allergies
"I think I have chronic sinusitis, with allergy symptoms all day long, year round. Zyrtec used to help, but doesn't anymore.
I thought that Xyzal was going to be the answer and it was until I started experiencing stomach aches that had no apparent cause in what I had eaten. 
Very reluctantly I stopped the Xyzal and the stomach aches stopped.
But if you don't experience any of the side effects, then it works better than any other medication for allergies that I have ever taken!"