What is an antihistamine? Antihistamines are a drug that tries to reduce or eliminate the effects of histamines which are chemical mediators secreted normally during allergic reactions. Their main function is to destroy these foreign substances, but the molecule also intervenes in other processes. Antihistamines is effective against some symptoms that occur during an allergic reaction, including itching, runny noses, watering eyes, sneezing… However, it is by no means effective on asthma. Antihistamines: how do they work? Antihistamines are inhibitors of the histamine receptors. As a consequence, the production of histamine isn’t affected, but the receptors specific to this molecule are blocked, which prevents them from working. Furthermore, there are two types of antihistamines, H1and H2 antihistamines. They must not be confused, due to the pharmacological targets of the drugs being different. - the H1 antihistamine target the H1 receptors, which are situated throughout the entire body, in order to fight against allergic phenomena. They are indicated in the symptomatic treatment of seasonal allergic rhinitis (hay fever, urticarial, allergic conjunctivitis etc.). They particularly have a dilatory effect in the blood vessels and they also prevent oedematous reactions. - the H2 antihistamine acts on the H2 receptors, mainly found in the gastric lining in the stomach. They are used for problems linked to stomach ulcers and duodenum, as well as in cases of gastro-oesophageal reflux, where they serve to reduce gastric acid. H2 antihistamines, however, only have a limited effect on the support of allergic reactions but can be indicated in treatment of some atopic conditions, such as chronic urticaria. It is important to know that antihistamines aren’t a symptomatic treatment, as they do not act on the cause of the allergy, but on the contrary, on the desensitisation (remedial treatment of the allergy).