Sulfamethoxazole, smz, is an antibiotic which is classified as a Biopharmaceutics Classification System (BCS) class IV, low solubility and low permeability drug. Co-crystal formation has been examined in an attempt to improve solubility. Both ball milling and crystallization from solution have been examined. Ball milling showed that thirteen co-formers gave new crystalline X-ray powder patterns and four gave X-ray amorphous patterns while crystallization from solution gave single crystals of four co-crystals and a salt. The co-formers which gave the co-crystals and the salt have better H-bond acceptors than the sulfonyl oxygens of smz. The 4,4-dipyridyl cocrystal has an interesting high Z'' structure. It crystallized in space group P1 with four smz and six 4,4'-dipyridyl molecules in the asymmetric unit. The highest dissolution rate among the smz co-crystal and co-amorphous systems was shown by the 1,3-di(4-pyridyl)propane co-crystal.

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