**4. Conclusions**

In this investigation, six new multicomponent forms, three for lidocaine and three for lidocaine hydrochloride, with hydroquinone, resorcinol, and pyrogallol, were obtained and characterised by different techniques. LAG allowed the synthesis of six new crystal forms, and the stability assays revealed the high stability of the **(lidhcl)** forms compared with **(lid)** phases, which were selected for further analysis. The SC-XRD structure solution allowed a comparative structural analysis, highlighting how **(lidhcl)** increases the stability of the new forms compared to **(lid)** through a steric protection effect. The structural information obtained revealed the important role that chloride ions play in the stabilisation of the **(lidhcl)** new multicomponent forms, allowing an improved oxidation behaviour.

There was a tendency between melting points and solubility for the new **(lidhcl)** phases. The solubility of the new crystal forms of **(lidhcl)** was lower than **(lidhcl)** itself, which, far from being a drawback, can be an interesting way to modulate the **(lidhcl)** solubility and stability, as can be concluded from the DSC of the new phases showing an increase in thermal stability. In this concern, the synthesis of molecular or ionic cocrystals can be taken as a valuable approach to tuning the properties of the API to adapt it to the requirements.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/cryst12060798/s1: Figure S1. Calculated vs. experimental PXRD patterns for the six new phases; Figure S2. **(lidhcl)** vs. **(lid)** new phases: visual comparative slurry stabilities. Left: **(lidhcl)** forms, right: **(lid)** forms.

**Author Contributions:** Conceptualisation and methodology, C.V.-E. and D.C.-L.; formal analysis and investigation, C.V.-E., C.A.-P., F.J.A.-M., R.F.-P., D.C.-L. and A.D.-M.; writing—original draft preparation, C.V.-E.; writing—review and editing, C.V.-E. and A.D.-M.; funding acquisition, D.C.-L. and A.D.-M.; supervision, C.V.-E. and A.D.-M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Spanish Agencia Estatal de Investigación of the Ministerio de Ciencia, Innovación, y Universidades (MICIU) and co-funded by FEDER, UE, Project PGC2018- 102047-B-I00 (MCIU/AEI/FEDER, UE) and Project B-FQM-478-UGR20 (FEDER-Universidad de Granada, Spain).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** F.J.A.-M. wants to acknowledge an FPI grant (Ref. PRE2019-088832). C.V.-E. acknowledges Project PTA2020-019483-I funded by the Spanish Agencia Estatal de Investigación of the Ministerio de Ciencia e Innovación.

**Conflicts of Interest:** The authors declare no conflict of interest.
