How long does Nyquil last?

Symptom relief with Nyquil lasts for about 6 hours after a dose, so you will need to take Nyquil every 6 hours if you want to maintain consistent symptom relief.

Drowsiness or dizziness caused by an ingredient contained in some Nyquil preparations called doxylamine may persist for longer, sometimes up to 8 hours. This is because doxylamine can last in the body for a long time, especially following the administration of multiple doses. The half-life of doxylamine is 10 hours, which means it can take up to 50 hours (over 2 days) for your body to totally eliminate doxylamine. However, the effects of doxylamine do not usually persist for much longer than 8 hours, even though it may be detectable in your body for longer.