Montelukast
"My son has had asthma from the beginning, he was placed on Singulair at 18 months.  It did seem to help, but he has now developed side effects and we have taken him off of it.  He becomes very agitated, and aggressive.  He is normally a very calm and sweet child.  It took a while to put two and two together.  We took him off, and his sweet nature came back.  Truthfully, it scared me how much this medicine affected his mood.  It works great for asthma, but we won't use it anymore."