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HomemedicationsAnastrozole (Arimidex) vs. Letrozole (Femara): Side Effects of Breast Cancer Drugs

Anastrozole (Arimidex) vs. Letrozole (Femara): Side Effects of Breast Cancer Drugs

Anastrozole vs. Letrozole

What are anastrozole and letrozole?

Anastrozole is an aromatase inhibitor, an anti-cancer medication use for the adjuvant or initial treatment of breast cancer in post-menopausal women. Estrogen causes or increases growth of certain breast cancers. Anastrozole works by blocking aromatase enzyme, which is involved in estrogen production in the body. This leads to decreased tumor size or delayed progression of tumor growth in some women.

Letrozole is an oral, anti-estrogen drug that is used for treating postmenopausal women with breast cancer. The growth of some breast cancers in postmenopausal women is promoted by estrogens that circulate in the blood, and the adrenal glands are the main source of these circulating estrogens. Letrozole inhibits the enzyme in the adrenal glands (aromatase) that produces the estrogens, estradiol and estrone.

What are the side effects of anastrozole and letrozole?


Side effects of anastrozole are:


The most common side effects with letrozole are:

Cholesterol levels may increase during letrozole therapy. Cholesterol levels should be monitored and some patients may require treatment for high cholesterol levels. Letrozole decreases bone mineral density, increasing the risk of osteoporosis and fractures.

What is the dosage of anastrozole vs. letrozole?


The recommended dose of anastrozole is one tablet (1 mg) by mouth once daily.


The recommended dose is 2.5 mg once daily with or without food. Patients with severe liver impairment should receive 2.5 mg every other day.

What drugs interact with anastrozole and letrozole?


Anastrozole should not be used with tamoxifen because tamoxifen can reduce anastrozole levels in the body and decrease its therapeutic effects. Estrogens decrease the effect of anastrozole by increasing estrogen levels in the body.


Tamoxifen reduces blood levels of letrozole when both drugs are administered together. However, in clinical studies the benefit of letrozole was not reduced when administered immediately after tamoxifen. Letrozole is an anti-estrogen drug. Therefore, estrogen containing products counteract the effect of letrozole.

Are anastrozole and letrozole safe to use while pregnant or breastfeeding?


Anastrozole is not recommended for pregnant mothers under any circumstances. Anastrozole may cause fetal harm and terminate pregnancy. It is not known whether anastrozole enters breast milk. It should not be administered to nursing mothers to avoid any harm to the newborn.


Letrozole damages the fetus. It should not be taken by pregnant women. It is not known if letrozole is secreted into breast milk.


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