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Anastrozole: 7 things you should know

Medically reviewed by Carmen Pope, BPharm. Last updated on June 21, 2023.

1. How it works

  • Anastrozole is a medicine that may be used for the treatment of hormone-receptor-positive breast cancer in postmenopausal women.
  • The growth of many cancers of the breast is stimulated or maintained by estrogens. Anastrozole lowers estrogen levels by preventing the synthesis of estrogen from adrenal androgens (primarily androstenedione and testosterone). Anastrozole does this by inhibiting the enzyme aromatase, which converts these androgens into estrogen. This slows the growth of tumors that require estrogen to grow. Anastrozole is selective, which means that it only affects blood estradiol concentrations, and has no effect on the formation of adrenal corticosteroids or aldosterone.
  • Anastrozole belongs to the class of medicines called aromatase inhibitors.

2. Upsides

  • Used in addition to other medications to treat hormone-receptor-positive early breast cancer in postmenopausal women. May also be used to treat hormone-receptor-unknown, locally advanced, or metastatic, breast cancer.
  • May also be used in the treatment of advanced breast cancer in women who have disease progression following tamoxifen treatment.
  • Anastrozole reduces the risk of breast cancer by 53% after 7 years of follow-up, and 50% after 10.9 years of follow-up.
  • Although hot flushes (flashes) are a common side effect, anastrozole is less likely than tamoxifen to cause these.
  • No dosage adjustment is needed for women with kidney disease, mild-to-moderate liver disease, or seniors.
  • Available as a generic.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Anastrozole is usually taken once daily for five years, although some women may take it for a shorter period.
  • Side effects may start within a day of starting Anastrozole. This is because Anastrozole lowers total body estrogen levels by about 70% within 24 hours of starting the drug, which causes side effects similar to those of menopause, such as hot flashes or difficulty sleeping.
  • Other side effects include asthenia (weakness or lack of energy; back, chest, joint, muscle, stomach, or pelvic pain; constipation; cough; diarrhea; headache; nausea and vomiting; rash; or vaginal dryness.
  • Some side effects take longer to develop and may include bone pain; carpal tunnel syndrome (a nerve condition that affects your wrist, causing pain, tingling, or numbness); depression or low mood; high blood pressure, high cholesterol; osteoporosis (brittle bones) and fractures; shortness of breath; swelling in the ankles and feet (peripheral edema); swollen lymph nodes (lymphedema); weight gain.
  • Women with pre-existing heart disease have an increased risk of developing high blood pressure or vasodilatation when taking anastrozole. The risk is higher than with tamoxifen (risk 17% with anastrozole compared with 10% with tamoxifen).
  • Monitor bone mineral density in patients taking anastrozole because trials have shown a decrease in both lumbar spine and total hip bone mineral density. An increase in cholesterol levels was noted in 9% of people taking anastrozole compared with 3.5% of people taking tamoxifen.
  • The most common reason for discontinuing anastrozole has been hot flashes, although the incidence of this is less than with tamoxifen.
  • Serious side effects that occur in less than 1% of women include skin reactions, such as lesions, ulcers, or blisters; severe allergic reactions with swelling of the face, lips, tongue, or throat; liver toxicity, including liver inflammation and changes in liver function tests.
  • Women who have ER-negative disease or who have not responded to previous tamoxifen treatment are unlikely to respond to anastrozole. Anastrozole also offers no benefit to premenopausal women with breast cancer and may cause harm to the fetus when administered to pregnant women. Anastrozole is contraindicated in pregnancy.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Anastrozole is a medication that may be used to treat postmenopausal breast cancer and works by lowering estrogen levels in the body. It is usually taken for five years and hot flushes are the most common side effect. It reduces the risk of breast cancer by 53% after 7 years of follow-up, and 50% after 10.9 years of follow-up.

5. Tips

  • Take anastrozole every day for as long as your doctor has recommended you take it (five years is the usual length of treatment).
  • Anastrozole can be taken with or without food.
  • If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip that dose and take the next regularly scheduled dose. Do not take two doses at a time.
  • Many women worry that stopping anastrozole after five years will cause their cancer to come back; however, the International Breast Cancer Intervention Study II (IBIS-II) Prevention trial reports that breast cancer incidence among postmenopausal women at high risk for breast cancer continued to be significantly reduced 5.9 years after stopping five years of anastrozole (average of 10.9 years post breast cancer treatment). The trial is ongoing.
  • Once your treatment team has decided that you can stop taking anastrozole, you can stop it completely; you won’t need to taper down the dose gradually.
  • Do not take anastrozole if you are pregnant or could become pregnant. Anastrozole is not recommended for use in premenopausal women.
  • Be alert for allergic-type reactions, and seek urgent medical attention if you develop swelling of the face, lips, tongue, or throat.
  • Tell your doctor if you experience new or worsening chest pain, shortness of breath, tingling, or numbness in your hands or feet.
  • Anastrozole may lower your bone density and put you at an increased risk of fractures. If you sustain a fracture tell the attending doctor that you take anastrozole.
  • Anastrozole may also increase your cholesterol. Your doctor may choose to monitor this.

6. Response and effectiveness

  • Anastrozole reduces the risk of breast cancer by 53% after 7 years of follow-up, and 50% after 10.9 years of follow-up.
  • Anastrozole also reduced the risk of other cancers compared with placebo, such as endometrial cancer, ovarian cancer, lung cancer, and melanomas.
  • In the IBIS-II trial, too few deaths were reported in the anastrozole Vs placebo arms (2 versus 3 respectively) to make any conclusions about overall survival differences.
  • Invasive ER-positive breast cancer was reduced by 54% with anastrozole treatment. A nonsignificant effect was observed for invasive ER-negative breast cancer.
  • Overall, anastrozole also reduced the risk of ductal carcinoma in situ (DCIS), with the biggest reduction seen in cases known to be ER-positive.
  • Breast cancer incidence among postmenopausal women at high risk for breast cancer continued to be significantly reduced 5.9 years after stopping five years of the anastrozole (average of 10.9 years post breast cancer treatment)
  • Anastrozole reduced the likelihood of breast cancer by 50% in women considered at high risk, at 10.9 years follow-up.
  • An estimated 29 women need to be treated with anastrozole for 5 years to prevent one woman from developing breast cancer during treatment and within the next 5 years. For tamoxifen, 49 women need to be treated to prevent one developing breast cancer.

7. Interactions

Medicines that interact with anastrozole may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with anastrozole. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with anastrozole include:

  • estrogen-containing treatments, such as estrone or estradiol
  • fostamatinib or tucatunib
  • tamoxifen (do not take together)
  • warfarin.

Note that this list is not all-inclusive and includes only common medications that may interact with anastrozole. You should refer to the prescribing information for anastrozole for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use anastrozole only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: June 21, 2023.