Promethazine plain oral solution is a medication that temporarily treats runny nose, coughing, and sneezing caused by the common cold, allergies, hay fever, and other breathing illnesses. It is a phenothiazine that works like an antihistamine, blocking the effects of histamine, a substance released by the body during an allergic reaction. It can also help you relax, go to sleep, and relieve nausea, vomiting, or pain before or after procedures2. Promethazine plain oral solution is contraindicated in comatose states, and in individuals known to be hypersensitive or to have had an idiosyncratic reaction to promethazine or to other phenothiazines.