*3.6. Stability Studies*

The stability of cocrystals was studied in this work by performing aqueous slurry experiments at 25 ◦C and storing them at accelerated ageing conditions (40 ◦C and 75% relative humidity). The thermodynamic stability of cocrystals was first evaluated by slurry experiments at 25 ◦C. In these experiments, excess solids of the cocrystal powders were stirred in deionized water for 24 h. The resulting filtered and air-dried samples were analyzed by PXRD to evaluate their phase purity, and it was observed that the two cocrystals were stable upon slurrying. These observations suggest that the cocrystals are thermodynamically stable at room temperature. Likewise, results of the stability tests suggest that the two new solid forms remained the same after storage for two months (Figure 7). The stability of the cocrystals at accelerated test conditions is consistent with the thermodynamic stability observed in the slurry experiments.

**Figure 7.** PXRD patterns of FUR–ETZ (**a**) and FUR–PRX (**b**) with respect to the stability under accelerated ageing conditions (40 ◦C, 75% RH) at different time intervals.
