Antihistamines (H1 receptor antagonists)

This chapter discusses the adverse effects of antihistamines (H1 receptor antagonists). Antihistamines act as inverse agonists that combine with H1 receptors, stabilizing them in the inactive form and shifting the equilibrium towards the inactive state. The cardiotoxic effects of antihistamines are of concern in patients with long QT syndrome who also have allergies. Fixed drug eruptions are the most frequent types of adverse cutaneous drug reaction; although fixed drug eruptions caused by systemic antihistamines are very rare, some have been previously reported with both cetirizine and levocetirizine, and another has been reported. First-generation sedating antihistamines such as dexchlorpheniramine and diphenhydramine readily cross the blood–brain barrier, resulting in blockade of histamine H1 receptors in the brain, which are responsible for maintaining a state of arousal, leading to drowsiness, fatigue, and psychomotor disturbances.