Key takeaways:
Singulair (montelukast) is an oral medication that treats asthma and allergies. It’s usually recommended as an alternate or add-on treatment if other medications aren’t enough.
Common Singulair side effects include upper respiratory tract infections, fever, and headache. Singulair also has a boxed warning for mental health-related side effects. But, this risk is rare overall.
Talk to a healthcare professional if you’re concerned about Singulair side effects. They can offer tips on how to prevent and manage them.
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Many asthma treatments come as inhalers, but Singulair (montelukast) stands out from the crowd. It’s an oral, prescription-only medication that’s FDA approved to treat asthma and manage allergy symptoms. It’s also approved to treat exercise-induced asthma. Singulair is usually reserved for managing symptoms when other treatments aren’t doing enough.
If you’re taking Singulair for the first time, it's important to know what to expect and how to manage side effects you might experience.
Singulair (montelukast) side effects at a glance
Singulair is beneficial for many people. But potential side effects might be a factor when deciding if Singulair is a good choice for you.
Many Singulair side effects are mild and get better as your body adjusts to the medication. But serious side effects are also possible, and some might require medical attention. Knowing what to expect can help you take Singulair safely and effectively.
Common Singulair side effects include:
Upper respiratory tract infection
Fever
Headache
Sore throat
Cough
Stomach pain
Diarrhea
Earache or ear infection
Runny nose
Sinus infection
Less common side effects include:
Tiredness or weakness
Dental pain
Dizziness
Nasal congestion
Rash
Serious Singulair risks and side effects include:
Muscle and joint pain
Heart palpitations
Serious allergic reactions, such as anaphylaxis
While rare, Singulair also has a boxed warning for serious mood and behavior changes. A boxed warning is the most serious warning the FDA places on a medication.
Below, we’ll cover seven notable Singulair side effects and tips for managing them.
1. Upper respiratory infections, including common cold symptoms
Singulair can increase the risk of developing an upper respiratory tract infection (URTI). The upper respiratory tract includes the nose, sinuses, and throat. A URTI occurs when an infection develops in any of these areas.
These infections tend to cause symptoms that resemble the common cold or a sinus infection. This might include a runny or stuffy nose, sneezing, or a sore throat. These symptoms are usually mild and go away on their own.
To help manage these symptoms, get plenty of rest and drink lots of fluids to stay hydrated. These tried-and-true tips help your body recover naturally. You can also try an over-the-counter (OTC) cold medicine that your prescriber or pharmacist recommends.
2. Headache
Headaches are a common side effect of Singulair. You might notice a mild or dull headache when you start the medication.
Most headaches improve once your body gets used to Singulair. But an OTC pain reliever might be helpful if a healthcare professional gives you the OK to take one.
3. Upset stomach
Singulair can sometimes cause an upset stomach. You might feel some stomach pain, indigestion, or nausea while taking it.
Taking Singulair with food can help reduce discomfort. Eating smaller, more frequent meals and drinking fluids might also help ease symptoms. Avoid greasy, spicy, and heavy foods to help prevent an upset stomach.
4. Diarrhea
Diarrhea can happen while taking Singulair. It usually gets better once your body adjusts to the medication.
In the meantime, there are some things you can try to help manage your symptoms, such as drinking clear fluids like water, electrolyte drinks, or broth. A healthcare professional might also recommend taking an OTC anti-diarrhea medicine to help with your symptoms.
5. Muscle and joint pain
Singulair can sometimes cause pain in your muscles and joints. This can feel like soreness, stiffness, or general discomfort.
Doing gentle stretches might help with pain or discomfort. OTC pain relievers might also be helpful. A healthcare professional can give you a recommendation on a safe one to try.
6. Heart palpitations
Heart palpitations are a less common side effect of Singulair. You might feel like your heart is racing, pounding, or skipping beats. Palpitations are typically a mild concern, but you should make note of them if they happen often or last for a long time.
To be safe, let your prescriber know if palpitations happen — even if they feel mild. They can walk you through the situation and let you know if any next steps are warranted. Go to the nearest ER right away, though, if you also experience symptoms such as chest tightness, shortness of breath, and sudden dizziness.
7. Mental health-related side effects
The FDA added a boxed warning to Singulair’s labeling in March 2020. This warning came after they received reports of serious behavior and mood-related changes, such as:
Agitation, aggression, or hostility
Attention problems
Restlessness
Nightmares
Depression
Anxiousness
Hallucinations
Memory problems
Sleepwalking
Trouble sleeping
Suicidal thoughts
These side effects could happen while taking Singulair or shortly after stopping it. They’re not common, but they should always be taken seriously. If you experience any of these side effects while taking Singulair, you should stop the medication and tell your prescriber right away.
Tips for managing and preventing Singulair (montelukast) side effects
Not everyone will experience Singulair side effects. But if you do experience them, it’s good to know how to respond.
Here are five quick tips for taking Singulair safely and to lower your risk of side effects.
1. Follow your prescriber’s prescription instructions closely
Do your best to take Singulair exactly as prescribed. Don’t lower or raise your dose without your prescriber’s approval.
Lowering your dose might make Singulair less effective. And raising the dose might raise your risk of side effects.
2. Take Singulair at the same time each day
Following a set schedule with your Singulair prescription gives it the best chance of success.
If you’re taking Singulair for asthma, it’s recommended to take your medication at the same time every day, usually in the evening. This is because asthma symptoms often get worse at night and early in the morning. So you’re likely to get the most benefits from taking Singulair in the evening.
