Structural features and interactions of new sulfamethazine salt and cocrystal

•A new salt (sulfamethazine-dichlofenac acid) and a new cocrystal (sulfamethazine-piperazine) of sulfamethazine were prepared for the first time.•Difference between the salt and cocrystal presented through solid-state characterization and density functional calculations.•Sulfamethazine interaction sites: the sulfonamide group, pyrimidinyl, and amino group.•Sulfamethazine can transfer between hydrogen bond donor and acceptor simultaneously.