Psilocybin’s effects don’t rely on a single serotonin pathway. This mouse study shows that the serotonin 1B (5-HT1B) receptor is required for several of psilocybin’s behavioral and brain-activity effects, including changes tied to flexibility and responsiveness, while other effects remain 5-HT2A-dependent. The findings clarify that psilocybin works through a network of serotonin receptors—not just one—helping explain why its effects are broad, nuanced, and harder to replicate with single-target drugs.