Question: how long does it take for liquid cough medicine to work?
Answer: Regardless of formulation, an adult should never take more than 120 mg of dextromethorphan in a day, and maximum daily doses are even lower for some children. Dextromethorphan's medicinal effects typically begin after 15 and 30 minutes, reach their maximum around 2 to 3 hours, and end at or before 6 hours.