1-Hydroxy-ethyl-2-methyl-5-nitro-imidazolium 3-carb-oxy-4-hydroxy-benzene-sulfonate.

Cocrystallization of 1-hydroxy-ethyl-2-methyl-5-nitroimidazole (metronidazole) and 5-sulfosalicylic acid (5-H(2)SSA) from methanol solution yields the title salt, C(6)H(10)N(3)O(3) (+)·C(7)H(5)O(6)S(-). In the crystal structure, the ions are linked by a combination of inter-molecular O-H⋯O, N-H⋯O and C-H⋯O hydrogen bonds, forming a three-dimensional framework. The hydroxyl group of the cation is disordered over two sites in a 0.860 (4):0.140 (4) ratio.