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What happens when you stop taking Arimidex?

Medically reviewed by Carmen Pope, BPharm. Last updated on April 26, 2023.

Official answer

  • The protective effects of Arimidex continue even after you stop taking it, so long as you have been taking it for around five years.
  • The latest results from the IBIS-II prevention trial reports Arimidex continues to significantly reduce the incidence of breast cancer for at least 5.9 years (the current duration of the trial). The trial is ongoing.
  • Some side effects of Arimidex, such as nausea, hot flashes, pain, chest discomfort, dry eyes, a reduced sex drive, and weakness, should resolve within a few weeks of discontinuing Arimidex.
  • However, some side effects, such as high cholesterol, lymphedema, osteoporosis, and vaginal dryness may persist despite stopping Arimidex.
  • It can also take several weeks or months to reverse bad sleeping habits caused by Arimidex.
  • Talk to your doctor if side effects from Arimidex are persisting despite stopping it.

Arimidex (anastrozole) is a medication that is used to treat breast cancer in postmenopausal women. In most cases, Arimidex is taken for five years, although some women may take it for a shorter period.

Many women worry that stopping Arimidex 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 post menopausal women at high risk for breast cancer continued to be significantly reduced 5.9 years after stopping five years of the Arimidex (average of 10.9 years post breast cancer treatment). The trial is ongoing.
  • The IBIS-II Prevention trial enrolled 3,864 postmenopausal women at increased risk for developing breast cancer from 2003 to 2012.
  • Women were considered high risk if they had two or more blood relatives with breast cancer, a mother or sister who developed breast cancer before the age of 50, or a mother or sister who had breast cancer in both breasts.
  • Among the participants, 1,920 were randomly assigned to anastrozole for five years and 1,944 to placebo.
  • Five-year adherence was 74.6 percent for Arimidex and 77.0 percent for placebo, which is not significantly different.

Once your treatment team has decided that you can stop taking Arimidex, you can stop it completely; you won’t need to taper down the dose gradually.

How long do side effects continue once Arimidex is stopped?

Many side effects of Arimidex will improve within a couple of weeks of stopping arimidex; however, some may persist long term. No new side effects have been reported in clinical trials after stopping Arimidex.

Side effects that should resolve quickly include:

  • Chest discomfort
  • Dizziness
  • Dry eyes
  • Headache
  • Heartburn
  • Hot flashes
  • Nausea
  • Pain
  • Sex drive
  • Weakness.

Side effects that may persist for a few months, or that may still require medications to treat them include:

  • Carpal tunnel syndrome and other nerve-related conditions
  • Dry hair
  • High blood pressure
  • High cholesterol
  • Lymphedema
  • Osteoporosis
  • Sleeplessness (it can take months to resolve bad sleep habits)
  • Vaginal dryness (tends to be the result of vaginal atrophy).

Unfortunately, no formal clinical studies have been done investigating the persistence of side effects from Arimidex.

How long does Arimidex last in your body for?

The half life of Arimidex is 30 to 60 hours. Generally it takes four to five half-lives for a drug to be eliminated from the body, so in the case of Arimidex this would be 150 to 300 hours, or six to 12 days.

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).

It 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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