Antihistamine
The vast majority of marketed H-antihistamines are receptor antagonists and only a minority of marketed compounds are inverse agonists at the receptor. Clinically, H-antihistamines are used to treat allergic reactions and mast cell-related disorders. Sedation is a common side effect of H-antihistamines that readily cross the blood–brain barrier; some of these drugs, such as diphenhydramine and doxylamine, are therefore used to treat insomnia. H-antihistamines can also reduce inflammation, since the expression of NF-κB, the transcription factor the regulates inflammatory processes, is promoted by both the receptor's constitutive activity and agonist (i.e., histamine) binding at the H receptor.
Adverse effect
Most drugs have a large list of nonsevere or mild adverse effects which do not rule out continued usage. These effects, which have a widely variable incidence according to individual sensitivity, include nausea, dizziness, diarrhea, malaise, vomiting, headache, dermatitis, dry mouth, etc. These can be considered a form of pseudo-allergic reaction, as not all users experience these effects; many users experience none at all.