The present disclosure relates to novel lamotrigine salts, co-crystals, and compositions. In some embodiments, the lamotrigine salt is a lamotrigine dicarboxylic acid salt or hydrate or solvate thereof. In some embodiments, the lamotrigine co-crystal is a co-crystal of lamotrigine and a dicarboxylic acid, or a hydrate or solvate thereof. In some embodiments, the compositions comprise lamotrigine or a salt or co-crystal thereof, or a hydrate or solvate thereof. In some embodiments, the composition is in the form of a powder. In some embodiments, the composition is in the form of a liquid. The present disclosure also relates to methods of preparation and use in medical therapy, for example, for the treatment of a subject with epilepsy or bipolar disorder.