Anastrozole (Arimidex)

How Anastrozole Works

After menopause, the ovaries stop making most of the body's estrogen. A smaller amount is still made in fat, muscle, skin and breast tissue, where an enzyme called aromatase turns androgens (hormones made by the adrenal glands) into estrogen.

Anastrozole blocks aromatase. In postmenopausal women, a 1 mg daily dose lowers estradiol (the main estrogen) by roughly 70% or more within a day, and levels stay low as long as you keep taking it. Hormone receptor-positive breast cancer cells need estrogen to grow, so lowering estrogen can slow or stop their growth and lower the chance that cancer returns.

Why it does not work alone before menopause

Before menopause, the ovaries make large amounts of estrogen. Blocking aromatase in that setting can make the brain signal the ovaries to work even harder. That is why anastrozole is only used in premenopausal women together with ovarian suppression (monthly or 3-monthly injections such as goserelin or leuprolide, or surgery to remove the ovaries).

Nonsteroidal vs. steroidal

Anastrozole and letrozole are nonsteroidal aromatase inhibitors: they block the enzyme reversibly. Exemestane is steroidal and inactivates the enzyme permanently. All three work about equally well against breast cancer.

FDA-Approved Uses

The FDA has approved anastrozole for postmenopausal women with:

• Early breast cancer (adjuvant treatment): Hormone receptor-positive breast cancer after surgery, to lower the chance of recurrence.
• Advanced breast cancer, first treatment: Hormone receptor-positive or receptor-unknown, locally advanced or metastatic breast cancer.
• Advanced breast cancer after tamoxifen: Cancer that has grown during or after tamoxifen treatment.

The label notes that people whose cancer is hormone receptor-negative, or whose cancer did not respond to tamoxifen before, rarely respond to anastrozole.

Guideline-supported uses

• Premenopausal women with ovarian suppression: An aromatase inhibitor plus ovarian suppression is an option for higher-risk premenopausal early breast cancer (ASCO and NCCN guidelines).
• Breast cancer risk reduction: In the IBIS-II trial, 5 years of anastrozole lowered breast cancer risk by about half in high-risk postmenopausal women, with the benefit lasting after treatment ended. ASCO lists anastrozole as an option for risk reduction, although this use is not on the FDA label.
• Extended therapy: Some women continue an aromatase inhibitor beyond 5 years, depending on recurrence risk and side effects.
• With targeted drugs: In advanced breast cancer, an aromatase inhibitor is often combined with a CDK4/6 inhibitor such as palbociclib (Ibrance).

Key evidence

The ATAC trial compared anastrozole, tamoxifen, and the combination in more than 9,000 postmenopausal women. After 10 years of follow-up, anastrozole lowered the risk of recurrence more than tamoxifen, with fewer blood clots and fewer endometrial cancers, but more fractures and joint symptoms. Taking anastrozole and tamoxifen together worked no better than tamoxifen alone, so the two are never combined.

Dosing and How to Take It

• Dose: 1 mg once a day by mouth, at about the same time each day
• Food: With or without food
• Kidney or mild-to-moderate liver problems: No dose change needed (the label notes limited study in severe liver impairment)
• Older adults: No dose change based on age
• Missed dose: Take it when you remember that day. If it is nearly time for the next dose, skip it. Never take two at once.
• Length: Usually 5 years for early cancer. Some women take tamoxifen first and switch, or continue for longer. For advanced cancer, treatment continues as long as it is working.

Side Effects

Anastrozole side effects mostly come from low estrogen. In the ATAC trial and the FDA label, the most common were:

Less common but important

• Fractures and osteoporosis (see Protecting Your Bones)
• Carpal tunnel syndrome (numbness or tingling in the hand) and trigger finger
• Raised cholesterol
• Heart events in women with existing heart disease
• Liver test changes; rarely, hepatitis
• Severe skin reactions and allergic reactions (rare)
• Hair thinning

What anastrozole does not do

Unlike tamoxifen, anastrozole does not raise the risk of uterine cancer, and blood clots and strokes were less frequent than with tamoxifen in ATAC. It does not cause hair loss like chemotherapy or lower blood counts.

