Table 1.

The H1R and H2R antagonists that are currently clinically used.

Types	Drugs	Structure	Refs.
H1R antagonists
First-generation	Diphenhydramine	Ethanolamine	[11,12,13]
	Clemastine	Ethanolamine	[14]
	Chlorpheniramine	Hydrocarbon amine	[15]
	Pyrilamine/Mepyramine	Ethylenediamine	[16,17]
	Cyproheptadine	Piperidine	[18]
Second-generation	Cetirizine	Piperazine	[19,20,21]
	Terfenadine	Piperidine	[22,23,24]
Third-generation	Fexofenadine	Piperidine	[25,26]
H2R antagonists
First-generation	Cimetidine	Imidazole	[11,13,14,15,27]
Second-generation	Ranitidine	Furan	[12,16,17,22,23]
	Lafutidine/loxtidine	Furan	[28]
Third-generation	Famotidine	Thiazole	[19]
Fourth generation	Roxatidine	Piperidine toluene	[24]