This information is about a hormonal therapy called megestrol acetate, which is commonly known as Megace®. It is usually used to treat breast cancer that has spread. The information should ideally be read with our general information about secondary breast cancer.

Megace is a man-made substance called a progestogen which is similar to the female hormone progesterone. Hormones are substances that are produced naturally in the body. They act as chemical messengers and help to control the activity of cells and organs. Megace is known as a hormonal therapy.

Although it is mainly used to treat breast cancer Megace is sometimes used to treat cancer of the womb and occasionally to treat prostate cancer.

Megace can also be prescribed to reduce hot flushes in women during the menopause, and in men having hot flushes as a result of treatment for prostate cancer. Megace can also help to stimulate appetite so it's sometimes helpful for people who have eating problems due to advanced cancer.

The way in which Megace works is not yet fully understood, but it is thought that in some way it interferes with the action of particular female hormones in the body.

Many breast cancers rely on supplies of specific sex hormones, such as oestrogen, to grow. On the surface of the cancer cells are proteins called receptors, which are sensitive to the sex hormones. When the sex hormones come into contact with the receptors, they activate the cancer cells to divide and the tumour grows.

Megace disrupts this process in some way. It may be that it prevents the cancer cells from maintaining the receptors on their surface. Alternatively it may have a more direct way of destroying cancer cells that has not yet been identified.

Your doctor will consider a number of different factors before deciding whether Megace is the most appropriate treatment for you.

For breast cancer, Megace is mainly used as a treatment for women whose cancer has returned after a period of time during which it was successfully controlled with other hormonal therapies such as tamoxifen.

Megace may also be used by women whose cancer has not responded to other hormonal therapies, but whose cancer cells have oestrogen receptors on their surface.

Megace is taken as a tablet. The tablets are available in two different strengths: 40mg and 160mg. It is commonly taken once a day but can be prescribed in smaller doses to be taken at regular intervals during the day. When a single daily dose is prescribed it should be taken at the same time each day. When it is prescribed in divided doses (to be taken more than once per day) the doses should be evenly spaced.

Each person's reaction to any medication is unique. As people react to drugs in different ways it is not possible to predict who is going to have side effects. Many women who take Megace have no side effects, while others will experience them. Very rarely, if side effects are severe, you may have to stop taking the drug and a different hormonal drug may be prescribed.

Usually, there are few side effects of Megace and these are mild:

Increased appetite The most commonly reported side effect is an increase in appetite, causing some people to put on weight. Dieting can help to control this, but you should consult your doctor before making any changes to your diet.

Fluid retention Some women have experienced mild ankle swelling caused by fluid retention. This is not harmful but can be uncomfortable.

Feeling sick (nausea) Occasionally people feel sick for the first few weeks of taking Megace, but this usually goes away.

Very rarely, women have reported allergic reactions, jaundice and raised blood pressure.

Like most drugs, Megace should not be taken during pregnancy. Even though women may find that their periods stop while taking Megace, it is not a contraceptive, so reliable contraception must be used while having treatment.

People with diabetes will need to monitor their blood-sugar more closely and may need to adjust their anti-diabetic medication while on treatment with Megace. They should contact their doctor if there are any problems with the control of their diabetes.

People who have any history of blood clots should discuss this with their doctor before taking Megace. Rarely it increases the risk of getting a clot.

• Keep the tablets in a safe place out of the reach of children, as Megace could harm them.
• If your doctor decides to stop the treatment, return any remaining tablets to the pharmacist. Do not flush them down the toilet or throw them away.
• Don't worry if you forget to take your tablet. Do not take a double dose. Let your doctor or nurse know. The levels of the drug in your blood will not change very much, but try not to miss more than one or two tablets in a row.
• Remember to get a new prescription a few weeks before you run out of tablets. Make sure that you have plenty for holidays.

This section has been compiled using information from a number of reliable sources including:

• Martindale: The Complete Drug Reference (35th edition). Sweetman et al. Pharmaceutical Press, 2007.

• British National Formulary (56th edition). British Medical Association and Royal Pharmaceutical Society of Great Britain, September 2008.

For further references, please see the general bibliography.