Side effects of NyQuil include drowsiness, blurred vision, nausea, constipation and dizziness, according to WebMD. NyQuil may also cause nervousness and dryness in the mouth, throat or nose. If any of these symptoms persist, tell a doctor or pharmacist.
NyQuil is a medication used to treat the flu, allergies or the common cold, explains WebMD. The active ingredients are dextromethorphan, acetaminophen and doxylamine succinate, states Drugs.com. Acetaminophen is a pain reliever and fever reducer, dextromethorphan is a cough suppressant, and doxylamine succinate is an antihistamine. Individuals taking NyQuil should not take more than the recommended dose because taking too much acetaminophen can cause liver disease, advises WebMD.