An aromatase inhibitor is a medication that blocks the body’s production of estrogen by disabling the enzyme responsible for making it. These drugs can reduce estrogen levels by as much as 95%, and they are primarily used to treat hormone-sensitive breast cancer in postmenopausal women. They work differently from older estrogen-blocking drugs like tamoxifen, which let estrogen circulate but prevent it from attaching to cancer cells. Aromatase inhibitors cut off estrogen production at its source.
How Aromatase Inhibitors Work
Your body makes estrogen through an enzyme called aromatase, which converts androgens (a type of hormone) into estrogen. Before menopause, the ovaries produce most of the body’s estrogen. After menopause, the ovaries stop making estrogen, but the hormone is still produced in smaller amounts by fat tissue, breast tissue, bone, and even the brain. Aromatase is the enzyme doing that conversion in all of those sites.
Aromatase inhibitors shut down this enzyme, starving estrogen-dependent tissues of the hormone. In breast cancer, this matters because roughly two-thirds of breast cancers rely on estrogen to grow. When aromatase is blocked inside and around a tumor, there is a steep drop in the estrogen concentration within the tumor itself, not just in the bloodstream. That local effect is a key part of why these drugs work so well against breast cancer that has estrogen receptors.
Why Only Postmenopausal Women
Aromatase inhibitors do not stop the ovaries from making estrogen. They only block the enzyme in non-ovarian tissues like fat cells. In a premenopausal woman whose ovaries are still active, the ovaries would simply keep producing estrogen, making the drug ineffective. That’s why these medications are only prescribed to women who have gone through menopause, whether naturally, through surgical removal of the ovaries, or through medications that suppress ovarian function.
Premenopausal women with hormone-sensitive breast cancer are typically prescribed tamoxifen instead. In some cases, younger women can receive drugs or surgery to shut down their ovaries first, which then makes them eligible for an aromatase inhibitor.
The Three Main Drugs
There are three aromatase inhibitors in widespread clinical use, and they fall into two categories based on how they interact with the enzyme.
Nonsteroidal (reversible) inhibitors: Anastrozole and letrozole. These drugs sit in the active site of the aromatase enzyme and block androgens from binding to it. The blockade is reversible, meaning if the drug is removed, the enzyme can resume functioning. These are sometimes called type II inhibitors.
Steroidal (irreversible) inhibitor: Exemestane. This drug mimics the natural substance that aromatase normally acts on. The enzyme recognizes exemestane as a substrate and begins processing it, but in doing so, the drug permanently bonds to the enzyme and destroys it. This is why exemestane is sometimes called a “suicide inhibitor”: the enzyme’s own activity is what causes its permanent inactivation. New aromatase molecules must be produced before estrogen synthesis can resume.
Despite these mechanistic differences, all three drugs are effective at suppressing estrogen. The choice between them often depends on a patient’s side effect profile and how well they tolerate a given medication.
How Well They Work Against Breast Cancer
Aromatase inhibitors have become the standard hormonal treatment for postmenopausal women with hormone-receptor-positive breast cancer. Compared with tamoxifen, they reduce the risk of cancer recurrence. Real-world data shows that nonsteroidal aromatase inhibitors (with or without ovarian suppression) improve disease-free survival compared to tamoxifen-based regimens, a benefit that holds regardless of menopausal status when ovarian function is suppressed.
How Long Treatment Lasts
For decades, five years was the standard recommendation for hormonal therapy after breast cancer surgery. That has evolved. Current guidelines now tailor the duration to the individual’s risk level.
- Lower-risk cancers (stage I, favorable features): Five years of endocrine therapy is generally sufficient, particularly if an aromatase inhibitor was part of the regimen.
- Moderate-risk cancers (stage II, limited lymph node involvement): Around seven years of total treatment appears to offer an advantage over five.
- Higher-risk cancers (advanced stage or aggressive biology): Up to 10 years remains the standard recommendation.
Collectively, research supports seven to eight years as a practical “sweet spot” for women with average-risk breast cancers. The tradeoff is that longer treatment means longer exposure to side effects, so the decision is a balance between reducing recurrence risk and maintaining quality of life.
Common Side Effects
Because aromatase inhibitors suppress estrogen throughout the body, they can cause side effects in any tissue where estrogen normally plays a protective role.
Joint pain and stiffness are the most common complaints, often severe enough that some women consider stopping treatment. The pain typically affects the hands, wrists, knees, and hips, and it tends to develop within the first few months of starting the medication.
Bone loss is a well-documented consequence. Estrogen helps maintain bone density, so suppressing it accelerates bone breakdown. Women on aromatase inhibitors have a measurably higher fracture risk, and that risk increases with extended treatment beyond five years. Bone density monitoring is a routine part of care for women on these drugs.
Other common effects include hot flashes, fatigue, and mood changes, many of which overlap with menopausal symptoms since the drugs are essentially deepening the estrogen deprivation that menopause already started.
Managing Joint Pain and Stiffness
Joint symptoms are the side effect most likely to make women stop taking their medication, so managing them well matters. Several approaches have evidence behind them.
Exercise is one of the most effective interventions. In a controlled trial of physically inactive women on aromatase inhibitors, those assigned to 150 minutes of weekly aerobic exercise plus two strength-training sessions saw their worst joint pain scores drop by 1.6 points over 12 months, while women receiving usual care actually got slightly worse. Yoga has also shown benefits for balance, flexibility, and pain in smaller studies.
Acupuncture has shown modest benefit. In a trial comparing real acupuncture to sham treatment, 52% of women receiving true acupuncture achieved at least a 30% improvement in pain, compared with 33% in the sham group. Sessions were given twice weekly for six weeks, then weekly for another six weeks.
Over-the-counter pain relievers like acetaminophen or anti-inflammatory drugs can help with short-term flare-ups, though they are not recommended for long-term daily use. Opioids are not recommended for this type of pain.
Omega-3 fatty acids may help some women. A trial found that high-dose fish oil supplements reduced pain scores in women with a BMI of 30 or higher, though no benefit was seen in women at lower body weights.
Some women also develop carpal tunnel syndrome or trigger finger while on aromatase inhibitors. These are generally mild and can be managed with nighttime wrist splints or, if needed, minor procedures.
Uses Beyond Breast Cancer
While breast cancer treatment is their primary role, aromatase inhibitors are also used off-label in several other situations, particularly in men. Because aromatase converts testosterone to estrogen in male bodies too, blocking it raises testosterone levels while lowering estrogen.
In men with obesity-related low testosterone, aromatase inhibitors can reduce the excessive estrogen production that fat tissue drives, allowing testosterone and related hormones to rise. They have also been used to treat gynecomastia (enlarged breast tissue in men), where the drugs have been shown to reduce breast size. In adolescent boys with short stature or delayed puberty, aromatase inhibitors have been used to slow the closure of growth plates, which estrogen normally triggers, potentially allowing more time for growth.
These uses are less well-established than breast cancer treatment, and the drugs are not formally approved for most of them, but they reflect the broad biological role that estrogen and aromatase play across the body.