A group of drugs that stop the action of the ENZYME, aromatase. This enzyme converts androgens (see ANDROGEN) to OESTROGENS. By inhibiting conversion, the concentrations of oestrogens in the body are reduced – so these drugs operate against tumours, such as breast cancer, that depend on oestrogen for their growth. Aromatase inhibitors include anastrazole, exemestane and letrozole and are usually recommended as part of the treatment of early stage hormone receptor positive breast cancer in postmenopausal women as an alternative to TAMOXIFEN. If used in premenopausal women, the drugs are combined with an ovarian suppressant.