Eczema, urticaria, contact dermatitis and other allergic diseases we often use anti-sensitizers as the first clinical drug of choice. Anti-sensitizers are actually called “antihistamines” by our doctors, and the antihistamines we commonly use are divided into the first and second generation according to the drowsiness caused by the drug crossing the blood-brain barrier. The commonly used first generation anti-sensitizers include chlorpheniramine maleate (paracetamol), diphenhydramine, cycloheximide hydrochloride, ketotifen, etc. The second generation anti-sensitizers include cetirizine hydrochloride, loratadine, imipramine, etc. The first generation of antisensitizers easily cross the blood-brain barrier, resulting in drowsiness, fatigue, sleepiness, dizziness, inattention, etc. Some drugs also have anticholinergic effects, resulting in dryness of mucous membranes, difficulty urinating, dilated pupils, and should be prohibited or used with caution in high-altitude work and drivers, and should also be used with caution in glaucoma and prostate hypertrophy. The second generation of anti-sensitizers are not easy to cross the blood-brain barrier, do not produce drowsiness or only have a slight sleepy effect, while the anticholinergic effect is smaller, the drug is absorbed quickly, the action time is longer, and generally can be taken once a day. Many years ago it was the first generation of antihistamines that were used clinically as the main drug, while currently it is the second generation of antihistamines that are widely used clinically, and cetirizine hydrochloride is supposed to be the most widely used antihistamine in clinical practice.