NyQuil typically starts working within 30 minutes of taking it and its symptom-relieving effects generally last about 4 to 6 hours. However, the drowsiness caused by NyQuil can last longer, up to 6 to 8 hours depending on the person. The exact duration can vary based on individual metabolism, liver function, and other factors. NyQuil's active ingredients, including acetaminophen, dextromethorphan, and doxylamine, have different half-lives which contribute to these varied durations. For most people, it is safe to take another dose every 6 hours as needed, but it should not be used continuously for long periods without medical advice.
