There is a large heterogeneity among individuals in their therapeutic responses to the same drug and in the occurrence of adverse events. A key factor increasingly recognized to contribute to this variability is the gut microbiome. The gut microbiome can be regarded as a second genome, holding significant metabolic capacity. Consequently, the field of pharmacomicrobiomics has emerged as a natural extension of pharmacogenomics for studying variations in drug responses. Pharmacomicrobiomics explores the interaction of microbiome variation with drug response and disposition. The interaction between microbes and drugs is, however, complex and bidirectional. While drugs can directly alter microbial growth or influence gut microbiome composition and functionality, the gut microbiome also modulates drug responses directly through enzymatic activities and indirectly via host-mediated immune and metabolic mechanisms. Here we review recent studies that demonstrate the interaction between drugs and the gut microbiome, focusing on cancer immunotherapy and immunomodulation in the context of inflammatory bowel disease and solid organ transplantation. Because the gut microbiome is modifiable, pharmacomicrobiomics presents promising opportunities for optimizing therapeutic outcomes, with recent clinical trials highlighting fecal microbiota transplantation as a strategy to enhance the efficacy of immune checkpoint blockade. We also shed light on the future perspectives for patients arising from this field. Although multiple lines of evidence already demonstrate that the gut microbiome interacts with drugs, and vice versa, thereby affecting treatment efficacy and safety, well-designed clinical studies and integrated in vivo and ex vivo models are necessary to obtain consistent results, improve clinical translation, and further unlock the gut microbiome's potential to improve drug responses.