Hallucinogens are currently defined as drugs that alter perceptions of the natural world, perceptions of passage of time, and processes of thinking. There is a great diversity to the substances that cause hallucinations, but many are not guaranteed to create strong hallucinogenic reactions. The current issues with street and prescription hallucinogens involve the ongoing release of new or modified versions of the drugs that may or may not behave as expected.
Substances that put a person into a state of hallucination are as old as mankind's to ingest substances of nature by accident and later, by design. It is very understandable that the effects of hallucinogens were interpreted to mean that a superior state of supernatural, creative intellectual or physical capacity was being experienced and that the substance was the gateway to those states. From there, it is understandable how such substances became incorporated into many religious and spiritual rituals as enhancements to human "interaction" with the supernatural world.
Today, the facts that are known about such substances are enough to demonstrate that it is the mind, not the supernatural world, that causes the fanciful workings of time travel, gateways to alternate worlds, extra normal powers, or interactions with fanciful creatures. But to the person who willingly and frequently ingests the substances, the events and abilities appear to be quite real, and there is awareness that the substance is the cause.
Today, these substances, whether found in hundreds of plants or are synthesized in labs, are still used for religious experiences and for pleasurable, but illegal "trips" to alternate states of being. The widespread delivery and use of substances that are synthesized and distributed illegally as "street drugs" creates enormous hazards as they can be of wildly varying quality and safety, giving completely hazardous and unexpected results to many. Wide distribution and use can occur long before new versions of the drugs are even identified by those who are trained to look for them, especially in the schools, law enforcement and the medical community.
The known and classified hallucinogens are Lysergic acid diethylamide (LSD), ecstacy (MDMA), mescaline and psilocybin.
Phencyclidine (PCP, angel dust), marijuana and amphetamines are known to produce hallucinations, but are not considered as classical hallucinogens. Also, many prescription and over the counter drugs can cause hallucinations, even if such events are rare. Finally, certain medical conditions can result in hallucinations and delusions.
There is a serotonin receptor in the brain called 5HT (subscripted 2). This receptor helps with nerve impulse transmissions and is associated with well being and physiological actions. The drugs bond with the receptor sites for 5HT2 and block serotonin from getting through, leaving free serotonin in the brain. Problems with distortions of the senses of sound, sight, touch develop. The perceptions of time and space and alterations of mood are also involved.
With the stronger substances, such as LSD, the symptoms can be serious and are called "bad trips". The famous LSD flashbacks and alterations that can last for months are indications of the power of the drug to alter the brain for a time, if not permanently.
According to the DSM-IV-TR, section 4.6, substance related disorders, the official hallucinogen disorders are listed below.
hallucinogen persistent perception disorder (flashbacks)
hallucinogen intoxication delirium
hallucinogen-induced psychotic disorder with delusions
hallucinogen-induced psychotic disorder with hallucinations
hallucinogen-induced mood disorder
hallucinogen-induced anxiety disorder
hallucinogen-related disorder not otherwise specified