[13] Common side effects include diarrhea, nausea, vomiting, mild rashes, asymptomatic elevations in liver enzymes, and fever.

Uncommon side effects include fatigue and malaise, behavioral changes, paresthesias and seizures, muscle cramps, and nose bleeds.

[3][14] In 2019 and 2020, concerns for neuropsychiatric reactions were added to the prescription labels in the United Kingdom and the United States where the most frequently suspected were nightmares, depression, insomnia (may affect between 1 in 100 to 1 in 1,000 people taking montelukast); aggression, anxiety and abnormal behavior or changes in behavior (may affect between 1 in 1,000 and 1 in 10,000 people taking montelukast).

[20][21][16] The boxed warning advises health care providers to avoid prescribing montelukast to patients with mild symptoms, particularly those with allergic rhinitis, because there are many other allergy medicines that can safely and effectively manage this condition.

[20] In the FDA's data analysis, in comparison to case reports that based on people's self-reports, the propensity of developing neuropsychiatric disorders after montelukast use did not outpace that of inhaled corticosteroids; and there were no statistically significant risks of new-onset neuropsychiatric disorders among males, females, patients 12 years and older, patients with a psychiatric history, or after the 2008 FDA communication and prescribing information changes that first publicized the concern.

[21] In addition, the FDA's analysis summary of its findings said "exposure to montelukast was significantly associated with a decreased risk of treated outpatient depressive disorder and the decreased risks were seen among patients with a history of a psychiatric disorder, in patients 12 to 17 years as well as 18 years and older, and in both females and males.

[21] In 2024, following reports of night terrors, uncontrollable aggression, intrusive thoughts, depression and rare cases of hallucinations and suicidal behavior in children, the UK Medicines and Healthcare products Regulatory Agency (MHRA) was reviewing the risks of montelukast after identifying "further concerns".

[23] In a limited audience review of some findings of this internal group to the American College of Toxicology, on November 20, 2024 in Austin, TX, Jessica Oliphant, deputy director for FDA's National Center for Toxicological Research, said "that laboratory tests showed “significant binding” of montelukast to multiple receptors found in the brain"[23] and that "These data indicate that montelukast is highest in brain regions known to be involved in (psychiatric effects),”.

The references were submitted through Article One Partners, an online research community focused on finding literature relating to existing patents.

[34] Montelukast is sold under a variety of brand names including Monalast (Ziska Pharmaceuticals Ltd) Montenaaf (NAAFCO Pharma) Montelon-10 (Apex), Montene (Square), Montair-10, Lukotas,[35] Montelo-10, Monteflo, and Tukast L in India, Reversair (ACI Bangladesh), Monas, Miralust, Montiva, Provair, Montril, Lumona, Lumenta, Arokast and Trilock in Bangladesh, Ventair in Nepal, Montika in Pakistan, Montelair in Brazil, Zykast in the Philippines though combined with levocetirizine, Desmont, Levmont, Aircomb and Notta in Turkey, Topraz and Monte-Air[36] in South Africa, AirOn in Venezuela, and AirFast in Saudi Arabia.