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.
Promethazine (the active ingredient in Phenergan) will start to make you feel sleepy around 20 minutes after you take it and may work for up to 12 hours. If you're taking it for a cough or cold symptoms, allergies or nausea, your symptoms should start to improve within 20 minutes. The medicine should work for around 4 to 6 hours. Please take note of dosage requirements.
Phenergan also works directly on the brain to help you feel more relaxed. Many people find Phenergan helps them when travelling with motion sickness. Phenergan Tablets are used to treat the following conditions: For short term use: to treat adults with difficulty sleeping (insomnia).