Psychedelics are drugs which have an impact on brain chemistry, altering normal brain function to cause fundamental shifts in consciousness. As a general rule, people use the term “psychedelic” to refer specifically to drugs which have this function, but no clear medical use. Several prescription drugs have effects which could be considered psychedelic, but because they have medical value, they are not usually classed with other psychedelics, unless they are being used recreationally.
This class of drugs is part of a larger family of drugs known as hallucinogens. Hallucinogens are drugs which alter perception in some way. In the case of psychedelics, the drug alters the user's consciousness, causing a dramatic shift in how the user perceives his or her environment. Other hallucinogens may cause a disconnect from the surrounding environment, as in the case of opiates, or they may trigger a form of delirium. Some people believe that psychedelics can be used to access parts of the consciousness which are usually hidden.
Humans have a long history of psychedelic drug use, taking advantage of the psychoactive components of numerous plants such as peyote, Amanita muscaria mushrooms, and ergot. Many human cultures used psychedelics for religious reasons, believing that the drugs connected them with the divine, and the use of such drugs was often reserved for shamans, priests, and other religious figures. As humans learned to synthesize psychedelics in the 20th century, use of this class of drugs expanded radically, and became more recreational than religious in nature.
Many countries treat psychedelic drugs as controlled substances, under the rationale that they are potentially dangerous, and they have no known medical value. Controlled substance laws are designed to make it difficult to access such drugs, protecting people from harmful effects such as permanent alteration of brain chemistry, injuries acquired while under the influence of drugs, and so forth. Some psychedelics are protected under laws which permit religious use, allowing specific qualified individuals to produce and use the drugs, but not to sell them.
Psychedelics can be very dangerous. Drugs of plant origin have inconsistent dosages influenced by how the plants were grown, harvested, and processed. Synthetic drugs are typically much stronger, with more regular dosages, but they can be adulterated with harmful substances which mimic the effects of the psychedelic, but cause permanent harm to the user.