Aromatase inhibitor

How they work

After menopause, the ovaries make little estrogen, and most of what remains is produced in fat, muscle, skin and other tissues by aromatase. Blocking that enzyme cuts estrogen levels sharply.

There are two types. Anastrozole and letrozole are nonsteroidal and bind to the enzyme reversibly. Exemestane is steroidal and inactivates the enzyme permanently.

Main approved use

Aromatase inhibitors are mainstays of treatment for hormone receptor–positive breast cancer, mostly in postmenopausal women. They're used after surgery to lower the chance of recurrence, sometimes before surgery, and for advanced disease. Premenopausal women can take them only together with a medicine that shuts down the ovaries, because in a woman who still cycles, lower estrogen signals the brain to stimulate the ovaries even harder.

Side effects

Because they push estrogen very low, side effects resemble an intensified menopause:

• Joint and muscle pain and stiffness, the most common reason people stop.
• Bone loss, osteoporosis and fractures; bone density is usually monitored.
• Hot flashes, vaginal dryness and lower sex drive.
• Changes in cholesterol, raised blood pressure, fatigue and sleep problems.

Aromatase inhibitors in men's health

You may see anastrozole mentioned in testosterone clinics. Some prescribers add it to testosterone therapy to keep estradiol from rising, and the American Urological Association lists aromatase inhibitors among options that may be used, alone or with other medicines, to help men preserve fertility. These are off-label uses; aromatase inhibitors are not FDA-approved for men.

Using them routinely 'to control estrogen' on testosterone therapy is controversial. Men need some estradiol for bone density, libido and erections, and driving it too low can cause joint pain, weaker bones and sexual side effects. Any use should be based on symptoms and lab results, with a clear reason and follow-up.

Who should not take them

Aromatase inhibitors are not used in pregnancy and can harm a fetus. Before starting, clinicians usually check bone density, vitamin D and cholesterol, and they review other medicines: estrogen-containing products such as hormone therapy or birth control can work against an aromatase inhibitor, so your clinician needs to know about them. If you're taking one for breast cancer, never stop or change the dose without talking to your oncology team.

Joint pain and bone loss can often be managed rather than simply endured. Exercise, calcium and vitamin D, bone-protecting medicines and, in some cases, switching to a different aromatase inhibitor are options your care team may discuss.