Antihistamine
The vast majority of marketed H-antihistamines are receptor antagonists and only a minority of marketed compounds are inverse agonists at the receptor. Clinically, H-antihistamines are used to treat allergic reactions and mast cell-related disorders. Sedation is a common side effect of H-antihistamines that readily cross the blood–brain barrier; some of these drugs, such as diphenhydramine and doxylamine, are therefore used to treat insomnia. H-antihistamines can also reduce inflammation, since the expression of NF-κB, the transcription factor the regulates inflammatory processes, is promoted by both the receptor's constitutive activity and agonist (i.e., histamine) binding at the H receptor.
QT interval
Prolongation of the QT interval may be due to an adverse drug reaction. Many drugs, such as haloperidol, vemurafenib, ziprasidone, methadone and sertindole, can prolong the QT interval. Some antiarrhythmic drugs, like amiodarone or sotalol, work by getting a pharmacological QT prolongation. Also, some second-generation antihistamines, such as astemizole, have this effect. In addition, high blood alcohol concentrations prolong the QT interval. A possible interaction between selective serotonin reuptake inhibitors and thiazide diuretics is associated with QT prolongation. Macrolide and fluoroquinolone antibiotics are also suspected to prolong the QT interval. It was discovered recently that azithromycin is associated with an increase in cardiovascular death.