Hallucinogens are drugs that alter sensory input to the brain. This creates an altered sense of reality, as well as a change in emotions and thought patterns. Hallucinogens can connect the conscious mind to the unconscious, delivering thoughts and feelings that would otherwise remain out of our conscious awareness.
Naturally occuring hallucinogen
Hallucinogenic mushrooms often have a characteristic blue bruising on their stems.
Hallucinogens are divided into three categories: psychedelics, dissociatives, and deliriants.
Psychedelics
Psychedelics are distinguished from the other two subclasses by their lack of addictive qualities, as well as the remarkable experiences one may have under their influence. Most (but not all) psychedelics are non-toxic and have a very high LD50. This means that the user would have to ingest or take very large amounts of the drug, many times that of a recreational dose, in order to kill themselves. The resulting experiences, however, can be extremely overwhelming, often driving users to seek emergency assistance.
Effect of Psychedelics on the Brain
Most psychedelics work by interacting with serotonin receptors in the brain. Effects include increased breathing and heart rate, dilated pupils, dehydration, increased color perception, a state of empathetic well-being (feeling as though one is at peace with everyone and everything), and visual distortion: things may appear to move, shapes may appear on textures and exhibit a kaleidoscope-like effect, or lighting may dramatically change for no apparent reason.
Dissociatives
Dissociatives are a subclass of hallucinogens that work by blocking or altering sensory perception (including senses that are perceived by the unconscious mind), creating a feeling of disconnection and depersonalization from one's body and reality in general. Many dissociatives are strong depressants, and large doses can slow down the heart or breathing to the point of causing death. Generally they are not physically addictive, but can be habit forming if used repeatedly in a short time span.
Effect of Dissociatives on the Brain
Most dissociative drugs simulate a dream-like experience. As with psychedelics, a mixing of the senses can lead users to believe they can see sounds or taste textures. Primary dissociatives are NMDA antagonists, which block glutamate from entering its receptors and regulating brain function.
Deliriants
Deliriants are very similar to dissociatives, and are considered to be true hallucinogens because the visuals they produce are hard or impossible to distinguish from reality. Under their influence, users will often have entire, rational conversations with people who aren't actually there. Some people may see their own reflection and believe it is another person copying their actions. Users often retain awareness of their physical surroundings, but are consciously impaired. Almost all deliriants are toxic enough that there is a very small difference, in terms of quantity used, between a recreational dose and a lethal one. Both Benadryl and Dramamine are deliriants when taken in extremely high quantities.
Effect of Deliriants on the Brain
Deliriants work by inhibiting acetylcholine, a neurotransmitter responsible for cognition and stimulation. By contrast, when you drink a cup of coffee, your acetylcholine increases, leading to enhanced alertness and focus.
Common Hallucinogens
Psychedelics
Psilocybin mushrooms, also known as "shrooms," are mushrooms that contain the psychedelic compounds psilocybin and psilocin. They are mainly used as recreational drugs, whose effects include euphoria, altered thinking processes, closed- and open-eye visuals, synesthesia, an altered sense of time, and spiritual experiences.
LSD, also a psychedelic, blocks serotonin from the brain, which regulates mood, perception, muscle contraction, and other cognitive functions. LSD blocks serotonin because it is, structurally, similar to serotonin. For this reason, the brain mistakes LSD for serotonin and directs it to the synaptic cleft (instead of actual serotonin). LSD causes a number of alterations in perception by affecting both cognitive and visual sensory systems, and it changes the sense of time, body-image, and ego. Memory is also greatly affected. A typical "trip" can last anywhere between six and ten hours.
Dissociatives
PCP (or angel dust), a dissociative, prevents the actions normally caused when a neurotransmitter called glutamate is able to attach to its receptor in the brain. It also disrupts the actions of other neurotransmitters. This drug is addictive, and its effects are very unpredictable. For example, it may make some people hallucinate and become aggressive, while others may become drowsy and passive.
Dextromethorphan is the active ingredient in most over-the-counter and prescription cough medicines. When used in excess of specified maximum dosages, dextromethorphan acts as a dissociative. It can produce effects similar to the dissociative states created by other dissociative anaesthetics such as ketamine and phencyclidine.
Nitrous oxide, commonly known as laughing gas, is used in surgery and dentistry for its anaesthetic and analgesic effects. It is known as laughing gas due to the euphoric effects of inhaling it, a property that has led to its recreational use as a dissociative hallucinogen.
Deliriants
While deliriants are often naturally occuring (in plant species), synthetic compounds such asdiphenhydramine (Benadryl) and dimenhydrinate (Dramamine) are also deliriants. Uncured tobacco is also a deliriant due to its very high nicotine content, resulting in a delirious hallucinogenic intoxication. Despite the fully legal status of several common deliriant plants, deliriants are largely unpopular as recreational drugs because of their unfavorable effects.