breeding season. This does not include however canids or other carnivorous species due to the potential for progestin side effects addressed in the corresponding taxonomic sections below.

Precautions – Progestins likely cause weight gain in all species. Possible deleterious effects on uterine and mammary tissues vary greatly by species; see cautions for each taxon.

Reporting requirements - All institutions using megestrol acetate are asked to complete a Contraception Survey for the AZA Reproductive Management Center. It is essential that accurate records of doses and intervals be maintained and results reported to the Reproductive Management Center Database to contribute to dosage development.

For questions about the Contraception Survey, contact:
Ashley Franklin, Program Analyst
AZA Reproductive Management Center
Saint Louis Zoo
1 Government Drive
St. Louis, MO 63110
314-646-4732; fax: 314-646-5534