How does anastrozole work, and who is it for?

Anastrozole works by lowering the amount of oestrogen in the body. About two in three breast cancers are hormone-receptor-positive, meaning their cells carry receptors for oestrogen or progesterone and use these hormones as a signal to grow and divide. Before menopause, most oestrogen comes from the ovaries. After menopause, the ovaries largely stop, but the body keeps making a smaller amount of oestrogen in fat, muscle, skin and breast tissue. It does this with an enzyme called aromatase, which converts hormones made by the adrenal glands, called androgens, into oestrogen. Anastrozole attaches to aromatase and blocks it, which lowers oestrogen levels in the blood and in breast tissue to very low levels. Without this growth signal, hormone-sensitive cancer cells are less able to grow, and tiny cancer deposits that may remain after surgery are less likely to develop into a recurrence. Because anastrozole cannot stop the ovaries themselves, it is meant for women who are past menopause, or younger women whose ovaries have been switched off with injections or surgery. In a woman whose ovaries are active, blocking aromatase can even prompt the brain to signal the ovaries to work harder. Anastrozole does nothing for cancers that are hormone-receptor-negative, because those cells do not rely on oestrogen. Your pathology report shows the receptor status, which is how your oncologist decides whether anastrozole is suitable.

It lowers oestrogen after menopause

Anastrozole blocks the enzyme that makes most oestrogen once the ovaries have stopped.

It only helps hormone-sensitive cancers

Your pathology report shows whether your cancer has oestrogen or progesterone receptors.

The ovaries must be inactive

Natural menopause or ovarian suppression is needed for anastrozole to work well.

The way anastrozole works is well understood, but biology varies between cancers.

Some cancers find other routes

Over time, some hormone-positive cancers learn to grow without oestrogen. This is called endocrine resistance, and it is the reason treatment may need to change in advanced disease.

Receptor strength matters

Cancers with strongly positive receptors tend to respond better than those with weak or low positivity.

Lowering oestrogen affects the whole body

The same drop in oestrogen that helps against cancer also causes joint pain, hot flushes and bone thinning.

What this page cannot tell you

It cannot read your pathology report. Your oncologist can explain your receptor results.

The pathology report from your biopsy or surgery describes how many cancer cells carry hormone receptors and how strongly.

Scores and percentages

Reports may give a score, such as an Allred score, or the proportion of cells that stain positive. Higher scores mean more hormone sensitivity.

Low-positive results

Cancers with only a small proportion of positive cells may gain less from hormone therapy, and your oncologist will weigh this carefully.

HER2 status

Your report also shows HER2, which guides whether targeted treatment is added alongside hormone therapy.