Abstract
Dear Editors – I read with great interest Dr Romagnoli’s comments on the use of megestrol in his recent JFMS review on progestins to control feline reproduction. 1 This is a drug, under the trade name Ovaban, that I started using in the 1970s for canine contraception. We went off-label and used it for feline contraception as he described. Its long-term use serendipitously uncovered that megestrol has powerful soft tissue anti-inflammatory properties in cats that are not found in dogs. It came to be used widely for atopy, flea allergic dermatitis, lymphoplasmacytic stomatitis, vomiting due to inflammatory bowel disease (or ‘idiopathic vomiting’ as it was then called), eosinophilic keratitis, rodent ulcers, eosinophilic granulomas and dysuria due to idiopathic cystitis. We reached a point where we said humorously that no cat should be allowed to die without a test dose of Ovaban; it was called the ‘feline silver bullet’.
However, its widespread use eventually uncovered some notable side effects that I would like to list for the benefit of veterinarians who may wish to use this drug for feline contraception. The most common doses were 5 mg/cat 1–3 times per week.
Increased appetite and weight gain. This is the most consistent side effect. Megestrol can produce significant weight gain in just a few months. Conversely, it has been used to stimulate the appetite of cats (and humans) with anorexia due to severe illness, especially neoplasia.
Personality changes. Aggressive cats can become very calm and docile; very laid-back cats can become aggressive.
Mammary hyperplasia. This can be very severe. Discontinuation of the drug can cause reversal, but some cats require mastectomy.
Mammary adenocarcinoma. This occurs very infrequently, but when it does occur it is usually fatal.
Onset of diabetes. This is the most dramatic of the common side effects. During the 1980s we thought that megestrol caused diabetes since it impairs glucose metabolism. In retrospect, it is more likely that the onset of diabetes was only in cats that were borderline diabetics already. Those cats would have become diabetic within a few months, even if megestrol had not been given. Most of the megestrol-associated diabetics went into diabetic remission if the drug was promptly withdrawn. However, those cats usually became diabetic again because they had impaired insulin secretion prior to the use of megestrol.
These side effects, especially diabetes, gave the drug a very bad reputation in the 1980s and 1990s. Many condemned its use and those who prescribed it. My contact with US graduates in the past 5 years tells me that it is no longer even discussed in veterinary schools. Therefore, I am writing this to give new graduates an understanding of the history of this drug. I still use it occasionally, and only short-term, for situations that are not well treated with other drugs. However, I am still in the hope that another ‘silver bullet’ will emerge so I no longer feel that I have to use megestrol.