[1] 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.
[1] Chronic allergies increase the risk of health problems which antihistamines might not treat, including asthma, sinusitis, and lower respiratory tract infection.
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.
[6] 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.
[2][8] 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.
Sedation is a common side effect of H1-antihistamines that readily cross the blood–brain barrier; some of these drugs, such as diphenhydramine and doxylamine, may therefore be used to treat insomnia.
However, upon high doses second-generation antihistamines will begin to act on the central nervous system and thus can induce drowsiness when ingested in higher quantity.
H2-antihistamines are among first-line therapy to treat gastrointestinal conditions including peptic ulcers and gastroesophageal reflux disease.
H3 receptors are primarily found in the brain and are inhibitory autoreceptors located on histaminergic nerve terminals, which modulate the release of histamine.
[22][27] Subsequently, other non-sedating antihistamines like loratadine (Claritin), cetirizine (Zyrtec), and fexofenadine (Allegra) were developed and introduced.
[1] The United States government removed two second generation antihistamines, terfenadine and astemizole, from the market based on evidence that they could cause heart problems.
[1] Another gap in the research is in information reporting the health effects for individuals with long-term allergies who take antihistamines for a long period of time.
[1] Continuous and/or cumulative use of anticholinergic medications, including first-generation antihistamines, is associated with higher risk for cognitive decline and dementia in older people.
[1] Research regarding the effects of commonly used medications upon certain cancer therapies has suggested that when consumed in conjunction with immune checkpoint inhibitors some may influence the response of subjects to that particular treatment whose T-cell functions were failing in anti-tumor activity.
Upon study of records in mouse studies associated with 40 common medications ranging from antibiotics, antihistamines, aspirin, and hydrocortisone, that for subjects with melanoma and lung cancers, fexofenadine, one of three medications, along with loratadine, and cetirizine, that target histamine receptor H1 (HRH1), demonstrated significantly higher survival rates and had experienced restored T-cell anti-tumor activity, ultimately inhibiting tumor growth in the subject animals.