Key takeaways

  • Montelukast can cause side effects, such as fever or headache.
  • Montelukast has a boxed warning for the risk of mental health changes. A boxed warning is the most serious warning from the Food and Drug Administration (FDA).
  • If side effects of montelukast become difficult to tolerate, talk with your doctor or pharmacist.

Montelukast is a generic prescription drug used for the following purposes in adults and some children:

You may also see this medication referred to as montelukast sodium. (Sodium is an inactive ingredient in the medication that helps it dissolve and be absorbed in your body.) Montelukast is the generic version of the brand-name drug Singulair.

This drug has a boxed warning for the risk of serious mood and behavior changes. A boxed warning is the most serious warning from the Food and Drug Administration (FDA). It alerts doctors and patients about drug effects that may be dangerous. For details, see the “Boxed warning for montelukast” section.

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

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

  • cough
  • fever
  • belly pain
  • dizziness
  • upper respiratory infection, such as the common cold or flu
  • sore throat
  • diarrhea
  • ear infection or earache
  • runny nose
  • nosebleed
  • sinusitis (swelling of the sinuses)
  • sinus headache

These side effects may be temporary, lasting a few days to weeks. However, if the side effects continue, bother you, or become severe, be sure to talk with your doctor or pharmacist.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. Sharing your experience helps make medications safer for everyone by giving doctors and researchers more information about how the drug works in real life. If you develop a side effect while taking montelukast and want to tell the FDA about it, visit MedWatch or call 800-FDA-1088.

Many common side effects of montelukast are mild. But in rare cases, some people may have serious side effects from this drug.

Serious side effects that have been reported with montelukast include:

  • high levels of eosinophils (a type of white blood cells)
  • liver-related side effects
  • low level of platelets (cells that help with blood clotting)
  • risk of serious mood and behavior changes

If you develop serious side effects while taking montelukast, 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.

Allergic reaction

Allergic reaction

For some people, montelukast can cause an allergic reaction.

This side effect occurred after the drug became available on the market.

In general, symptoms of allergic reaction can be mild or serious.

Ways to manage

For mild allergic reaction symptoms, such as a mild rash, call your doctor right away. They may recommend treatments to help manage your symptoms. They’ll also let you know whether you should keep taking the medication.

For severe allergic reaction symptoms, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms require immediate medical care because they can become life threatening. If you’ve had a serious allergic reaction to montelukast, your doctor may recommend taking a different medication instead.

Montelukast has a boxed warning. A boxed warning is the most serious warning from the Food and Drug Administration (FDA).

Risk of serious mood or behavior changes

Montelukast has a boxed warning for the risk of serious psychiatric side effects and mental health changes. These mood or behavior changes may include:

What you can do

Due to the risk of serious mood or behavior changes, doctors will only prescribe montelukast when the potential benefits of the treatment outweigh the potential risks. And they’ll only prescribe this medication for people with allergic rhinitis if they can’t use any other treatment or have tried other treatments that haven’t worked.

If you or those close to you notice any serious changes in your mood or behavior while you’re taking montelukast, let your doctor know right away. They’ll likely have you stop taking this medication and switch to a different treatment. Depending on your symptoms, your doctor may also recommend treatment for these side effects.

Help is out there

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

  • Call or text the 988 Suicide and Crisis Lifeline at 988 or chat at 988lifeline.org. Caring counselors are available to listen and provide free and confidential support 24/7.
  • Text HOME to the Crisis Text Line at 741741 to connect with a volunteer crisis counselor for free and confidential support 24/7.
  • Not in the United States? Find a helpline in your country with Befrienders Worldwide.
  • Call 911 or your local emergency services number if you feel safe to do so.

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’re not in the same household, stay on the phone with them until help arrives.

Here are answers to some commonly asked questions about montelukast’s side effects.

No. Montelukast shouldn’t cause weight gain or weight loss. These side effects weren’t reported in studies of people taking the drug.

It’s possible that other drugs used to treat asthma, such as prednisone, may cause weight gain. But this isn’t a side effect of montelukast.

Keep in mind that other side effects of montelukast, such as belly pain, the flu, or sore throat, may decrease your appetite. In some cases, this could lead to weight loss.

If you have concerns about weight changes while you’re taking montelukast, talk with your doctor.

Montelukast 10 milligrams (mg) is the only recommended dose for adults and for children ages 15 years and older.

If your child is younger than 15 years old and takes montelukast, their doctor may prescribe a chewable tablet in a dose lower than 10 mg. In this case, they may have a higher risk of side effects if they take any dose higher than their doctor prescribes.

It’s important to take montelukast exactly as prescribed. If you have questions about the montelukast dosage, talk with your doctor.

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

Liver-related side effects

Montelukast may increase liver enzyme levels.

Typically, mildly elevated liver enzyme levels do not cause symptoms. But very high liver enzyme levels could be a sign of serious liver problems, including liver damage.

Serious liver-related side effects were not reported in studies of people taking montelukast. Some people have reported liver-related side effects after the drug was approved, but it’s not known how often these side effects have occurred.

Examples of liver-related side effects include liver cell damage and cholestatic hepatitis. With cholestatic hepatitis, the flow of bile (a fluid produced by your liver that helps you digest fats) from your liver is blocked or slowed. This can result in a buildup of bilirubin and lead to liver inflammation (swelling and damage).

Most people who reported serious liver-related side effects with montelukast had other factors or conditions that increased their risk of liver damage. Examples include excessive alcohol use or other types of hepatitis.

Symptoms of serious liver-related side effects include:

  • fatigue
  • loss of appetite
  • nausea
  • yellowing of your skin or the whites of your eyes
  • itchy skin
  • dark urine
  • light-colored stools

What might help

Call your doctor right away if you have symptoms of liver side effects, such as the ones listed above. They’ll help you get treatment if needed. If you have serious liver problems, your doctor will likely have you stop taking montelukast and switch to a different treatment.

Sinus headache

Montelukast may cause a sinus headache. Headache was a common side effect reported in studies of people taking montelukast. But sinus headache was only reported by adults and children ages 15 years and older who took montelukast for year-long allergic rhinitis.

With sinus headache, the sinus areas behind your nose, eyes, cheeks, and forehead are inflamed or congested. This causes pain and pressure.

Symptoms of sinus headache can include:

  • a feeling of pain or pressure behind your forehead, eyes, nose, or cheeks
  • pain that worsens when you lean forward
  • aching in your top jaw
  • aching teeth

What might help

Sinus headaches can often go away on their own without prescription medication. Applying a warm wet cloth over the painful area may help relieve pressure and inflammation that causes pain.

Rinsing your nose and sinuses with nasal saline may also help relieve congestion.

If your sinus headache doesn’t go away or is very severe, let your doctor know. They may recommend taking an over-the-counter pain reliever, such as ibuprofen (Advil, Motrin) or acetaminophen (Tylenol).

You should talk with your doctor or pharmacist before taking any OTC medications. They’ll let you know if they’re safe to take with montelukast and with your condition.

Consider the following important information before starting montelukast.

Montelukast has a boxed warning. A boxed warning is the most serious warning from the Food and Drug Administration (FDA). For details, see the ”Montelukast: Boxed warnings” section.

Other precautions

Before taking montelukast, discuss your health history with your doctor. Montelukast may not be right for you if you have certain medical conditions or other factors affecting your health. Be sure to talk with your doctor if any of the following apply to you:

  • sensitivity to aspirin or other non-steroidal anti-inflammatory drugs (NSAIDs)
  • liver problems
  • phenylketonuria
  • previous allergic reaction
  • pregnancy
  • breastfeeding

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.