If you’re taking Singulair for allergies, you can take it any time of day. Studies suggest Singulair is equally effective for this use whether it’s taken in the morning or evening. Being consistent helps you get the best results.
There is one exception. If you take Singulair for exercise-induced asthma, be sure to take it 2 hours before exercise. Taking it too early or too late could mean that the medication won’t give you enough protection by the time you’re ready for physical activity.
3. Do not double up if you miss a dose
If you forget to take your daily Singulair dosage, skip the missed dose. Then, take the next dose at your regular time.
Don’t double up on your Singulair dose. Doing so can raise your risk of side effects without any extra benefits.
4. Let your pharmacist know about other medications and supplements you’re taking
Several medications and supplements interact with Singulair.
Some medications — such as phenobarbital and rifampin (Rifadin) — may lower Singulair levels in your body. This may lead to a less effective treatment. Medications such as gemfibrozil (Lopid) may raise Singulair levels and increase the risk of side effects.
Tell your pharmacist about all the prescription and OTC medications you take. Be sure to mention supplements, too. It’s especially helpful if you maintain and share an updated medication list with them so they can check for potential interactions.
5. Tell a healthcare professional right away if you start to notice behavior or mood-related changes
As mentioned above, there’s a chance you might experience side effects that affect your mood or behavior while taking Singulair. If you notice these, you should stop your medication and let your prescriber know as soon as possible. They’ll help you create a plan of action that works best for you.
For additional resources or to connect with mental health services in your area, call SAMHSA’s National Helpline at 1-800-662-4357. For immediate assistance, call the 988 Suicide & Crisis Lifeline at 988, or text HOME to 741-741 to reach the Crisis Text Line.
How to save on Singulair
There are ways to save on Singulair, which is available as both a brand-name and generic medication. GoodRx can help you save over 80% off the average retail price of the generic version. The price of 30 tablets of generic montelukast 10 mg at certain pharmacies is as low as $8.73 with a free GoodRx discount.
Frequently asked questions
Weight gain wasn’t observed in Singulair’s initial clinical trials. If you notice unexpected changes in your body weight after starting it, keep track of when the changes began and any other symptoms you’re experiencing. Then reach out to a healthcare professional to review your medications, diet, and activity levels. They can help figure out what’s likely causing the change and decide whether any adjustments are needed.
Certain people might need to avoid Singulair or use it with caution. This may include people who have certain mental health conditions. Or those who have experienced mental health side effects from Singulair in the past. This is because Singulair itself can cause serious mental health side effects.
Healthcare professionals will often recommend other treatments first to manage your asthma or allergy symptoms. Be sure to tell your prescriber about any current or past mental health concerns. They can help you decide if Singulair is a safe option for you.
Singulair is generally safe for most people when taken as directed. It’s not typically considered a high-risk medication, but no medication is completely risk-free. The FDA placed a boxed warning on Singulair due to its risk of causing mental health side effects. Because of this risk, healthcare professionals carefully weigh the decision to prescribe Singulair. Talk with your prescriber if you have any concerns about taking Singulair.
Weight gain wasn’t observed in Singulair’s initial clinical trials. If you notice unexpected changes in your body weight after starting it, keep track of when the changes began and any other symptoms you’re experiencing. Then reach out to a healthcare professional to review your medications, diet, and activity levels. They can help figure out what’s likely causing the change and decide whether any adjustments are needed.
Certain people might need to avoid Singulair or use it with caution. This may include people who have certain mental health conditions. Or those who have experienced mental health side effects from Singulair in the past. This is because Singulair itself can cause serious mental health side effects.
Healthcare professionals will often recommend other treatments first to manage your asthma or allergy symptoms. Be sure to tell your prescriber about any current or past mental health concerns. They can help you decide if Singulair is a safe option for you.
Singulair is generally safe for most people when taken as directed. It’s not typically considered a high-risk medication, but no medication is completely risk-free. The FDA placed a boxed warning on Singulair due to its risk of causing mental health side effects. Because of this risk, healthcare professionals carefully weigh the decision to prescribe Singulair. Talk with your prescriber if you have any concerns about taking Singulair.
The bottom line
Singulair (montelukast) is FDA approved to treat asthma and allergies. But it’s usually not the first-choice medication for either condition. This is mainly due to potential side effects. Common side effects include upper respiratory tract infections, fever, and headache. Singulair also carries a boxed warning for rare but serious mental health side effects.
Talk with a healthcare professional if you’re concerned about Singulair side effects. They can help you decide if Singulair is a good option for you.
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References
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Arthritis Foundation. (n.d.). Relieve tension and stiffness with stretching.
A-S Medical Solutions. (2026). MONTELUKAST SODIUM- montelukast sodium tablet, chewable [package insert]. DailyMed.
Bisgaard, H., et al. (2009). Safety and tolerability of montelukast in placebo-controlled pediatric studies and their open-label extensions. Pediatric Pulmonology.
Global Initiative for Asthma. (2023). Global strategy for asthma management and prevention.
Jordan, A., et al. (2023). Psychiatric adverse effects of montelukast - A nationwide cohort study. The Journal of Allergy and Clinical Immunology: In Practice.
MedlinePlus. (2025). Upper respiratory tract. National Library of Medicine.
U.S. Food and Drug Administration. (2020). FDA requires boxed warning about serious mental health side effects for asthma and allergy drug montelukast (Singulair); advises restricting use for allergic rhinitis.
