Solid solutions could represent a viable alternative to better understand and control structure–property relationships of drugs, in order to optimize their properties for practical applications. Lamivudine (3TC) and emtricitabine (FTC) are nucleoside analogue reverse transcriptase inhibitor antiretroviral drugs, which have similar molecular structures, differing by a single fluorine atom, which is only present in the FTC molecule. Due to these similarities in structure and molecular resemblances, lamivudine and emtricitabine are good candidates for producing a solid solution with physicochemical properties controlled by the stoichiometry. Following this hypothesis, the formation of a nonconventional solid solution was verified, whose crystalline structure is not defined by any of the individual constituents but by the one of the lamivudine hydrate with emtricitabine as a solute. The crystalline structures of two members of the solid solution were determined showing a nonuniform distribution of the solute among the independent molecules of the asymmetric unit of the lamivudine hydrate structure. Thermal analysis investigations confirmed that physicochemical properties could be controlled through the variation of the emtricitabine content.

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