Tamoxifen has been used to treat breast cancer for more than 30 years. It is a very successful treatment and we know that it reduces the risk of the cancer coming back. However, more recently another group of drugs have been developed for breast cancer. They are called aromatase inhibitors and include exemestane (Aromasin), anastrozole (Arimidex®) and letrozole (Femara).

Tamoxifen and aromatase inhibitors are both types of hormone therapy for breast cancer, but they work in different ways. Tamoxifen blocks oestrogen from reaching oestrogen receptors on breast cancer cells. It can be used to treat pre-menopausal and post-menopausal women.

Aromatase inhibitors (AIs) block oestrogen from being made outside the ovaries. After the menopause the ovaries stop making oestrogen but it is still made outside the ovaries. Aromatase inhibitors are only suitable for women who are postmenopausal.

Generally the side effects of both drugs are similar. But, there are some important differences. Women taking tamoxifen are at slightly higher risk of developing blood clots or womb problems. While women on exemestane are more at risk of bone thinning and slightly more at risk of bone fractures.

Taking tamoxifen for 5 years greatly reduces the risk of breast cancer coming back. But, in some women breast cancer comes back even with tamoxifen . One reason for this may be that the breast cancer cells become resistant to tamoxifen. So doctors have looked at switching women from tamoxifen to an aromatase inhibitor drug, such as exemestane, after 2 or 3 years of treatment.

A trial called the IES trial compared 5 years of treatment with tamoxifen to switching from tamoxifen to exemestane after 2 or 3 years. The results showed that switching to exemestane reduced the risk of cancer coming back more than tamoxifen alone.

Overall the results of the IES study are encouraging and add to results from other studies that show switching to an aromatase inhibitor (exemestane, letrozole or anastrozole) can further reduce the chance of breast cancer coming back. So, this may be why your doctor has suggested you change to exemestane.

NICE (the organisation that gives advice on the use of new treatments in England and Wales) has looked at how aromatase inhibitors can be used to treat women with early breast cancer. They say that treatment with aromatase inhibitors should be considered for all post menopausal women with early oestrogen sensitive breast cancer. They also state that your doctor should discuss with you which treatment approach is likely to be best for you, taking into account:

• whether you have already had tamoxifen
• your individual risk of breast cancer coming back
• whether you have had the menopausethe side effects and benefits of each treatment
• how these may be affected by your own health factors (such as an increased risk of blood clots or osteoporosis).