HealthTap says it takes four to six hours for the effects of NyQuil to fade. Drugs.com says that NyQuil is intended to be taken every six hours as needed to treat cold symptoms.
Drugs.com explains that NyQuil is a combination of acetaminophen, dextromethorphan, doxylamine and pseudoephedrine. The combination is intended to treat the pain, coughing and congestion associated with upper respiratory tract infections and allergies. It blocks the action of histamine and constricts blood vessels in the nasal passages.
According to the manufacturer, NyQuil comes in liquid syrups or in capsules. The traditional form of NyQuil contains alcohol, but alcohol-free preparations are also available.