There are a few key differences between these two over-the-counter (OTC) medicines. Delsym is an extended-release cough syrup, which means the medicine is released slowly over time. This makes it effective for treating coughing that lasts all day or night. Robitussin, on the other hand, is a short-acting medication that's meant to be taken every four hours as needed.
Both medications contain dextromethorphan, a cough suppressant that works by reducing the activity of the part of the brain that controls coughing. However, Delsym also contains alcohol and glycerin, which helps to keep the medicine from being absorbed too quickly into your system. This extended release action is what makes Delsym more effective at treating persistent coughs than Robitussin.