Cocrystal formation between nicotinamide and five fenamic acid derivative drugs (flufenamic acid, niflumic acid, tolfenamic acid, mefenamic acid and meclofenamic acid) was investigated using solution-based and solid-state preparation methods. It was anticipated that the well-known acid–aromatic nitrogen heterosynthon would provide a sufficient driving force for cocrystallization. The experiments yielded cocrystals with four of the five acids. Although the structures of these molecules are similar, they showed marked differences in both the stability and the stoichiometry of the cocrystals. A detailed analysis of the structures and properties of both the starting materials and the cocrystals allows a tentative explanation of these differences, but it also shows that even though all four cocrystals utilize one of the most predictable supramolecular synthons (COOH···N), their structures and properties remain elusive to design.