**Preface to "Clinical Utility of Applying PGx and Deprescribing-Based Decision Support in Polypharmacy: Future Perspectives"**

According to the World Health Organization (WHO), polypharmacy is one of the global key challenges for medication today. Polypharmacy is a necessary and important aspect of drug treatment; however, it becomes a challenge when the medication risks outweigh the benefits for an individual patient. Drug–drug interactions and the introduction of prescribing cascades are common features of polypharmacy, which can lead to ineffectiveness and increased risk of adverse drug reactions (ADR). Genes encoding CYP450 isozymes and other drug-related biomarkers have attracted considerable attention as targets for pharmacogenetic (PGx) testing due to their impact on drug metabolism and response. This Special Issue is devoted to explore the status and initiatives taken to improve medication and to reduce ADR in polypharmacy patients. The collection of articles spans many expert areas and disciplines dealing with drug–drug interactions and consequences thereof in therapeutic management, including PK- and PD-profiling; the application of PGx-based guidance and/or decision tools for drug–gene and drug–drug gene interactions; medication reviews; development and application of deprescribing tools; and drivers and barriers to overcome for successful implementation in the healthcare system. We hope by this Special Issue to stimulate to further actions and initiatives to be taken for the benefit of polypharmacy patients. We thank the authors of the individual articles for their creativity and ingenuity and for paving the way forward in developing new tools and approaches to the understanding of polypharmacy.

> **Charlotte Vermehren and Niels Westergaard** *Editors*
