Aromatase inhibitors (anastrozole, exemestane and letrozole)

Aromatase inhibitors are a group of drugs used to treat breast cancer in post-menopausal women (women who have gone through the menopause).

There are three types of aromatase inhibitors:

Taken as a tablet once a day, aromatase inhibitors are a type of hormone therapy (also known as endocrine therapy).

How do aromatase inhibitors work?

Some breast cancers are stimulated to grow by the hormone oestrogen. In women who have been through the menopause, oestrogen is no longer produced by the ovaries. But some oestrogen is still made in body fat using an enzyme (a type of protein) called aromatase. Aromatase inhibitors stop this enzyme from working so there’s less oestrogen in the body.

An aromatase inhibitor will only be prescribed if your breast cancer has receptors within the cell that bind to the hormone oestrogen, known as oestrogen receptor positive or ER+ breast cancer. All breast cancers are tested for oestrogen receptors using tissue from a biopsy or after surgery. When oestrogen binds to these receptors, it can stimulate the cancer to grow.    

When oestrogen receptors are not found (oestrogen receptor negative or ER- breast cancer) tests may be done for progesterone (another hormone) receptors. The benefits of hormone therapy are less clear for people whose breast cancer is only progesterone receptor positive (PR+ and ER-). Very few breast cancers fall into this category. However, if this is the case for you your specialist will discuss with you whether an aromatase inhibitor is appropriate.

If your cancer is hormone receptor negative, then an aromatase inhibitor will not be of any benefit.

Information on different aromatase inhibitors

Anastrozole, exemestane and letrozole all work in similar ways to each other and share many of the same side effects, although there are some differences.

To find out more about each of these, including when they are prescribed, how long they are taken for and possible side effects, see our separate anastrozole, exemestane and letrozole web guides.


Last reviewed: June 2016
Next planned review begins 2018

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