Aromatase catalyzes the conversion of testosterone (an androgen) to estradiol (an estrogen) in many tissues including the adrenal glands, ovaries, placenta, testicles, adipose (fat) tissue, and brain. Estrogen is produced directly by the ovaries and is also made by the body using aromatase. Aromatase inhibitors cannot do anything about estrogen produced by the ovaries, but they do interfere with the body's use of aromatase.
The growth of many breast cancers is promoted by estrogens. Most estrogen after menopause comes from the action of aromatase. Aromatase inhibitors may therefore be used to treat estrogen-dependent tumors after the menopause. Aromatase inhibitors are used mostly in women who have reached menopause, when the ovaries are no longer producing estrogen.