Anastrozole (INN) (marketed under the trade name Arimidex by AstraZeneca) is a non-steroidal aromatase-inhibiting drug approved for treatment of breast cancer after surgery, as well as for metastasis in both pre and post-menopausal women. The severity of breast cancer can be increased by estrogen, as sex hormones cause hyperplasia, and differentiation at estrogen receptor sites. Anastrozole works by inhibiting the synthesis of estrogen. The patent on Arimidex by AstraZeneca expired June 2010. It is on the World Health Organization's List of Essential Medicines, a list of the most important medications needed in a basic health system.