Anastrozole vs. Letrozole

• Anastrozole and letrozole are both aromatase inhibitors, anti-cancer medications used for treating postmenopausal women with breast cancer.
• The brand name for anastrozole is Arimidex and the brand name for letrozole is Femara. Both drugs are available in generic form.
• Side effects of anastrozole and letrozole that are similar include nausea, vomiting, and headache.
• Side effects of anastrozole that are different from letrozole include hot flashes, pain, arthritis, increased blood pressure, depression, bone pain, weakened bones, sleeplessness, and rash.
• Side effects of letrozole that are different from anastrozole include fatigue, muscle aches, diarrhea, constipation, and chest pain.
• Both anastrozole and letrozole should not be used with tamoxifen because tamoxifen can reduce anastrozole and letrozole levels in the body and decrease their therapeutic effects.

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.

Anastrozole

Side effects of anastrozole are:

• hot flashes,
• pain,
• arthritis,
• headache,
• increased blood pressure,
• depression,
• nausea,
• vomiting,
• bone pain,
• weakened bones,
• sleeplessness, and
• rash.

Letrozole

The most common side effects with letrozole are:

• nausea,
• vomiting,
• fatigue,
• headache,
• muscle aches,
• diarrhea,
• constipation, and
• chest pain.

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.

Anastrozole

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

Letrozole

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.

Anastrozole

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.

Letrozole

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.

Anastrozole

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

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