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.
5-HT3 antagonist
There are few side effects related to the use of 5-HT antagonists; the most common are constipation or diarrhea, headache, and dizziness. Unlike antihistamines with antiemetic properties such as cyclizine, 5-HT antagonists do not produce sedation, nor do they cause extrapyramidal effects, as phenothiazines (such as prochlorperazine) sometimes do.