H1antihistaminer

H1 antihistamines, or H1 antihistamines, are a class of medications that block histamine H1 receptors to prevent or reduce allergy-related symptoms. By acting as competitive antagonists at these receptors, they help decrease symptoms such as sneezing, itching, rhinorrhea, and conjunctival irritation, and they can also reduce capillary permeability during allergic reactions.

There are two broad generations of H1 antihistamines. First-generation agents, such as diphenhydramine and chlorpheniramine, readily cross the blood-brain barrier and often cause drowsiness and anticholinergic effects like dry mouth and urinary retention. Second-generation agents, including cetirizine, loratadine, fexofenadine, desloratadine, and levocetirizine, have reduced central nervous system penetration and generally produce less sedation, though mild drowsiness can occur in some individuals.

Common indications include allergic rhinitis, conjunctivitis, and chronic urticaria. Some first-generation antihistamines have additional uses as antiemetics or sedatives, and they may be employed for motion sickness in certain contexts. Second-generation agents are typically preferred for chronic, non-sedating management of allergic symptoms.

Pharmacokinetics vary by compound but most H1 antihistamines are administered orally. Onset and duration depend on the specific drug, with metabolism mainly in the liver and elimination through urine or feces. Interactions with alcohol and other central nervous system depressants can enhance sedation, particularly with first-generation agents. Safety considerations include potential anticholinergic effects and cautions in glaucoma, urinary retention, pregnancy, or in the elderly.