Water is generally regarded as a universal plasticizer of amorphous drugs or amorphous drug-containing systems. A decrease in glass-transition temperature (T g) is considered the general result of this plasticizing effect. A recent study exhibits that water can increase the T g of amorphous prilocaine (PRL) and thus shows an anti-plasticizing effect. The structurally similar drug lidocaine (LID) might show similar interactions with water, and thus an anti-plasticizing effect of water is hypothesized to also occur in amorphous LID. However, the influence of water on the T g of LID cannot be determined directly due to the very low stability of LID in the amorphous form. It is possible to predict the T g of LID from a co-amorphous system of PRL–LID using the Gordon–Taylor equation. Interactions were observed between PRL and LID based on the deviations between the experimental T gs and the T gs calculated by the conventional use of the Gordon–Taylor equation. A modified use of the Gordon–Taylor equation was applied using the optimal co-amorphous system as a separate component and the excess drug as the other component. The predicted T g of fully hydrated LID could thus be determined and was found to be increased by 0.9 ± 0.7 K compared with the T g of water-free amorphous LID. It could be shown that water exhibited a small anti-plasticizing effect on LID.

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