I've been taking Zyrtec daily for years. I'm always tired but I don't think it's due to the Zyrtec. I think it makes my mouth drier when I wake up. About weight gain. I don't know. I think I just eat too much and I'm older so I gain more. I never heard of antihistamines causing weight gain. I have to take it because of bad allergies, even with allergy shots too.