Exemestane is a medication used to treat hormone-sensitive breast cancer in postmenopausal women. Sold under the brand name Aromasin, it works by permanently shutting down the enzyme your body uses to produce estrogen, starving cancer cells of the hormone they need to grow. It’s taken as a single 25 mg tablet once daily with food.
FDA-Approved Uses
Exemestane has two primary approved uses, both in postmenopausal women with breast cancer that is estrogen-receptor positive (meaning the cancer grows in response to estrogen).
The first is early-stage breast cancer. In this setting, exemestane is used as part of a sequential approach: women take tamoxifen for two to three years, then switch to exemestane to complete a total of five consecutive years of hormonal therapy. This switch strategy reduces cancer recurrence and improves long-term survival compared to tamoxifen alone.
The second is advanced breast cancer. For postmenopausal women whose cancer has progressed despite tamoxifen treatment, exemestane serves as a next-line therapy to slow or stop disease progression.
Breast Cancer Risk Reduction
Beyond treating existing cancer, exemestane has shown striking results in preventing breast cancer in high-risk postmenopausal women. A major clinical trial of 4,560 women found that those who took exemestane were 65 percent less likely to develop invasive breast cancer over three years compared to women taking a placebo. In raw numbers, only 11 women on exemestane developed invasive breast cancer versus 32 on placebo. Exemestane also significantly reduced the development of ductal carcinoma in situ (DCIS), a precancerous condition.
About 30 percent of participants in that trial stopped taking exemestane due to side effects, roughly 10 percent dropping out each year.
How Exemestane Works
Your body produces estrogen through an enzyme called aromatase, which converts androgens (a type of hormone) into estrogen. Exemestane binds permanently to aromatase and destroys it. This is what makes it a “suicide inhibitor”: once exemestane attaches to the enzyme, that enzyme molecule is permanently deactivated. Your body has to produce entirely new aromatase molecules to resume estrogen production.
This permanent binding is what distinguishes exemestane from other aromatase inhibitors like letrozole and anastrozole, which attach temporarily and can release from the enzyme. In practical terms, all three drugs effectively suppress estrogen, but exemestane’s irreversible mechanism means it works even after it’s been cleared from your system.
Why It Only Works After Menopause
Before menopause, the ovaries are the body’s main estrogen factory, and they contain far more aromatase than exemestane can block. The drug simply can’t keep up. After menopause, the ovaries largely stop producing estrogen, and the remaining estrogen comes from smaller amounts of aromatase in fat tissue, muscle, and other organs. Exemestane can effectively suppress these lower levels of production, which is why it’s prescribed only for postmenopausal women.
Common Side Effects
Because exemestane works by dramatically lowering estrogen levels throughout the body, its side effects largely mirror the effects of estrogen loss. Hot flashes and joint pain are among the most frequently reported. These menopausal-type symptoms are generally mild, though they’re bothersome enough that a meaningful number of women choose to stop treatment.
Fatigue, increased sweating, insomnia, and headache are also common. Joint stiffness and musculoskeletal pain tend to be more prominent with aromatase inhibitors than with tamoxifen, and for some women this is the most disruptive side effect of daily treatment.
Effects on Bone Health
Estrogen plays a key role in maintaining bone density, so suppressing it with exemestane can accelerate bone loss. Research on a subgroup of women in the MAP.3 prevention trial showed that even with calcium and vitamin D supplementation, women taking exemestane experienced greater decreases in bone mineral density after two years compared to those on placebo.
Expert guidelines recommend a baseline bone density scan (called a DXA scan) before starting any aromatase inhibitor, with repeat scans every one to two years during treatment. The National Comprehensive Cancer Network advises scanning at least every two years, while ASCO guidelines recommend annual screening. If bone density drops into the osteoporosis range, bone-protecting medications can be added to the treatment plan.
Notably, in the large prevention trial, the rates of osteoporosis, bone fractures, and cardiac events were identical between the exemestane and placebo groups over three years of follow-up. This suggests that while bone density does decline, the clinical consequences may take longer to emerge or may be manageable with monitoring.
How It Differs From Tamoxifen
Tamoxifen and exemestane both fight hormone-sensitive breast cancer, but they do it in fundamentally different ways. Tamoxifen blocks estrogen from attaching to cancer cells, like putting a key in a lock without turning it. Estrogen is still circulating in the body; it just can’t reach the receptors on the tumor. Exemestane, by contrast, stops the body from making estrogen in the first place.
This difference matters clinically. The standard treatment sequence for early-stage breast cancer uses both drugs: tamoxifen first for two to three years, then a switch to exemestane. Research from the National Cancer Institute confirmed that this sequential approach reduces breast cancer recurrences and prolongs survival compared to taking tamoxifen alone for the full five years. The two drugs also have different side effect profiles. Tamoxifen carries risks related to blood clots and uterine cancer, while exemestane is more associated with bone loss and joint pain.
Taking Exemestane
The standard dose is 25 mg once daily, taken after a meal. Food increases the drug’s absorption, so taking it on an empty stomach reduces its effectiveness. For women taking certain other medications that speed up the body’s metabolism of exemestane (such as rifampin or phenytoin), the dose may be increased to 50 mg daily to compensate.
In the adjuvant setting for early breast cancer, exemestane is typically taken for two to three years to complete a total of five years of hormonal therapy (following the initial years on tamoxifen). For advanced breast cancer, treatment generally continues as long as the cancer responds and side effects remain manageable.