Cetirizine (Zyrtec) and Nyquil Cold and Flu (acetaminophen/dextromethorphan/doxylamine) are both used to relieve symptoms, but they serve different purposes and have different ingredients. Cetirizine is an antihistamine that helps with allergy symptoms like runny nose and itchy eyes. It is available in various forms, including tablets and liquids, and is typically taken once a day. Nyquil Cold and Flu, on the other hand, is a combination medication containing acetaminophen, dextromethorphan, and doxylamine. It is used to treat cold and flu symptoms such as cough, sore throat, and fever, and is taken every 6 hours. While both medications can cause sleepiness, Nyquil Cold and Flu is more likely to make you extremely sleepy due to the antihistamine doxylamine. Additionally, Nyquil Cold and Flu should not be taken with other medications containing acetaminophen to avoid liver damage, whereas cetirizine does not have this restriction.