The most common adverse effect is sedation; this "side-effect" is utilized in many OTC sleeping-aid preparations.
Other common adverse effects in first-generation H-antihistamines include dizziness, tinnitus, blurred vision, euphoria, uncoordination, anxiety, increased appetite leading to weight gain, insomnia, tremor, nausea and vomiting, constipation, diarrhea, dry mouth, and dry cough.
Infrequent adverse effects include urinary retention, palpitations, hypotension, headache, hallucination, and psychosis.-receptors and have a better tolerability profile compared to the first-generation agents.
Agents where the main therapeutic effect is mediated by negative modulation of histamine receptors are termed antihistamines; other agents may have antihistaminergic action but are not true antihistamines.
In common use, the term "antihistamine" refers only to H In type I hypersensitivity allergic reactions, an allergen (a type of antigen) interacts with and cross-links surface Ig E antibodies on mast cells and basophils.