Anastrozole: what it treats, how to take it, side effects
Last updated September 3, 2026.
Anastrozole (brand name Arimidex) is a once-daily aromatase inhibitor that starves hormone-driven breast cancer of estrogen. It is a cornerstone of treatment for hormone receptor-positive breast cancer in postmenopausal women, usually taken for years.
What does it treat?
It treats hormone receptor-positive breast cancer in postmenopausal women: as adjuvant therapy after surgery for early-stage disease, as first-line treatment for locally advanced or metastatic disease, and after tamoxifen for advanced disease. By blocking the aromatase enzyme, it drops estrogen production by over 90%, removing the signal the cancer grows on.
How do you take it?
One 1 mg tablet once a day, with or without food, at the same time daily. Adjuvant treatment typically runs for 5 years, sometimes longer; do not stop early on your own, because the protection accumulates over the full course. If you miss a dose, take it when remembered unless the next dose is near, and never double up. Your team will periodically check bone density and cholesterol, both of which anastrozole can move in the wrong direction.
Side effects to know
- Common: hot flashes, joint and muscle pain, weakness, headache, nausea, sweating, mood changes, insomnia, and vaginal dryness; basically induced-menopause effects plus achiness.
- Serious: bone thinning and fractures (density monitoring is standard), rising cholesterol, and a higher rate of heart-related events seen in women with pre-existing heart disease in trials. It can harm a fetus, so reliable contraception matters for anyone who could still become pregnant.
Who should not take it?
It is only for women who have completed menopause; it does not suppress ovaries, so it cannot work as intended in premenopausal women unless combined with ovarian suppression. Do not take it while pregnant or breastfeeding. Flag osteoporosis, high cholesterol, liver disease, or heart disease to your oncologist before starting. Tamoxifen reduces its levels, so the two are not combined.
When is it an emergency?
Call 911 for chest pain, sudden weakness or numbness on one side, or trouble speaking, and for signs of a severe allergic reaction like facial swelling or trouble breathing. A fracture from a minor fall, or severe new back pain suggesting a spinal compression, needs prompt care. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why was a man prescribed anastrozole?
Off-label. Anastrozole is FDA-approved only for breast cancer in postmenopausal women, but some clinicians prescribe it to men to lower estrogen conversion: for gynecomastia, certain fertility protocols, or to manage estradiol during testosterone therapy. Evidence in men is thinner, monitoring is essential, and the label notes it failed to show efficacy for pubertal gynecomastia. It is a specialist decision, not a self-start.
How long do I have to take anastrozole?
For early-stage breast cancer after surgery, the standard is 5 years of daily therapy, and some patients are advised to extend further. The benefit accumulates across the whole course, so stopping early because side effects annoy you is a decision to make with your oncologist, not alone.
Does anastrozole cause bone loss?
Yes, it is one of the main monitored effects: less estrogen means faster bone thinning and more fractures. Expect a baseline and periodic bone-density scans, calcium and vitamin D attention, weight-bearing exercise, and sometimes a bone-protecting drug if density drops.
Anastrozole vs tamoxifen: why one over the other?
Both treat hormone receptor-positive breast cancer, differently: tamoxifen blocks estrogen receptors, anastrozole cuts estrogen production. In postmenopausal women, aromatase inhibitors generally prevent recurrences somewhat better but cost more in bone and joint side effects. Premenopausal women start with tamoxifen, because anastrozole alone cannot stop ovarian estrogen.
Can anastrozole raise cholesterol?
It can. Increases in total cholesterol were seen in trials, and the label suggests monitoring. If cholesterol climbs on treatment, your team may add a statin or adjust cardiovascular risk management alongside the cancer therapy.
What can I do about the joint pain?
First, report it, because it is the most common reason people quit. Options your team may use: exercise programs (which trials show genuinely help), switching to a different aromatase inhibitor, short breaks, or pain-management strategies. Many find the aches ease after the first months.