I used to take zytec but my dr. switched me to allegra. I took the zyrtec at night as it made me sleepy for the first few weeks. then I got used to it and I didn't notice any other effects. I take Allgegra as it is supposed to work better for pet allergies. But this med also causes drowsiness. Most antihisamines do cause drowsiness but if you take them at night you won't have the sleepiness during the day.