Hi. My Son Is 5 Years Old And Suffers From

Hi. My son is 5 years old and suffers from allergies - runny nose, stuffy nose, sneezing and watery eyes. He also coughs a lot at night...some nights it just won't let up. I believe the cough is from the drainage, as he really only coughs this way at night when he is laying down. He has been on singulair since we was 1 1/2 for allergies, however it doesn't seem to be helping anymore. Do you have any suggestions for settling the cough so he can sleep at night? Thank you!