My 4yr old son has recently started on Montelukast 5mg tablet. However since he started it (to the day) he has changed and appears to be falling apart on me. Complete exhaustion (ie falling asleep late afternoon to following morning), dreadful behaviour (which isn't normally like him), sore throat which is hoarse. Has anyone else's children had this? I thought it might be a virus but I don't think it is anymore.