Montelukast Side Effects: What to Expect and When to Call Your Doctor

Montelukast (brand name: Singulair) is generally well-tolerated by most patients, but it carries an important FDA black box warning for neuropsychiatric side effects — the most serious warning level the FDA issues for prescription medications. Understanding what side effects to watch for, and when they require immediate attention, is essential for anyone taking or considering montelukast.

The FDA Black Box Warning: Neuropsychiatric Events

In March 2020, the FDA required a black box warning on montelukast for serious mental health side effects. This was added after ongoing reports in the FDA Adverse Event Reporting System (FAERS), including completed suicides and other severe behavioral changes in patients taking the drug.

Neuropsychiatric events reported in montelukast patients include:

Suicidal thoughts or actions (most serious; requires immediate action)

Depression and worsening mood

Agitation, aggression, and hostility

Anxiety and restlessness

Hallucinations and vivid or disturbing dreams/nightmares

Insomnia and sleep disturbances

Confusion, attention problems, and memory difficulties

Tremor and uncontrolled muscle movements

Stuttering

Most reported cases occurred during montelukast treatment, but some occurred after stopping the medication. These side effects can occur in any patient, including children, adults, and the elderly, and in patients with no prior history of mental illness.

What Should You Do If You Notice Neuropsychiatric Symptoms?

Stop montelukast immediately and contact your doctor if you or a family member notices any mood or behavioral changes while taking the medication. If there is any concern about suicidal thoughts or intent, call 988 (Suicide and Crisis Lifeline) or go to the nearest emergency room.

Parents and caregivers of children taking montelukast should watch especially closely for changes in mood, behavior, or sleep patterns, as children may not always be able to articulate what they are experiencing.

Common Side Effects of Montelukast

The most commonly reported side effects in clinical trials are generally mild and often resolve on their own:

Headache

Upper respiratory infection (cold-like symptoms)

Cough

Stomach pain or abdominal discomfort

Diarrhea

Ear pain or ear infection (especially in children)

Sore throat

Fever

Flu-like symptoms and runny nose

Sinus infection

Other Serious Side Effects

Beyond neuropsychiatric events, other serious side effects (though less common) include:

Allergic reactions: Anaphylaxis, hives, swelling of the lips/tongue/throat, and difficulty breathing. Seek emergency care immediately.

Churg-Strauss syndrome (eosinophilic granulomatosis with polyangiitis): A rare but serious inflammatory vasculitis. Signs include flu-like symptoms, severe sinus pain, skin rash, and numbness/tingling in the arms or legs. Report to your doctor immediately.

Eosinophilia: Elevated eosinophils sometimes seen in asthma patients reducing oral corticosteroids while on montelukast. Monitor if transitioning off systemic steroids.

Important Warnings for Specific Populations

Children and adolescents: The neuropsychiatric risk appears in all ages. Nightmares, aggression, and behavioral changes are commonly reported in pediatric patients. Caregivers should monitor closely.

Patients with PKU: Montelukast chewable tablets contain aspartame (phenylalanine). Patients with phenylketonuria should not use chewable tablets.

Aspirin-sensitive asthma: Patients with aspirin-sensitive asthma should continue to avoid aspirin and NSAIDs while taking montelukast.

When to Call Your Doctor Immediately

Any unusual changes in mood or behavior — agitation, depression, aggression, or confusion

New or worsening sleep problems, including nightmares or insomnia

Any thoughts of self-harm or suicide (call 988 or go to the ER immediately)

Signs of an allergic reaction (hives, swelling, difficulty breathing)

Flu-like symptoms with severe sinus pain, skin rash, or numbness in the limbs (possible Churg-Strauss syndrome)

To learn about medications that can interact with montelukast, see our guide to montelukast drug interactions. If you're having trouble filling your prescription, medfinder can locate it at a nearby pharmacy.