If you have breast cancer, you may be interested in learning more about letrozole. It’s a generic prescription drug used to treat the following kinds of breast cancer in certain situations:

For this purpose, the drug is prescribed to adult females* who have gone through menopause. It is not used in adult males* to treat breast cancer.

Letrozole comes as a 2.5-milligram (mg) oral tablet (a tablet that you swallow). It’s also available as the brand-name drug Femara.

If you and your doctor determine that letrozole is safe and effective for you, you’ll likely take it long term. But doctors usually don’t prescribe this medication for longer than 5 years.

This article describes letrozole’s side effects. For more information about letrozole, including details about its uses, see this in-depth article.

* In this article, we use the terms “female” and “male” to refer to someone’s sex assigned at birth. For information about the difference between sex and gender, see this article.

Some people may experience mild to serious side effects during their letrozole treatment. Examples of letrozole’s commonly reported side effects include:

* To learn more about this side effect, see the “Letrozole’s side effects explained” section below.

Some people may have mild side effects while taking letrozole.

Examples of mild side effects that have been reported with letrozole include:

* To learn more about this side effect, see the “Letrozole’s side effects explained” section below.
† An allergic reaction is possible after taking letrozole. But it’s not clear whether this side effect occurred in studies.

In most cases, these side effects should be temporary. And some may be easily managed. But if you have any symptoms that are ongoing or bother you, talk with your doctor or pharmacist. And do not stop taking letrozole unless your doctor recommends it.

Letrozole may cause mild side effects other than the ones listed above. See the drug’s prescribing information for details.

It’s possible to have serious side effects from letrozole, but most are rare.

Serious side effects that have been reported with letrozole include:

* To learn more about this side effect, see the “Letrozole’s side effects explained” section below.
† An allergic reaction is possible after taking letrozole. But it’s not clear whether this side effect occurred in studies.

If you develop serious side effects while taking letrozole, call your doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you’d like to notify the FDA about a side effect you’ve had with letrozole, visit MedWatch.

Learn more about some of the side effects letrozole may cause.

Weight gain

Letrozole can cause weight gain. This was a commonly reported side effect in studies of people taking this drug.

Swelling, usually in your arms, hands, legs, or feet, is another possible side effect of letrozole. If you have swelling while taking letrozole, weight gain may be a symptom of this side effect.

What might help

Talk with your doctor if you have concerns about weight changes while taking letrozole. They can recommend ways to maintain a weight that’s healthy for you. If you have troublesome swelling from letrozole, your doctor may have you stop taking this medication and suggest a different treatment option for you.

Hair loss

In studies of letrozole, hair loss was reported as a common side effect. This side effect happens because letrozole decreases estrogen in your body.

What might help

It may be helpful to be gentle with your hair to help reduce hair loss. Here are a few tips that may help protect your hair:

  • Try to limit your hair washing to no more than twice each week.
  • Use mild, fragrance-free shampoo and conditioner.
  • Avoid using hot water to wash and rinse your hair.
  • Pat your hair dry instead of rubbing it.
  • Use a soft hairbrush or wide-toothed comb to gently brush or comb your hair.

If you have concerns about hair loss with letrozole, talk with your doctor. They can recommend more ways to help with hair loss.

Headache

Letrozole can cause headaches. This was one of the more commonly reported side effects in studies of the drug.

What might help

Over-the-counter (OTC) pain relievers, such as Advil (ibuprofen) or Tylenol (acetaminophen), may help relieve mild headaches. But be sure to check with your doctor or pharmacist before taking any OTC medications. They can let you know whether the medications are safe to take with letrozole.

If you have a sudden, severe headache while taking letrozole, call 911 or a local emergency number immediately. This may be a symptom of stroke, which is a possible serious side effect of letrozole.

Joint pain

In studies, joint pain was commonly reported in people taking letrozole. In addition, letrozole may cause muscle and bone pain.

What might help

If you have joint pain while taking letrozole, talk with your doctor. To relieve this side effect, they may recommend OTC nonsteroidal anti-inflammatory drugs (NSAIDs), such as Advil (ibuprofen) or Aleve (naproxen). They may also recommend other OTC pain relievers, such as Tylenol (acetaminophen).

Allergic reaction

Like most drugs, letrozole can cause an allergic reaction in some people. But it’s not clear whether this side effect occurred in studies.

Symptoms can be mild to serious and can include:

  • skin rash
  • itchiness
  • flushing (temporary warmth, redness, or deepening of skin color)
  • swelling under your skin, usually in your eyelids, lips, hands, or feet
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

What might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. They may suggest a treatment to manage your symptoms. Examples include:

  • an antihistamine that you swallow, such as Benadryl (diphenhydramine)
  • a product you apply to your skin, such as hydrocortisone cream

If your doctor confirms you’ve had a mild allergic reaction to letrozole, they’ll decide whether you should continue taking it.

If you have symptoms of a severe allergic reaction, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms could be life threatening and require immediate medical care.

If your doctor confirms you’ve had a serious allergic reaction to letrozole, they may have you switch to a different treatment.

Keeping track of side effects

During your letrozole treatment, consider taking notes on any side effects you’re having. You can then share this information with your doctor. This is especially helpful when you first start taking new drugs or using a combination of treatments.

