I use Lysine for my Tuxie kitty. He has feline herpes and is prone to stuffiness, sneezing and minor URI. I buy the lysine at a health food store and make sure there are no additives in the capsules. For active flares I use 500 mg/daily for maintenance I use 250mg several times a week. There have been some studies that say Lysine does not help, but from personal experience it helps with my kitty and it has worked without having to use A/B. Lysine helps to boost the immune system, while zyrtec acts as an antihistamine. I have not used zyrtec personally, but hopefully others that have will weigh in.