Joint Pain (Aromatase Inhibitor Arthralgia)

Joint pain and stiffness are the side effects most likely to make people stop anastrozole. They usually start within the first few months. Typical features are stiffness on waking, pain in the hands, wrists, knees, hips or feet, and improvement once you get moving.

What helps

• Exercise: In the HOPE trial, a year of supervised aerobic and strength exercise reduced joint pain in women on aromatase inhibitors. Walking, swimming, cycling and gentle strength work all count.
• Pain relievers: Acetaminophen or an anti-inflammatory such as ibuprofen or naproxen, if safe for your kidneys, stomach and heart. Check with your team first.
• Duloxetine: In a SWOG trial, this medicine modestly reduced aromatase inhibitor joint pain.
• Acupuncture: A large SWOG trial found real acupuncture reduced joint pain more than sham or waiting-list care.
• Warmth, stretching and weight management can ease stiffness.
• Switching: If pain stays bad, a short break and then a switch to letrozole, exemestane or tamoxifen allows many people to continue hormone therapy.

Do not stop on your own. Tell your care team, so they can check for other causes and find a plan that lets you keep going. Read more on our cancer pain page.

Protecting Your Bones

Estrogen helps keep bones strong. Lowering it speeds up bone loss. In ATAC, fractures were more common with anastrozole than with tamoxifen during treatment (about 11% vs. 8% over 5 years), and the difference faded after treatment stopped.

• Baseline DEXA scan, then repeat about every 1 to 2 years while on treatment, or as your doctor advises.
• Calcium and vitamin D: Get enough from diet and supplements as recommended by your team. Many people need vitamin D levels checked.
• Weight-bearing and strength exercise, such as walking and resistance training.
• Limit alcohol and do not smoke.
• Bone-strengthening medicine: If you have osteoporosis or a high fracture risk, your doctor may recommend a bisphosphonate (such as alendronate or zoledronic acid) or denosumab. ASCO and Ontario Health guidelines also support considering bisphosphonates in postmenopausal women with early breast cancer, because they may lower the chance of cancer spreading to bone.
• Prevent falls: Good lighting, sturdy shoes, and removing trip hazards matter more as fracture risk rises.

Drug Interactions

• Tamoxifen: Do not take together. Tamoxifen lowers anastrozole blood levels by about 27%, and the combination was no better than tamoxifen alone.
• Estrogen-containing products: Hormone replacement therapy, birth control pills, and other estrogens counteract anastrozole and should not be used. Ask your oncologist before using any vaginal estrogen, even low dose.
• Supplements: Avoid concentrated soy isoflavone, red clover and other "menopause support" supplements unless your team approves them. Normal amounts of soy foods are considered safe.
• Other medicines: Anastrozole has few other clinically important interactions. It can be taken with CDK4/6 inhibitors, bisphosphonates, denosumab, most antidepressants, statins and blood pressure medicines.

Monitoring

• Bone density: Baseline and periodic DEXA scans
• Cholesterol: Periodic lipid panel, especially if you have heart risk factors
• Blood pressure at routine visits
• Vitamin D level if low levels are suspected
• Menopause status (blood estradiol and FSH) for women under 60 whose periods stopped with chemotherapy, or for women on ovarian suppression
• Symptoms: joint pain, mood, sleep, sexual health, and whether you are able to take every dose
• Cancer follow-up: mammograms and visits as planned by your oncologist

Anastrozole vs. Other Hormone Therapies

Cost and Financial Help

• Generic anastrozole is inexpensive. With insurance or a discount card, a month often costs under $20 to $30 in the U.S.; prices vary by pharmacy.
• Brand Arimidex costs much more and offers no medical advantage over generic.
• 90-day mail-order supplies often lower cost.
• Help paying: Ask your oncology social worker or pharmacist about patient assistance programs and nonprofit copay foundations.

Frequently Asked Questions