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.
Drug-induced QT prolongation
Drug-induced QT prolongation is seen with a QT interval above 450 ms on the ECG and is usually a result of treatment by anti-arrhythmic drugs, such as amiodarone and sotalol, or a number of other drugs that have been reported to cause this problem (e.g., cisapride). Some antipsychotic drugs, such as haloperidol and ziprasidone, have a prolonged QT interval as a rare side-effect. Antihistamines, erythromycin, and ciprofloxacin may also cause drug-induced LQT. Genetic mutations may make one more susceptible to drug-induced LQT. It is associated with hypokalaemia, hypocalcaemia and hypothermia and may lead to torsades de pointes.