The term "psychedelic" is used to refer to a group of substances and products2 which produce effects that are, precisely, psychedelic. These substances can be synthetic (LSD, ketamine) or natural (mushrooms of the Psilocybe or Amanita genus, ayahuasca), and the truth is that it is difficult to define these psychedelic effects. It is generally characterized by distortions in the senses (you may hear better, in more detail, or a wall may be perceived to move as if it were "breathing"), changes in thought (may increase speed of thought or creativity), or very sudden and intense changes in mood (burst into inconsolable tears one minute and be in a fit of laughter the next). Peak experiences, using Abraham Maslow's terminology, are also frequent at high doses. These are experiences that allow us to transcend the usual perception of oneself as a person and our environment. Maslow (1908-1970) was an American psychologist who founded humanistic psychology and, when he spoke of peak experiences he emphasized harmony. In these states it is usual to feel in harmony with oneself and with existence. The perception of time and space dims and a deep sense of well-being is heightened.

In short, the aim of this book on psychedelic pharmacology is to explain how psychedelic substances work, how they can have such dramatic effects on perception and thought, and what these substances do to our brain. These are some of the questions that we will try to answer from the perspective of pharmacology.

As for the origin of the word pharmacology, we are going to stop a little longer to comment on its fascinating etymology. First of all, in classical Greek phármakon means drug. Far from the clear pejorative connotations that this word currently has in our culture, originally the term was much more neutral, and therefore more complex. In fact, an almost identical and probably earlier term is pharmakós, which means "sacrificial lamb". Pharmakós was the name given to a person who was going to undergo some kind of sacrifice. It must be remembered that these sacrifices were not always lethal; it could be exile, stoning or other tormenting punishments, although the term referred to all the "victims" of these practices, all of them constituted part of a kind of purification ritual. When the community was hit by catastrophes or droughts, or individuals fell gravely ill, these events were interpreted as deregulations in the delicate balance between humans and gods, which evidenced the need to readjust said harmony by offering the gods the pharmakoi (plural of pharmakós), who carried all the impurities (or miasma) of the people. In other words, they would gather and concentrate "all that was bad" to later destroy it. But what relationship did the pharmakoi have with drugs or phármakon? Although it is mere conjecture, some authors suggest that, once sacrifices and other practices gave way to classical Greek rationality—including Hippocratic medicine—by the 5th century BC, the personification of the ills of the community for subsequent sacrifice was no longer considered essential. In fact, this magical act came to be conceived as a practice typical of charlatans. The pharmakós, the poor victim who would be sacrificed, became an impersonal phármakon (a botanical preparation, for example) capable of "purifying" a body without the need for another being to succumb in the process.

Interestingly, another similar word in Greek (although its relationship to phármakon is less clear) is pharmasso, which means "to temper iron", that is, when red hot iron is submerged in cold water. "To temper" has an unequivocal meaning in psychological terms, being synonymous with soothing, calming. In this sense, it may be that at some point the effects of the phármakon (at least when substances such as opium were used) were associated with the action of "tempering" the iron. Likewise, another close concept is pharmak, which is formed by a first part (Phar) that could derive from the Indo-European root bher (which means "to move", "to carry") and a second part (mak), also from an Indo-European root, which means "power". In that a case, the possibly oldest root of the term "drug" would be referring to a substance with the "power" of "moving" (impurities).

Regarding the term phármakon itself, its meaning was not limited exclusively to the concept of drug, as we have seen. Phármakon could refer to medicines, poisons or remedies. This is because sometimes the difference between a remedy and a poison, so (supposedly) evident today, was not so clear. As Paracelsus3 would say centuries later, the difference may lie in the dosage. Therefore, the word phármakon did not have the same negative connotations that the word drug currently has for us. It referred only to a chemical vehicle used to intervene in the functions of an organism.

The CNS is made up of the brain and the spinal cord. Both structures are protected by the skull and the spinal column, respectively, which already informs us of their crucial importance. The CNS is where most of the body's functions will be coordinated, and where we find the structures without which few of these functions would be possible. Forming part of the brain we find the cerebrum, cerebellum and medulla oblongata. In the brain is where functions as complex as the integration of sensory information, information processing, or emotional processing, among many others, are carried out. It is made up of two zones clearly distinguishable by their colors: gray matter and white matter. The first corresponds to sets of neuronal cell bodies, while white matter corresponds to nerve fibers that connect these neuronal bodies. Higher functions, such as reasoning or planning, seem to be located mostly in the cerebral cortex, a broad mantle about 3 mm thick that covers the entire surface of the brain. Regions or neurons that lie under this cortex are called subcortical.

The evolutionarily older regions of the brain are those that lie deeper, and are called limbic structures or limbic brain, while the regions that are more recent, from an evolutionary point of view, are closer to the cortex, being the neocortex the most recently developed structure. It should be noted that, apart from these regions and structures, the brain is an organ endowed with remarkable plasticity, so that it "constructs" itself in correspondence with the development of the individual and their experiences.

The cerebellum has been typically associated with motor functions, although it also performs notable cognitive and sensory functions. The medulla oblongata is located at the base of the brain and is a connecting area between the brain and the spinal cord. Also found in this structure are many nuclei responsible for automatic body functions, such as breathing or heart rate. In the medulla oblongata there is an important opiate receptor density, and that is why an overdose of this type of drugs can risk the life of the user, because, if the activity of these nuclei is inhibited with sufficient intensity, automatic processes like the ones mentioned above can be interrupted.

The ANS is called autonomous precisely because it coordinates functions that are generally beyond the reach of our will or even our consciousness. It is basically made up of the nerves that connect the CNS with the rest of our organism, so it can send and receive signals from any point in our body. It is made up of the sympathetic and parasympathetic nervous systems.

The sympathetic nervous system is mainly related to the sudden activation of our body in situations of need. When we are faced with some dangerous or threatening situation, it is this part of our nervous system that activates the necessary resources to succeed. It is responsible, for example, for emptying the bladder, for sending more blood to the extremities so you can run further, for releasing more adrenaline to better withstand possible pain, etc. In short, it prepares the body for fight or flight reactions.

The parasympathetic nervous system has the opposite function. The...