Arimidex® (also known as anastrozole) is a man-made drug which is used in the treatment of breast cancer in women who have had their menopause (change of life). It is a type of hormonal therapy.

Many breast cancers rely on supplies of the hormone oestrogen to grow. In women who have passed the menopause the main source of oestrogen is through changing androgens (sex hormones produced by the adrenal glands) into oestrogens. This is carried out by an enzyme called aromatase. This conversion process is known as aromatisation, and happens mainly in the fatty tissues of the body.

Anastrozole works by blocking this process and preventing this chemical change. This results in a reduction in the amount of oestrogen in the body. For this reason anastrozole is known as an aromatase inhibitor.

Anastrozole works in a different way to tamoxifen, as tamoxifen attaches itself to the oestrogen receptors in breast cancer cells and stops oestrogen from actually affecting the cells. This is known as an anti-oestrogen drug.

Clinical trials have shown that for women who have more advanced breast cancer (when the tumour has spread to other sites in the body) anastrozole and tamoxifen seem to give a similar chance of controlling the disease.

The trials that have been carried out with anastrozole have shown that it is a safe drug and it appears to have less side effects than tamoxifen. Despite this some people may have the following side effects in varying degrees:

• hot flushes and sweats. These are usually mild and may wear off after a period of time and become less of a problem.
• vaginal dryness. This may occur while using anastrozole. Gels are available which can help overcome the dryness and it will go back to normal after the medication is stopped.
• nausea (feeling of sickness), vomiting and diarrhoea. These side effects are uncommon but may occasionally happen. If they do occur they can usually be effectively treated so let your doctor know. Feeling sick can often be relieved by taking your tablet with food or at night. If you get diarrhoea it is important to drink plenty of fluids.
• hair thinning. Some women notice their hair becomes thinner while taking anastrozole. This is very uncommon and is usually mild and the hair goes back to normal at the end of treatment.
• headaches. Some people have headaches whilst taking anastrozole but this is not common. It is important to drink plenty of fluids. Let your doctor know if you are getting headaches, as medicines can be prescribed to help.
• skin rashes. Rarely, anastrozole can cause skin rashes.
• vaginal bleeding. Vaginal bleeding (usually in the first few weeks of treatment) has been reported. This is rare and has usually occurred after changing from existing hormone therapy to treatment with anastrozole. If bleeding continues, tell your doctor or breast care nurse.
• joint pains/ stiffness. This side effect is uncommon but some women experience pain and stiffness in their joints while taking anastrozole. Let your doctor know if these effects are troublesome. It may be helpful to take mild painkillers.
• tiredness and lethargy. Some people can have increased tiredness.
• weight gain. Weight gain can be a side effect of anastrozole. This is often due to water retention which is not harmful but can be uncomfortable.

Although this looks a long list it is important to remember that most women will have very few problems with the drug.

Anastrozole, like tamoxifen, is taken as a tablet once a day.

Currently in the UK, anastrozole is recommended as an alternative to tamoxifen in the treatment of women with advanced breast cancer. Most women with advanced breast cancer will start with tamoxifen and switch to anastrozole if their cancer is not controlled or if it starts to grow again after a period of time. If you are thinking about taking anastrozole rather than tamoxifen it is a good idea to discuss this with your specialist.