If your doctor mentioned montelukast for allergies, you probably left the appointment with a mix of relief and a nagging question. The relief because a once-daily pill sounds easier than juggling nasal sprays and antihistamines. The question because a friend, or the internet, or maybe the pharmacist warned you about mood-related side effects.
That warning is real. In 2020, the FDA added a boxed warning to montelukast about serious neuropsychiatric events. It’s the strongest safety label the agency uses. That doesn’t mean the drug is dangerous for everyone. It means the risk-benefit math for allergic rhinitis, in particular, has shifted.
Here are the five side effects worth understanding before you start, or continue, taking it.
How Montelukast Works (Briefly)
Montelukast (brand name Singulair) is a leukotriene receptor antagonist. Leukotrienes are inflammatory chemicals your immune system releases during an allergic response, and they’re a big part of why your nose runs, your eyes itch, and your airways tighten. Blocking them can meaningfully calm allergy and asthma symptoms.
It’s FDA-approved for asthma, exercise-induced bronchoconstriction, and allergic rhinitis. For seasonal allergies specifically, it works, but it’s usually not first-line anymore. Antihistamines and intranasal steroids typically come first, and montelukast for allergies is reserved for people who don’t respond well to those or who also have asthma.
1. Neuropsychiatric Effects (The Boxed Warning)
This is the one that changed the conversation. Reported effects include:
- New or worsening depression
- Anxiety and agitation
- Sleep disturbances, including vivid dreams and nightmares
- Irritability, especially in children
- Suicidal thoughts or behavior (rare, but documented)
The FDA’s review found these events can start within days of the first dose or emerge months later. Most resolve after stopping the drug, though not always immediately.
What I tell patients: if you or your child has a history of depression, anxiety, or ADHD, mention it before starting. It’s not an automatic no. But it’s a conversation worth having, and someone at home should know what to watch for during the first month.
2. Headaches
Headache is the most commonly reported side effect in clinical trials, showing up in roughly 18% of adults on montelukast versus about 17% on placebo. So the signal is small, but real for some people.
These tend to be mild and improve within the first two weeks. If you’re getting daily headaches that weren’t there before, that’s worth flagging. Especially if you’re also taking other allergy medications. Some patients do better switching to a different approach entirely, and comparisons like loratadine versus fexofenadine can help sort out which non-drowsy antihistamine might replace montelukast for straightforward seasonal symptoms.
3. Stomach Upset and Abdominal Pain
Gastrointestinal complaints show up in maybe 2 to 4% of users. What that looks like in practice:
- Nausea, usually mild
- Abdominal pain, often described as crampy
- Diarrhea or, less often, constipation
- Occasional heartburn
Taking montelukast in the evening with a small snack can help. If GI symptoms persist beyond a couple of weeks or interfere with eating, call your prescriber. Rarely, montelukast has been linked to liver enzyme elevations, so unexplained nausea plus fatigue or yellowing skin deserves prompt attention.
4. Upper Respiratory Symptoms and Sinus Infections
This one confuses people. You’re taking an allergy medication and getting more sinus infections? It happens.
Clinical trial data show slightly higher rates of upper respiratory infection, cough, and sinusitis in montelukast users compared to placebo. The mechanism isn’t fully understood. Some clinicians think it’s a subtle shift in local immune signaling in the airway. Others think it’s coincidental, because allergy patients get more sinus infections anyway.
Either way, if you’re catching every cold that goes around and it feels different from before, mention it. And if you’re weighing montelukast for allergies against a plain antihistamine strategy, the piece on montelukast versus cetirizine walks through the tradeoffs in more detail.
5. Rare but Serious: Churg-Strauss and Allergic Reactions
Two rare reactions deserve mention, even though most patients will never encounter them.
The first is eosinophilic conditions, including a vasculitis historically called Churg-Strauss syndrome (now eosinophilic granulomatosis with polyangiitis). Signs include flu-like symptoms, worsening pulmonary symptoms, cardiac complications, or a rash. It’s rare, and it’s more often associated with reducing oral steroids in asthma patients than with montelukast itself, but the Mayo Clinic lists it as a reaction to watch for.
The second is hypersensitivity reactions, including angioedema and, very rarely, anaphylaxis. Any facial swelling, tongue swelling, or trouble breathing after a dose warrants an ER visit, not a phone call.
Who Should Think Twice About Montelukast for Allergies
The 2020 boxed warning shifted prescribing patterns. For patients with mild to moderate allergic rhinitis and no asthma, most guidelines now favor intranasal steroids or second-generation antihistamines first. Montelukast for allergies still has a role, but the risk-benefit calculation has tightened.
Reasons to have a longer conversation with your prescriber:
- Personal or family history of depression, anxiety, or suicidal ideation
- Prior mental health side effects from other medications
- Children and adolescents (the neuropsychiatric signal is strongest here)
- Elderly patients already on multiple CNS-active medications
If cost is part of the reason you’re considering montelukast, know that generic versions are widely available and inexpensive. You can compare pricing on generic montelukast at edrugstore.com to see what your out-of-pocket looks like without insurance.
The Bottom Line
Montelukast for allergies works. It’s also a medication that carries the FDA’s strongest warning label for reasons that showed up mostly after it had been on the market for two decades. That’s not a reason to panic if you’re already taking it and doing fine. It is a reason to know what to watch for, to check in with the people around you about mood changes, and to ask whether a different first-line option might do the job just as well.
Talk to your pharmacist or physician about whether montelukast for allergies still makes sense for your specific situation, especially if antihistamines and nasal steroids haven’t been tried recently.
This article is for informational purposes only and is not medical advice. Always consult a licensed pharmacist or physician before starting, stopping, or changing any medication.
References
- FDA Boxed Warning on Montelukast, Official FDA safety communication on neuropsychiatric effects.
- MedlinePlus: Montelukast, NIH patient information on approved uses, dosing, and side effects.
- Mayo Clinic: Montelukast Side Effects, Comprehensive clinical reference on rare and common adverse reactions.
Dr Steven Schlosser
Dr. Schlosser graduated from Lafayette College Phi Beta Kappa and attended Georgetown Medical School. He had 4 years of medical residency training at Tufts University in Boston in both Gynecology and Internal Medicine. He has had a spotless medical career for the past 40+ years and is Board Certified. Learn More