Prevention of asthma symptoms, including exercise-induced asthma, usually alongside an inhaled steroid; and allergic rhinitis, particularly when asthma and hay fever occur together. Chewable tablets and granules are used for children.

In allergic asthma the immune system releases leukotrienes, chemicals that tighten the airway muscle, thicken the lining with mucus and draw in more inflammatory cells. Montelukast blocks the receptor those leukotrienes act on, so the airways stay more open and less inflamed over the following days and weeks.

Headache, stomach pain, thirst, and upper respiratory infections. In children, occasional diarrhoea, fever or rash.

Montelukast can cause changes in mood and behaviour, including agitation, anxiety, sleep problems, nightmares, low mood and, rarely, suicidal thoughts, in adults and children alike. Stop and contact a doctor if any of these appear. Keep taking your usual inhalers unless told otherwise, and never use montelukast to treat a sudden attack, which needs a reliever inhaler. It is a prescription medicine in Pakistan.