If you have a certain kind of lung or allergic condition, you may be interested in learning more about montelukast. It’s 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.

If you and your doctor decide that montelukast is safe and effective for you, you’ll likely take it long term.

Montelukast comes in the following forms:

  • oral tablet (a tablet that you swallow whole)
  • chewable tablet

This medication also comes in granules that you can take by mouth or mix in liquid or food and then swallow. But that form is not covered in this article.

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

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

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

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

* To learn more about this side effect, see the “Side effects explained” section below.
† An allergic reaction is possible after using montelukast. 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 montelukast unless your doctor recommends it.

Montelukast may cause mild side effects other than the ones listed above. For more details, see the prescribing information for montelukast oral tablet and chewable tablet.

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)
  • risk of serious mood and behavior changes*
  • liver-related side effects†
  • severe allergic reaction‡
  • low level of platelets (cells that help with blood clotting)§

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.

* Montelukast has a boxed warning for this side effect. This is the most serious warning from the Food and Drug Administration (FDA). To learn more, see the “Side effects explained” section below.
† To learn more about this side effect, see the “Side effects explained” section below.
‡ An allergic reaction is possible after using montelukast. But it’s not clear whether this side effect occurred in studies.
§ This side effect was not reported in studies of people taking montelukast oral tablets or montelukast chewable tablets. But it’s been reported since montelukast became available for use.

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 montelukast, visit MedWatch.

Montelukast oral tablets can be prescribed for children ages 15 years and older. The chewable tablets can be prescribed in children ages 2 years and older for asthma and allergic rhinitis, and ages 6 years and older for exercise-induced bronchoconstriction.

In montelukast studies, most side effects reported were the same in children as those seen in adults. But certain side effects were reported in children ages 2 to 5 years who took montelukast to treat asthma.

They include:

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

Does montelukast cause weight gain or weight loss in adults?

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

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.

Will I have a higher risk of side effects if I take the 10-mg dose of montelukast?

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 the 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 for your child to take montelukast exactly as their doctor prescribes it. If you have questions about increasing your child’s dosage, talk with their doctor.

Is hair loss a side effect of montelukast?

No. Hair loss was not reported as a side effect in studies of people taking the montelukast tablets or montelukast chewable tablets.

But a 2019 review of studies found that people with allergic rhinitis (which montelukast treats) may have an increased risk of hair loss. If you take montelukast for this use and experience hair loss, it may be related to the condition itself, not caused by montelukast.

If you have concerns about hair loss during your montelukast treatment, talk with your doctor.

Does montelukast cause joint pain?

It’s possible. But this side effect was not reported in studies of people taking the montelukast tablets or montelukast chewable tablets.

People taking montelukast have reported joint pain after the drug was released on the market. But it’s not known how often this has occurred or if montelukast was the cause of the joint pain.

If you have bothersome joint pain while taking montelukast, let your doctor know. They’ll try to determine the cause of your symptoms and recommend ways to manage the pain.

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

Risk of serious mood and behavior changes

Montelukast 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).

These mood or behavior changes may also be referred to as mental health changes and psychiatric side effects. Examples include:

These side effects were not reported in studies of people taking montelukast oral tablets or montelukast chewable tablets. People have reported having these side effects after the drug became available for use. But it’s not known how often this has occurred.

People who took montelukast have reported these mental health side effects while taking montelukast and in some cases, after they stopped taking the medication.

What might help

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:

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.

Liver-related side effects

Montelukast may increase liver enzyme levels. This side effect was reported in studies of people taking montelukast oral tablets and montelukast chewable tablets.

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 oral tablets or montelukast chewable tablets. 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 (low energy)
  • 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 oral tablets or montelukast chewable tablets. 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.

Allergic reaction

Like most drugs, montelukast 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 you take by mouth, 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 montelukast, they’ll decide if you should continue using 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 montelukast, they may have you switch to a different treatment.

Keeping track of side effects

During your montelukast 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 montelukast affects you. They can then use this information to adjust your treatment plan if needed.

Montelukast comes with some warnings, including a boxed warning.

Boxed warning: Risk of serious mood or behavior changes

Montelukast has a boxed warning for the risk of serious psychiatric side effects and mental health changes. A boxed warning is the most serious warning from the Food and Drug Administration (FDA).

Due to this risk, doctors may not prescribe montelukast to treat allergic rhinitis unless the benefits of the treatment outweigh the risks.

To learn more, see the “Side effects explained” section above.

Other warnings

Montelukast 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 montelukast is a good treatment option for you.

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

Sensitivity to aspirin or other non-steroidal anti-inflammatory drugs (NSAIDs). For some people, aspirin and other NSAIDs can trigger asthma symptoms. Examples of these drugs include ibuprofen (Advil, Motrin), naproxen (Aleve, Naprosyn), and meloxicam (Mobic). If you have an allergy to aspirin or NSAIDs, or if these drugs worsen your asthma, you should not take these medications during your montelukast treatment.

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

Liver problems. Montelukast can cause serious liver-related side effects. Most people who have had liver-related side effects with montelukast also had other conditions that increased their risk of liver problems. These included heavy alcohol use and hepatitis. Before you start montelukast treatment, tell your doctor if you have liver problems or consume large amounts of alcohol. They’ll help determine if this medication is right for you.

Phenylketonuria. Montelukast chewable tablets contain phenylalanine, which is a part of aspartame. If your child has phenylketonuria or is sensitive to aspartame, they should not take montelukast chewable tablets. Instead, talk with their doctor about the oral tablet form of montelukast or other treatment options.

Alcohol and montelukast

There are no known interactions between alcohol and montelukast. But keep in mind that drinking large amounts of alcohol over time can increase your risk of liver damage. Montelukast can also cause serious liver-related side effects. And some people who reported serious liver problems while taking montelukast also consumed a lot of alcohol.

If you drink alcohol, talk with your doctor to learn how much, if any, alcohol is safe to consume while taking montelukast.

Pregnancy and breastfeeding while taking montelukast

Montelukast is not known to be harmful if taken during pregnancy. Studies of montelukast use during pregnancy have not shown that montelukast increases the risk of birth abnormalities (commonly known as birth defects).

If you’re pregnant or planning to become pregnant, talk with your doctor before starting montelukast treatment. They can discuss with you the potential benefits and risks of taking this drug during pregnancy.

Montelukast is also not known to be harmful if used while breastfeeding. Montelukast passes into breast milk. Studies of people taking montelukast while breastfeeding have not shown the drug causes harm to the breastfed child. But it’s not known how montelukast affects breastmilk production.

Before taking montelukast, talk with your doctor if you’re breastfeeding or planning to do so. They’ll help determine if this drug is right for you.

If you have any questions about side effects that montelukast can cause, talk with your doctor. You can also ask them about Singulair, which is the brand-name version of montelukast.

A generic drug and its brand-name version are expected to have the same side effects because they contain the same active ingredient. (An active ingredient is what makes a drug work.)

Here are examples of questions you can ask your doctor:

  • How do the side effects of montelukast compare with those of prednisone?
  • Do I have a higher risk of side effects if I have kidney problems?
  • Can montelukast cause constipation?

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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.