Promethazine is a medicine called an antihistamine. It's classed as a drowsy (sedating) antihistamine. When you come into contact with something you're allergic to, such as pollen or animal hair, your body produces a chemical called histamine. This can cause symptoms such as a runny nose or skin rashes. Promethazine blocks the effects of histamine in your brain and this reduces symptoms. It enters the brain in large quantities and this can make you feel drowsy. Promethazine also blocks the effects of a chemical called dopamine, and this stops you feeling sick.