Can you overdose on tussin dm?

Absolutely: Dextromethophan is a cough suppressant but using too much can lead to many side effects including breathing problems, visual changes, blood pressure changes, stomach cramps, nausea, vomiting, sleepiness, dizzyness, convulsions, hallucinations, and more serious ones like coma in heavy doses. Minimize use following the label directions. If a cough persists, see a doctor.