The use of psychedelic compounds such as psilocybin—the hallucinogenic in magic mushrooms—and ayahuasca—a psychoactive mixture traditionally used by Amazonian Indigenous cultures—is already used in a therapeutic setting. Caspani was initially interested in the impact of the gut microbiome on different brain diseases, and later, in how the gut microbiome affects our brain and our mood. These lines of investigation seemed to point naturally at examining the relationship between psychedelics and the gut microbiome. There’s another reason to study how the gut microbiome handles psychedelic exposure. The gut microbiome could be the key to predicting those individual responses.