How Do Phenergan Tablets Work? Phenergan contains the active ingredient promethazine hydrochloride which is a type of medicine called a sedating antihistamine. It prevents the actions of histamine which causes an allergic reaction. Promethazine also helps to block the histamine receptors in the brain & prevents mixed signals from the inner ear being sent to the brain that cause nausea & vomiting. Due to its sedating qualities, Phenergan induces drowsiness & can aid sleep, useful in cases of insomnia. How To Take Phenergan Tablets Allergic conditions, I.e. hay fever; Children aged 5-10 years of age; 1 to 2 tablets to be taken at night, alternatively 1 tablet twice a day. Do not exceed the dose of 2 tablets in 24 hours. Children over 10 years & adults (including elderly); Begin with one tablet twice a day. If needed, can be gradually increased to two tablets three times a day if required. Travel sickness: Children aged 5-10 years; One tablet to be given at night, an additional tablet may be given after 6-8 hours if needed. Children over 10 years & adults & the elderly; Two tablets to be taken the night before travel. An additional two tablets can be taken if needed 6-8 hours after. If you are or think you might be pregnant, consult your doctor before taking Phenergan. They should not be taken two weeks before birth. Phenergan should not be taken while breast feeding as there is a chance that small amounts may pass into the mothers' milk which could be harmful to the baby. If you feel drowsy after taking this medicine do not drive or operate machinery. Consult your doctor or pharmacist before using if you are taking any of the following medications; Sedatives, tricyclic antidepressants, hypnotics, or anticholinergic agents.