Exemestane breast cancer drug inhibiting the enzyme aromatase, molecular model. Aromatase (white, green, blue and yellow) is an enzyme that produces oestrogen from androgen steroid hormones. Oestrogen is a hormone that stimulates the growth of certain breast cancers. Exemestane (centre, pink, red and white) is a drug taken orally as a treatment for breast cancer. It inhibits aromatase, reducing the production of oestrogen, thereby inhibiting the growth of some tumours. The drug also seems effective in reducing excessive oestrogen levels in men.