• Anastrozole may be used to treat hormone receptor-positive breast cancer in postmenopausal women.
• Hormone receptor-positive breast cancer is stimulated or maintained by estrogens. Anastrozole lowers estrogen levels by preventing the synthesis of estrogen from adrenal androgens.
• Side effects that tend to come on quickly include hot flashes, nausea, vomiting, headache, and pain. Many of these will improve after a few days or weeks.
• Other side effects, such as high blood pressure, high cholesterol, lymphedema, and osteoporosis (brittle bones) take much longer to develop (months to years).

Anastrozole is a medication that is used to treat hormone receptor-positive breast cancer in postmenopausal women. In most cases, anastrozole is taken for five years, although some women may take it for a shorter period. Anastrozole is usually taken in addition to other medications.

Anastrozole may also be used to prevent breast cancer in women who are at high risk of developing the disease.

How does anastrozole work?

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 too which means that it only affects blood estradiol concentrations, and has no effect on the formation of adrenal corticosteroids or aldosterone.

What are the side effects of anastrozole?

Anastrozole works quickly to lower estrogen and some side effects start within 24 hours 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. These may start within a day or two of taking anastrozole.

Side effects that tend to come on quickly include hot flashes, nausea, vomiting, headache, and pain. Many of these will improve after a few days or weeks.

Other side effects, such as high blood pressure, high cholesterol, lymphedema, and osteoporosis (brittle bones) take much longer (months to years) to develop and may be hard to notice, but your doctor will monitor you for these.

Some side effects (such as osteoporosis, high cholesterol, high blood pressure) will persist for as long as you take anastrozole, and you may need to take medications for these. Others, such as lack of energy, nausea, and vomiting, and a headache go away within a couple of days to a few weeks.

Talk with your doctor if you are having difficulty managing your side effects from anastrozole.

Common side effects of anastrozole that affect more than 5% of women and generally start soon after therapy begins include:

• Asthenia (weakness or lack of energy)
• Back, chest, muscle, stomach, or pelvic pain
• Constipation
• Cough
• Diarrhea
• Headache
• Hot flashes (flushes)
• Joint pain
• Nausea and vomiting
• Rash
• Sleeplessness
• Sore joints
• Sore throat
• Vaginal dryness.

Other side effects that generally take weeks or months to develop 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.

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.