H1 antagonists, also called H1 blockers, are a class of medications that block the action of histamine at the H1 receptor, helping to relieve allergic reactions.
First-generation antihistamines include diphenhydramine (Benadryl), carbinoxamine (Clistin), clemastine (Tavist), chlorpheniramine (Chlor-Trimeton), and brompheniramine (Dimetane).
Infrequent adverse effects include urinary retention, palpitations, hypotension, headache, hallucination, psychosis and erectile dysfunction.
[4] 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.
[citation needed] Histamine, acting on H1-receptors, produces pruritus, vasodilation, hypotension, flushing, headache, bradycardia, bronchoconstriction, increase in vascular permeability and potentiation of pain.
They are effective in the relief of allergic symptoms, but are typically moderately to highly potent muscarinic acetylcholine receptor (anticholinergic) antagonists as well.
The first H1-antihistamine discovered was piperoxan, by Ernest Fourneau and Daniel Bovet (1933) in their efforts to develop a guinea pig animal model for anaphylaxis at the Pasteur Institute in Paris.
This selectivity significantly reduces the occurrence of adverse drug reactions, such as sedation, while still providing effective relief of allergic conditions.