Ondansetron hydrochloride: a competitive serotonin 5-HT3 receptor blocker.

The methyl substituted imidazole ring in the title compound, 2-methyl-1-(9-methyl-4-oxo-2,3,4,9-tetrahydro-1H-carbazol-3-yl) imidazol-3-ium chloride dihydrate, C18H20N3O+.Cl-.2H2O, is approximately perpendicular to the carbazole plane [dihedral angle 87.0(1) degrees]. The water molecules are involved in an elaborate network of hydrogen bonds that reinforce the stability of the dihydrate and the cohesion of the structure.