Antiallergen

Antihistamine
Drug class
Histamine structure
Class identifiers
ATC code	R06
Mechanism of action	• Receptor antagonist  • Inverse agonist
Biological target	Histamine receptors  • HRH1  • HRH2  • HRH3  • HRH4
External links
MeSH	D006633

Antihistamines are drugs which treat allergic rhinitis and other allergies. Antihistamines can give relief when a person has nasal congestion, sneezing, or hives because of pollen, dust mites, or animal allergy. Typically people take antihistamines as an inexpensive, generic, over-the-counter drug with few side effects. As an alternative to taking an antihistamine, people who suffer from allergies can instead avoid the substance which irritates them. Antihistamines are usually for short-term treatment. Chronic allergies increase the risk of health problems which antihistamines might not treat, including asthma, sinusitis, and lower respiratory tract infection. Doctors recommend that people talk to them before any longer term use of antihistamines.

Although typical people use the word “antihistamine” to describe drugs for treating allergies, doctors and scientists use the term to describe a class of drug that opposes the activity of histamine receptors in the body. In this sense of the word, antihistamines are subclassified according to the histamine receptor that they act upon. The two largest classes of antihistamines are H₁-antihistamines and H₂-antihistamines. Antihistamines that target the histamine H₁-receptor are used to treat allergic reactions in the nose (e.g., itching, runny nose, and sneezing) as well as for insomnia. They are sometimes also used to treat motion sickness or vertigo caused by problems with the inner ear. Antihistamines that target the histamine H₂-receptor are used to treat gastric acid conditions (e.g., peptic ulcers and acid reflux). H₁-antihistamines work by binding to histamine H₁ receptors in mast cells, smooth muscle, and endothelium in the body as well as in the tuberomammillary nucleus in the brain; H₂-antihistamines bind to histamine H₂ receptors in the upper gastrointestinal tract, primarily in the stomach.