I'm really not sure what the difference is,,,I just wanted you to know I read your post and hope you get some answers. I know my asthma/allergy doc put me on Singulair for my Asthma,,,Zyrtec for my allergies. I'm allergic to about everything,,except cats,,,and I don't like cats! Figures! It's funny because I thought I didn't have any allergies,,,well,,,maybe dust,,,but before the doc could take the little pick things out of my back they were itching,,,knew that wasn't good!