My son is now 15 and he was on Singulair for three and a half years. It was a living hell. Plus he was taking Advair with it. They both do the same things. He was having horrible mood swings, he would get anrgy over the smallest things. Then there were the headaches. Then he started passing out for no reason. We thought it was stress due his some problems going on with his biological father. I've had a CT scan done on him and a MRI as well as about $8,000 worth the test done. All the test would come back fine.Then one day my father called me and told me that he saw a thing on TV that a child under the age of 18 should not take Singulair any longer than six months. The moment I got off the phone I got on the internet and looked up the side effects of the Singulair. That was over a month ago and that was the last time I let him take it. I will not let him take it again for any reason. I even asked his allergist if it could be the Singulair and she said no there was no way it was the Singulair. There is a web site I found that tells all of the side effects better than any of them I researched. Here it is www.canadadrugs.com/prescripition/product/Singulair+10mg/4501/. Or google it and the best one is all I did was type "Side effects to taking Singulair 10mg". The one I clickedon was Singulair-Medication Singulair. Side effects Buy Singulair Online. It gives all the effects of taking it. My son has had 58 of the side effects to this medicine. Tomorrow he goes back to the allergist(March 26,08). I know she will not like me when Ileave her office. Hope this information I gave helps someone. Marie Nester