Yes, NyQuil can make you feel sleepy. This effect is expected and is mainly due to one of its ingredients, doxylamine, which is known to cause drowsiness. NyQuil is designed as a nighttime cold and flu medicine, so feeling sleepy after taking it is part of how it works.
Doxylamine is an older type of antihistamine that affects the part of the brain involved in wakefulness. By blocking histamine, a chemical that helps keep the brain alert, it often leads to drowsiness and slower reaction time. The other common ingredients in NyQuil, such as acetaminophen for pain and fever or dextromethorphan for cough, do not directly cause sleepiness.
Sleepiness can begin within about 30 minutes after taking NyQuil and may last several hours. Some people may still feel groggy the next morning, especially if they are sensitive to antihistamines or take it late at night.
Because of these effects, NyQuil should only be taken when rest is possible, and activities like driving should be avoided. If strong or lingering drowsiness occurs, or if there are other health conditions or medications involved, it is important to speak with a doctor or pharmacist before using it again.