This comprehensive book on psychoactive substances delves deeply into their definition, classification, and the latest scientific advancements and innovative therapies. With a focus on the mechanisms of action in the central nervous system, it examines how stimulants, depressants, hallucinogens, and analgesics influence the brain and human behavior. Offering a thorough analysis that bridges neuroscience, medicine, and mental health, this work is an indispensable reference for scholars and professionals alike. Focusing on the therapeutic use of psychoactive substances, the book covers key topics such as medical cannabis, psychedelics in psychiatric therapies, and treating disorders like ADHD with stimulants. It also critically examines the medical and psychiatric risks, including chemical dependency, withdrawal syndromes, and the challenges of recreational use, all supported by cutting-edge scientific research. Exploring the intersections between psychoactive substances and mental health, this work discusses comorbidities such as anxiety, depression, and schizophrenia, as well as addressing dual diagnosis and bioethical and social issues. Reflections on the future of psychiatric therapies involving controlled substances and the medicinal use of cannabis and psychedelics make this book an essential read for understanding the innovations shaping modern medicine.