An antihistamine is not actually the opposite of a histamine. An antihistamine (antagonist) is a drug that binds to certain sites (receptors on cells) in the body, in order to prevent histamine from binding. Antihistamines act as competitive inhibitors of histamine (agonist) binding. Histamine is a major component of many venoms and sting secretions and is produced naturally by the immune system and released in response to tissue damage. In humans, histamine mediates immediate allergic and inflammatory responses, causes gastric acid release and functions as a central nervous system neurotransmitter. Most histamine is found in specialized cells called mast cells and basophils. Upon release histamine causes local vasodilation that allows other mediators of inflammation to enter the area.
Histamine causes inflammation by binding to H1 receptors on the smooth muscle cells within blood vessels causing them to dilate producing localized edema. Systemically, histamine causes smooth muscle in the lungs and gastrointestinal system to contract and causes vasodilation, low blood pressure (hypotension) and increased heart rate (tachycardia). Additional substances are released that contribute to the overall effect but this scenario is characteristic of anaphylactic shock.
The term antihistamine refers to a drug which blocks H1 (histamine, type 1) receptors in the body. (Drugs that are H2-receptor antagonists are those used to treat gastric ulcers as they block gastric-acid and enzyme secretion.) Antihistamines compete for the H1 receptors on blood-vessel endothelial cells and smooth muscle cells. As competitive inhibitors, the antihistamines bind to the receptors because there is more of the antihistamine than the natural histamine.
Side effects associated with H1 antagonists are sedation, nausea, vomiting, blurred vision, orthostatic hypotension, and local anesthesia. Some of these effects are exploited to treat other maladies besides inflammation and allergy. For instance, diphenhydramine can be used as a local anesthesia in persons allergic to lidocaine. Analgesics, asthma products, cold products, dermatologic preparations, menstrual products, motion sickness products, sleep aids and topical decongestants available over-the-counter (OTC) may contain hidden antihistamine.
Antihistamines are used to treat allergies, motion sickness, certain types of headaches, certain diseases (such as Crohn's disease and acute multiple sclerosis), some stomach secretory conditions, and so on. Antihistamines are generally well absorbed from the gut, metabolized primarily in the liver and most have a duration of action of about 4-6 hours. They are the drug of choice for treatment of colds and allergies but they do not cure a cold or allergy but rather provide relief from symptoms.
There are OTC antihistamines and those available in the U.S. by prescription only. First generation antihistamines (diphenhydramine, chlorepheniramine, cyproheptadine, hydroxyzine) are known as sedating. Other adverse affects associated with these OTC and prescription drugs include, dizziness, hallucinations, worsening of glaucoma and benign prostatic hyperplasia. Second generation antihistamines (fexofenadine, loratadine, cetirizine) are much less sedating.
Atarax (hydroxyzine) are antihistamines used to treat symptoms of itching due to allergies...