HealthCentral states that one of the significant differences between NyQuil and DayQuil is that NyQuil is in liquid suspension form and is intended for night time use only, whereas DayQuil has a different chemical structure which does not promote drowsiness, making it ideal for use any time of the day. Another notable difference is that NyQuil contains an antihistamine whereas DayQuil contains only a nasal decongestant, according to DailyMed.
While they may have slight differences, NyQuil and DayQuil also share much in common, notes Daily Med. They both contain the pain relieving agent acetaminophen as well as the cough suppressant component dextromethorphan.
Both NyQuil and DayQuil have noticeable side effects. HealthCentral notes that both medicines affect the nervous system and can cause drowsiness, nervousness, blurred vision and dizziness.
The Vicks website warns that it is not advisable to use NyQuil or DayQuil with other products containing acetaminophen or other ingredients used to treat cold and flu, as an acetaminophen overdose can have serious consequences to the liver. Patients who suffer from alcoholism, asthma, diabetes, glaucoma, heart disease, high blood pressure, liver or kidney disease, an enlarged prostate gland, chronic lung disease or thyroid disease should not take either NyQuil or DayQuil without the prior advice of a physician.