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How long do letrozole side effects last?

Medically reviewed by Carmen Fookes, BPharm. Last updated on Sep 27, 2021.

Official answer


Most side effects of letrozole are mild and temporary, lasting only a few days or weeks until your body gets used to taking the medication. Some other side effects of letrozole take a few months to develop and tend to persist long term until you stop taking letrozole. The half-life of letrozole is 2 days, so once you stop taking letrozole, you might continue to experience side effects for about a week to 10 days after your last dose.

Side effects that are more likely to be temporary and resolve within a few days or weeks include:

  • difficulty sleeping
  • dizziness
  • excessive sweating
  • fatigue (lack of energy)
  • feeling drowsy, weak, or tired
  • headache
  • hot flashes
  • nausea
  • vaginal bleeding or spotting.

Allergic reactions to letrozole are rare and symptoms include rash, itching, or swelling of the face, tongue, throat, or neck, severe dizziness, or trouble breathing and are most likely to happen within a few hours of starting letrozole.

Side effects that are more likely to take a few months to develop and persist long term include:

  • blood changes, such as decreases in white blood cell counts or platelets
  • edema (swelling that usually affects your hands, feet, legs, or arms)
  • hair loss
  • high cholesterol
  • low bone density (low levels of calcium and minerals in your bones)
  • low mood
  • weight gain
  • joint pain
  • muscle pain or bone pain.

Side effects that may take a while to resolve even after stopping letrozole include:

  • low bone density that may increase the risk of bone fractures and/or osteoporosis
  • weight gain.

Letrozole is not known to cause withdrawal symptoms on stopping it.

Talk to your doctor or pharmacist if your side effects last longer than expected, bother you, or become severe. Most people are given letrozole for 5 years, but some will take it for up to 10 years.

Letrozole is an oral prescription medication used for the treatment of breast cancer in postmenopausal women.

  • Letrozole Updated 11/2020. Natco Pharma Limited.

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