Contraceptive pills for female cats and dogs that contain megestrol acetate or medroxyprogesterone acetate (see table below) are effective, provided that users comply with the conditions for their use, particularly the timing during the sexual cycle and the frequency of administration. However, they have well-known side effects that have been documented in the scientific literature. Around 20 reports of adverse effects are received each year by the French Agency for Veterinary Medicinal Products (ANMV), part of ANSES. These side effects are due to the hormonal action of these products and are listed on the package leaflet. They are mainly genital (infections, uterine hypertrophies and tumours, mammary hypertrophies and tumours) and hormonal (diabetes mellitus). They mainly occur after prolonged use, but can also be triggered by short-term treatment. These side effects are potentially serious, requiring prompt surgical and/or medical treatment and may even lead to the death of the animal.

Precautions to be taken to limit adverse effects

The use of oral contraceptives in female cats and dogs is far from innocuous. To prevent or interrupt heat cycles in female cats and dogs and avoid unwanted pregnancies, veterinarians can recommend the most appropriate solution according to the profile of the animal concerned:  surgical sterilisation, which is the only definitive method, or medicated contraception, either in the form of injections (which can only be carried out by the veterinarian) or pills that can be administered by the owner.

In the latter case, the following precautions must be taken:

• It is essential to consult a veterinarian to determine the most appropriate treatment and limit the risk of adverse effects. As a reminder, since 2012, these medicines have only been available on prescription,
• Stop treatment immediately and contact a veterinarian if any adverse effects occur,
• As the risk of serious adverse effects increases with the duration of treatment, administration should be limited to what is strictly necessary,
• As the risk of serious adverse effects increases in the event of overdose, the animal should be weighed before each treatment and the dose adjusted to its weight,
• In the event of prolonged treatment (more than 3 months), the veterinarian should conduct regular checks.

At the ANMV's request, these recommendations will soon be included in the package leaflets and summaries of product characteristics of oral contraceptives. The SPCs are available on the website of the Index of veterinary medicinal products authorised in France.

Medicines concerned by these measures