Classic psychedelic substances (Psilocybin, LSD, DMT and Mescaline) as well as MDMA and Ketamine have shown extraordinary efficacy in the treatment of various psychological disorders such as depression, post-traumatic stress disorder (PTSD), anxiety disorders (especially for terminally ill patients) and addictions to alcohol, nicotine and other drugs. They have also shown promising results in the preventive and abortive treatment of migraines and cluster headaches. Incipient research points to possible applications in the treatment of anorexia nervosa, Parkinson's disease and Alzheimer's disease. The low toxicity of these substances means that their controlled use has attracted the attention of scientists from all over the world for their possible therapeutic application. After being used for more than 20 years, their medical use and research were banned in the 1970s and it was not until 1999 that licenses for their experimental use were granted again. Currently, more than a dozen countries have legalized the therapeutic use of MDMA, Ketamine and Psilocybin for the treatment of some psychological disorders such as treatment-resistant depression, post-traumatic stress disorder (PTSD) and anxiety. Adverse effects vary depending on the substance, but they are considered relatively safe. Contrary to popular belief, scientific evidence tells us that the risk of adverse effects in controlled situations is lower. Nevertheless, it does exist.