Psychedelics are a group of substances within the heterogeneous class of hallucinogenic drugs. Via binding to the serotonin (5-HT) 2A receptor, psychedelics exert profound alterations in various mental domains, including sensation, cognition, emotions, and self-perception. Psychedelics comprise phenethylamines (e.g., mescaline), tryptamines (e.g., psilocybin), and ergolines (e.g., LSD). These drugs have been used recreationally for decades but have also regained attention as potential treatments for various psychiatric as well as neurological illnesses. While psychedelics are generally considered to be relatively safe from a physiological standpoint, especially when compared to other recreational drugs, they are not without risks. The main safety concerns are lasting psychological adverse reactions such as persisting anxiety, dissociation, or flashbacks. This chapter provides a comprehensive overview of the pharmacology of classic psychedelics, including their origins, psychological and autonomic effects, interactions, and potential risks and side effects. Furthermore, the origin, dosing, and consumption methods are discussed. It differentiates psychedelics from other psychoactive drugs, such as MDMA and ketamine, and elaborates on their distinct receptor profiles. Overall, this chapter provides an overview of the pharmacological underpinnings necessary for understanding the harms caused by psychedelic drugs.