Phenergan Elixir Phenergan Elixir used to treat hayfever & other allergic conditions such as nettle rash & hives. They can also be used to treat insomnia in adults & to treat/prevent nausea & vomiting such as travel sickness. Phenergan Elixir contains promethazine hydrochloride, which works by blocking histamine, the natural substance that your body makes during an allergic reaction. It also works directly on the brain to help you feel more relaxed.