Antihistamine

Antihistamines are drugs that treat hay fever and other allergies. Typically, people take antihistamines as an inexpensive, generic (not patented) drug that can be bought without a prescription and provides relief from nasal congestion, sneezing, or hives caused by pollen, dust mites, or animal allergy with few side effects. 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. Consultation of a medical professional is recommended for those who intend to take antihistamines for longer-term use.

Although the general public typically uses the word "antihistamine" to describe drugs for treating allergies, physicians 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.

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. Antihistamines that target the histamine H₁-receptor are used to treat allergic reactions in the nose (e.g., itching, runny nose, and sneezing). In addition, they may be used to treat insomnia, motion sickness, or vertigo caused by problems with the inner ear. H₂-antihistamines bind to histamine H₂ receptors in the upper gastrointestinal tract, primarily in the stomach. Antihistamines that target the histamine H₂-receptor are used to treat gastric acid conditions (e.g., peptic ulcers and acid reflux). Other antihistamines also target H₃ receptors and H₄ receptors.

Histamine receptors exhibit constitutive activity, so antihistamines can function as either a neutral receptor antagonist or an inverse agonist at histamine receptors. Only a few currently marketed H₁-antihistamines are known to function as antagonists.

Histamine makes blood vessels more permeable (vascular permeability), causing fluid to escape from capillaries into tissues, which leads to the classic symptoms of an allergic reaction—a runny nose and watery eyes. Histamine also promotes angiogenesis.

Antihistamines suppress the histamine-induced wheal response (swelling) and flare response (vasodilation) by blocking the binding of histamine to its receptors or reducing histamine receptor activity on nerves, vascular smooth muscle, glandular cells, endothelium, and mast cells. Antihistamines can also help correct Eustachian Tube dysfunction, thereby helping correct problems such as muffled hearing, fullness in the ear and even tinnitus.

Itching, sneezing, and inflammatory responses are suppressed by antihistamines that act on H1-receptors. In 2014, antihistamines such as desloratadine were found to be effective to complement standardized treatment of acne due to their anti-inflammatory properties and their ability to suppress sebum production.

H₁-antihistamines refer to compounds that inhibit the activity of the H₁ receptor. Since the H₁ receptor exhibits constitutive activity, H₁-antihistamines can be either neutral receptor antagonists or inverse agonists. Normally, histamine binds to the H₁ receptor and heightens the receptor's activity; the receptor antagonists work by binding to the receptor and blocking the activation of the receptor by histamine; by comparison, the inverse agonists bind to the receptor and both block the binding of histamine, and reduce its constitutive activity, an effect which is opposite to histamine's. Most antihistamines are inverse agonists at the H₁ receptor, but it was previously thought that they were antagonists.