Can singulair (montelukast) cause weight gain?

Unlikely: Singulair/montelukast blocks substances called leukootrienes that are part of your immune system response. As you know, it is used to treat asthma and allergies. Reported side effects include upset stomach, nausea, diarrhea, fatigue, dizziness and headache, but not weight gain. Good wishes:)

Not known to: However, every drug can cause different side effects. If it is safe to stop it, based on your allergies, asthma then do and see it there is a difference. All the best.