Your side effect notes can include things such as:

  • what dose of the drug you were taking when you had the side effect
  • how soon you had the side effect after starting that dose
  • what your symptoms were
  • how it affected your daily activities
  • what other medications you were taking
  • any other information you feel is important

Keeping notes and sharing them with your doctor will help them learn more about how letrozole affects you. They can then use this information to adjust your treatment plan if needed.

Get answers to some frequently asked questions about letrozole’s side effects.

Can taking letrozole affect my teeth?

It’s not likely. Letrozole is not known to directly cause teeth-related side effects. Such side effects were not reported in studies of people taking this drug.

But letrozole can cause bone pain and joint pain. If you have joint pain in your jaw or bone pain in your face, it’s possible that you may experience pain in your teeth.

If you have troublesome tooth pain while taking letrozole, talk with your doctor. They may recommend over-the-counter (OTC) pain relievers, such as Tylenol (acetaminophen) or Advil (ibuprofen).

Always check with your doctor or pharmacist before taking any OTC medications. They can tell you whether the medications are safe to take with letrozole.

Will I experience mood changes from letrozole?

It’s possible. Depression was a side effect reported in studies of letrozole, but it was not common.

Mood changes with depression can include:

If you or someone close to you notices that you’re experiencing mood changes while you’re taking letrozole, let your doctor know. They can recommend ways to help you manage your symptoms. They can also recommend mental health resources if needed.

Help is out there

If you or someone you know is in crisis and considering suicide or self-harm, please seek support:

If you’re calling on behalf of someone else, stay with them until help arrives. You may remove weapons or substances that can cause harm if you can do so safely.

If you are not in the same household, stay on the phone with them until help arrives.

Was this helpful?

Does letrozole cause any long-term side effects?

Although rare, letrozole may cause some long-term side effects.

For example, studies of the drug reported osteoporosis as a side effect. With osteoporosis, you have thinning bones, which increases your risk of bone fractures. This side effect can last even after you stop taking letrozole.

Also, studies have found an increase in the risk of side effects from letrozole after 5 years of treatment. Specifically, your risk of bone fractures may increase after 5 years of taking this drug.

If you have questions about the long-term side effects of letrozole, talk with your doctor.

Letrozole may not be right for you if you have certain medical conditions. These are known as drug-condition interactions. Other factors may also affect whether letrozole is a good treatment option for you.

Talk with your doctor about your health history before starting letrozole. Factors to consider include those described below.

Osteoporosis. Letrozole can cause weakened or thinning bones. If you have osteoporosis, this medication can worsen your condition. Before you start letrozole treatment, let your doctor know if you have osteoporosis or any problems with your bones. They’ll likely recommend a different treatment option for you.

Allergic reaction. If you’ve had an allergic reaction to letrozole or any of its ingredients, your doctor will likely not prescribe letrozole. Ask them what other medications are better options for you.

Liver problems. Let your doctor know about any liver problems you have before you start taking letrozole. Liver problems may cause the drug to build up in your body, which increases your risk of side effects. If you have liver problems, your doctor may prescribe a lower dosage of letrozole.

High cholesterol. Letrozole can increase cholesterol levels. If you have high cholesterol, this medication may increase it even more. Tell your doctor whether you have problems with cholesterol before taking letrozole. They’ll likely check your cholesterol levels regularly during your letrozole treatment. If your cholesterol levels get too high, they may prescribe medication to help lower them.

Alcohol and letrozole

There aren’t any known interactions between alcohol and letrozole. But drinking alcohol can worsen or increase your risk of certain side effects of the drug. These include:

  • dizziness
  • sleepiness
  • nausea
  • fatigue (low energy)

If you have concerns about drinking alcohol while taking letrozole, talk with your doctor.

Pregnancy and breastfeeding while taking letrozole

Letrozole is not safe to take during pregnancy or while breastfeeding.

Although letrozole is prescribed to treat certain kinds of breast cancer in females* who have gone through menopause, the drug may be prescribed off-label for other uses. (With off-label use, doctors prescribe a drug for a purpose other than what it’s approved for.) In this case, before prescribing the drug, doctors will test for pregnancy anyone who can become pregnant.

If your doctor prescribes letrozole, they’ll advise that you use an effective method of birth control during your treatment. They’ll also recommend that you keep using it for at least 3 weeks after your last dose of letrozole.

Let your doctor know right away if you become pregnant while taking letrozole. They’ll have you stop taking this medication and recommend a different treatment.

You should not breastfeed while taking letrozole or for at least 3 weeks after your last dose.

If you’re planning to become pregnant or breastfeed, talk with your doctor about other treatments for your breast cancer.

* In this article, we use the term “female” to refer to someone’s sex assigned at birth. For information about the difference between sex and gender, see this article.

If you have any questions about side effects that letrozole can cause, talk with your doctor. You can also ask them about Femara, which is the brand-name version of letrozole. A generic drug and its brand-name version are expected to have similar side effects because they contain the same active ingredient. (An active ingredient is what makes a drug work.)

Here are a few questions you may want to ask your doctor:

  • Will stopping letrozole treatment suddenly cause withdrawal symptoms?
  • How do the side effects of letrozole compare with those of Arimidex (anastrozole)?
  • Does my risk of bone fractures depend on the dosage I take?
  • Am I at higher risk of side effects if I’m older than age 65 years?

To learn more about letrozole or Femara, see these articles:

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Disclaimer: Healthline has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.