Drug use is a real public health problem. Worldwide, it is estimated that more than 77 million people use cannabis, while the use of LSD is generally low and has been stable for several years. Drugs generally act on the reward system, leading to dependence. Their use causes numerous psychological and somatic problems. This book aims to show the effects of two psychoactive substances: cannabis, with THC as its active ingredient, which acts on the reward circuit, leading to dependence, and lysergic acid diethylamide (LSD), a hallucinogenic chemical that also acts on the reward circuit.