Singulair (montelukast) causes a range of side effects from mild to serious, with the most notable being neuropsychiatric symptoms that prompted the FDA to add its strongest safety warning to the drug in 2020. While many people take it without problems, the side effect profile is broader than most allergy and asthma medications, and some effects can persist even after stopping the drug.
How Singulair Works
Singulair blocks a receptor in your body that responds to leukotrienes, chemicals your immune system releases during an allergic or asthmatic reaction. When leukotrienes bind to their receptor, they cause your airways to tighten, blood vessels to leak fluid into surrounding tissue, and inflammatory cells to flood the area. By blocking that receptor, Singulair reduces airway inflammation and bronchoconstriction.
The drug also interacts with receptors in the central nervous system. Researchers have identified several additional targets for montelukast in the brain, which likely explains why its side effects extend well beyond the lungs and sinuses. The exact molecular basis for its neuropsychiatric effects is still not fully understood, but the toxicity appears to come from the drug’s interaction with biological pathways rather than from any toxic breakdown products.
Common Side Effects
The most frequently reported side effects overlap with the symptoms of the conditions Singulair treats, which can make them easy to miss or dismiss. They include:
- Headache
- Stomach pain
- Diarrhea
- Cough and sore throat
- Stuffy or runny nose
- Fever
- Unusual tiredness or weakness
- Ear pain, redness, or swelling
- Voice changes or loss of voice
Most of these are mild and may resolve on their own as your body adjusts. Diarrhea, for instance, is common enough that it’s generally considered manageable without stopping the medication. But persistent tiredness or recurring stomach pain is worth flagging to your prescriber, especially in children who may not articulate what they’re feeling.
The FDA’s Boxed Warning on Mental Health Effects
In March 2020, the FDA added a boxed warning (its most serious type) to Singulair’s label after reviewing years of reports linking the drug to significant mood and behavior changes. The agency also restricted its use for allergic rhinitis, advising that it should only be prescribed for allergies when other treatments have failed.
The neuropsychiatric side effects listed in the warning include:
- Agitation, including aggressive behavior or hostility
- Depression
- Suicidal thoughts and actions
- Hallucinations (seeing or hearing things that aren’t there)
- Vivid or bad dreams
- Trouble sleeping
- Sleepwalking
- Anxiety
- Irritability and restlessness
- Attention problems
- Memory problems
- Obsessive-compulsive symptoms
- Disorientation or confusion
- Tremor or shakiness
- Stuttering
- Uncontrolled muscle movements
The FDA identified 82 cases of completed suicide associated with montelukast. Of those, 45 were in adults, 19 were in patients 17 or younger, and 18 did not include the patient’s age. These numbers come from voluntary reporting systems, which generally capture only a fraction of actual cases.
Side Effects in Children
Singulair is approved for children as young as 12 months, which makes its neuropsychiatric profile especially concerning. Children may not be able to describe what they’re experiencing, so changes in behavior are often the only visible signal. Parents and caregivers should watch for sudden aggression, nightmares, sleepwalking, new anxiety, difficulty concentrating at school, or mood swings that seem out of character.
The FDA’s warning applies equally to children and adults, and the agency specifically asks caregivers to stop the medication and contact a prescriber immediately if mood or behavior changes appear. Some of these symptoms can emerge within days of starting the drug, while others develop gradually over weeks or months, making the connection to Singulair harder to recognize.
How Long Side Effects Last After Stopping
For many people, side effects improve within days to weeks of stopping Singulair. But reports from patient groups and data submitted to the FDA paint a more complicated picture for some. Members of the Montelukast Side Effects Support Group reported initial improvement after discontinuation followed by an intense relapse roughly a month later.
Some patients describe a protracted recovery period that includes long-term mental health difficulties and neurological complications. These can involve cognitive deficits, sensory or motor problems, emotional instability, significant behavioral changes, and in severe cases, seizures. This pattern does not appear to affect everyone, but the possibility of lingering effects is worth knowing before you start the medication and especially if you’re weighing whether to continue it.
Drug Interactions
Singulair is processed primarily through a liver enzyme called CYP2C8, which handles about 72% of its breakdown. Two other enzymes, CYP3A4 and CYP2C9, handle the remaining 16% and 12% respectively. This means any drug that competes for or inhibits these enzymes could change how quickly your body clears montelukast, potentially increasing its concentration and side effects. Gemfibrozil, a cholesterol-lowering drug, is one notable example because it strongly inhibits CYP2C8. If you take multiple medications, your pharmacist can check for interactions with these enzyme pathways.
A Rare but Serious Vascular Condition
Early reports linked Singulair to a rare condition called Churg-Strauss syndrome (now known as eosinophilic granulomatosis with polyangiitis), a type of blood vessel inflammation that can affect multiple organs. Symptoms include worsening asthma, numbness or tingling in the hands and feet, sinus problems, and skin rashes.
Research has since clarified that Singulair does not appear to directly cause this condition. Instead, what seems to happen is that patients with severe, hard-to-control asthma who were already on oral steroids get switched to Singulair. When the steroids are tapered, an underlying vasculitic condition that the steroids had been suppressing becomes apparent. The distinction matters: the syndrome was already developing, and the medication change simply unmasked it. When diffuse organ involvement occurs, particularly affecting the heart, kidneys, or central nervous system, the prognosis can be serious.
What to Watch For
If you or your child starts Singulair, pay close attention to mood, sleep, and behavior in the first several weeks. The FDA’s prescribing label instructs patients to notify their prescriber if any neuropsychiatric symptoms develop, and prescribers are expected to weigh the risks against the benefits before continuing treatment. In practice, this means the responsibility for early detection falls largely on you and your family. Keep a brief daily note of mood and sleep quality for the first month, especially for children. Any new anxiety, nightmares, aggression, or personality shifts that weren’t present before starting the drug should be taken seriously and reported promptly.