Phenergan Elixir is indicated: For short term use: To treat adults with difficulty sleeping (insomnia) To treat allergic conditions such as hayfever or rashes (like nettle rash or hives) To treat or stop you feeling sick (nausea) or being sick (vomiting) such as travel sickness For short term use: as a sedative for children aged 5 years and above How can the active ingredient promethazine hydrochloride help me? The active ingredient in Phenergan is promethazine hydrochloride, which is an antihistamine that belongs to a group of medicines known as phenothiazines. When you have an allergic reaction, your body creates a natural substance called histamine. Histamine sends signals around your body that trigger symptoms of allergy, such as a runny nose or watery eyes. Antihistamines work by blocking histamine, which prevents your symptoms so you won't have to tolerate an allergic reaction. How long does Phenergan make you sleepy? Promethazine (the active ingredient in Phenergan) will start t