D.W.
Look at the insert that is in the pamphlet for the singulair. I would think about making switching to zyrtec. i give it my two year old son and he does not have any issues. I think he has seasonal allergies and i started giving it to him and he stopped coughing with the post nasal drip and it really seemed to help. You can give it to children two years and older. The other good thing is that they sell it over the counter. I get two of the liquid zyrtec's for seventeen bucks at BJ's. There are always coupons out for it and it comes in bubble gum flavor. My son does not mind taking it.
talk to the doctor and see what he or she thinks. Good luck