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How soon do the side effects of Arimidex start?

Medically reviewed by Carmen Pope, BPharm. Last updated on April 25, 2023.

Official answer

  • Arimidex works quickly to lower estrogen and some side effects start within 24 hours of starting Arimidex.
  • 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.
  • Side effects that take longer to come on include high blood pressure, high cholesterol, osteoporosis, and lymphedema.

Arimidex (anastrozole) is a medication that is used to treat breast cancer in postmenopausal women.

Side effects may start within a day of starting Arimidex. This is because Arimidex 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 Arimidex.

Other side effects, such as high cholesterol 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 Arimidex, 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 Arimidex.

What are the side effects of Arimidex?

Common side effects of Arimidex 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 Arimidex 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.

How does Arimidex work?

The growth of many cancers of the breast is stimulated or maintained by estrogens. Arimidex lowers estrogen levels by preventing the synthesis of estrogen from adrenal androgens (primarily androstenedione and testosterone).

Arimidex does this by inhibiting the enzyme aromatase, which converts these androgens into estrogen. This slows the growth of tumors that require estrogen to grow. Arimidex is selective too which means that it only affects blood estradiol concentrations, and has no effect on the formation of adrenal corticosteroids or aldosterone.


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