H1 antagonist

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. 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.^{[citation needed]}

In common use, the term "antihistamine" refers only to H₁-antihistamines. Virtually all H₁-antihistamines function as inverse agonists at the histamine H₁-receptor, as opposed to neutral antagonists, as was previously believed.

H₁-antihistamines are used clinically to treat histamine-mediated allergic conditions. These indications may include:

First-generation H₁-antihistamines can act on the central nervous system. As a result, they are also used for:

H₁-antihistamines can be administered topically (through the skin, nose, or eyes) or systemically, based on the nature of the allergic condition.

The authors of the American College of Chest Physicians Updates on Cough Guidelines (2006) recommend that, for cough associated with the common cold, first-generation antihistamine-decongestants are more effective than newer, non-sedating antihistamines. First-generation antihistamines include diphenhydramine (Benadryl), carbinoxamine (Clistin), clemastine (Tavist), chlorpheniramine (Chlor-Trimeton), and brompheniramine (Dimetane). However, a 1955 study of "antihistaminic drugs for colds," carried out by the U.S. Army Medical Corps, reported that "there was no significant difference in the proportion of cures reported by patients receiving oral antihistaminic drugs and those receiving oral placebos. Furthermore, essentially the same proportion of patients reported no benefit from either type of treatment."

Adverse drug reactions are most commonly associated with the first-generation H₁-antihistamines. This is due to their relative lack of selectivity for the H₁-receptor and their ability to cross the blood–brain barrier.

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 H₁-antihistamines include dizziness, tinnitus, blurred vision, euphoria, incoordination, 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, psychosis and erectile dysfunction.