Lucy
I was diagnosed with asthma in May and began taking Singulair. I have NOT experienced vivid dreams with Singulair, but I had to stop the medicine after three months because of stomach upset. At first I just experienced nausea, but that turned into stomach pains every day. I was hardly able to eat and began to lose weight so my doctor took me off of Singulair. Now I'm taking Pulmicort Turbuhaler.
Hope this helps.
P.S. After I stopped taking the medicine I noticed that the ingredients included "lactose monohydrate". I'm not sure if the medicine contained enough to be the cause of the problem, but found it interesting because I cannot eat milk products.