The effect of promethazine is to block histamine receptors competitively and produce antihistamine effect. It can counteract the dilation of capillaries caused by antihistamine, reduce its permeability, and relieve wheezing caused by bronchial smooth muscle contraction, which is stronger and longer lasting than the effect of diphenhydramine hydrochloride. Because it can easily enter brain tissue, it has obvious sedative effect and can strengthen the central inhibitory effect of hypnotics, analgesics and anesthetics. Its anticholinergic effect is also stronger, and it has a better effect in preventing motion sickness. It is used for skin and mucous membrane allergy, as well as allergic rhinitis, asthma, food allergy, skin scarring, and seasickness, motion sickness, and airsickness, etc.