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Trade/other name(s): Femara

Why would this drug be used?

Letrozole is used to treat certain kinds of breast cancer in women are past menopause. It is also being studied for other uses.

How does this drug work?

Letrozole belongs to a general group of drugs called aromatase inhibitors. Normally in the body, some of the androgens (male hormones) that are made by the adrenal glands are converted to estrogen by an enzyme in fat tissue called aromatase. This drug blocks aromatase so that estrogen isn’t formed that way. This lowers estrogen levels in women who do not have functioning ovaries. This drug doesn’t affect estrogen production from the ovaries, so has little effect on estrogen levels in women whose ovaries are working. If estrogen levels are lowered, cancer cells that need estrogen are stopped from growing and may decrease in size.

Before taking this medicine

Tell your doctor…

  • If you are allergic to anything, including medicines, dyes, additives, or foods.
  • If you are pregnant, trying to get pregnant, or if there is any chance of pregnancy. There may be an increased risk of harm to the fetus if a woman takes this drug during pregnancy.
  • If you are breast-feeding. It is not known whether this drug passes into breast milk. If it does, it could harm the baby.
  • If you are still having menstrual periods. This drug is only for women who do not have functioning ovaries, such as those who are past menopause.
  • About any other prescription or over-the-counter medicines you are taking, including vitamins and herbs. In fact, keeping a written list of each of these medicines (including the doses of each and when you take them) with you in case of emergency may help prevent complications if you get sick.

Interactions with other drugs

Medicines that contain estrogen (hormone therapies, birth control pills or patches) raise estrogen levels and counter the action of anastrozole.

If tamoxifen is given with letrozole, it lowers the letrozole level in the blood.

Check with your doctor, nurse, or pharmacist about other medicines, vitamins, herbs, and supplements, and whether alcohol can cause problems with this medicine.

Interactions with foods

No serious interactions with food are known at this time. Check with your doctor, nurse, or pharmacist about whether foods may be a problem.

Tell all the doctors, dentists, nurses, and pharmacists you visit that you are taking this drug.

How is this drug taken or given?

For most patients, letrozole is a pill taken by mouth once a day. Patients with severe liver problems may only need to take this drug every other day. The dose is usually the same for all adults. Take with water, with or without food. Take this drug exactly as directed by your doctor. If you do not understand the instructions, ask your doctor or nurse to explain them to you.

Keep the medicine in a tightly closed container away from heat and moisture and out of the reach of children and pets.


Some people feel dizzy and tired when taking this drug, and a few notice feeling sleepy. This may affect driving. Find out how it affects you before driving or operating other dangerous machinery.

Because this drug lowers estrogen levels, it can weaken bones and lead to a condition called osteoporosis. Bones that become weak can break (fracture) more easily.

This drug may increase cholesterol levels, so your doctor may watch these levels while you are on treatment.

This drug is only helpful in lowering estrogen levels in women who do not have functioning ovaries (such as those who are past menopause). If you had your uterus removed (a hysterectomy) but still have your ovaries, blood tests of hormone levels may need to be done to see if your ovaries have stopped working (and you have truly gone through menopause).

Possible side effects

You will probably not have most of the following side effects, but if you have any talk to your doctor or nurse. They can help you understand the side effects and cope with them.


  • Hot flashes
  • Pain in bones, joints, or muscles
  • Feeling weak*

Less common

  • Feeling dizzy*
  • Increased sweating
  • Headache
  • Tiredness (fatigue)
  • Nausea and vomiting
  • Diarrhea
  • Constipation
  • Swelling, usually of arms, legs, or feet
  • Bone weakness or thinning, including osteoporosis*
  • Bones breaking (fractures)
  • Vaginal bleeding
  • Increase in cholesterol levels


  • Loss of appetite
  • Blood clots in veins of the arm or leg (deep venous thrombosis), the arteries of the lung (pulmonary embolus), or to the brain (stroke)
  • Heart attack or pain in the chest from the heart
  • Allergic reaction with swelling mouth or throat, skin welts, or trouble breathing
  • Death due to heart attack, stroke, pulmonary embolus, or some other cause

*See "Precautions" section for more detailed information.

Other side effects not listed above can also occur in some patients. Tell your doctor or nurse if you develop these or any other problems.

FDA approval

Yes – first approved in 1997.

Disclaimer: This information does not cover all possible uses, actions, precautions, side effects, or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for talking with your doctor, who is familiar with your medical needs.

Last Medical Review: 08/08/2014
Last Revised: 08/08/2014