Merck Manual

Please confirm that you are a health care professional

honeypot link


(Lysergic Acid Diethylamide; LSD; Psilocybin; Mescaline)


Gerald F. O’Malley

, DO, Grand Strand Regional Medical Center;

Rika O’Malley

, MD, Grand Strand Medical Center

Last full review/revision May 2020
Click here for Patient Education

Hallucinogens are a diverse group of drugs that can cause highly unpredictable, idiosyncratic reactions. Intoxication typically causes hallucinations, with altered perception, impaired judgment, ideas of reference, and depersonalization. There is no stereotypical withdrawal syndrome. Diagnosis is clinical. Treatment is supportive.

Traditional hallucinogens include lysergic acid diethylamide (LSD), psilocybin, and mescaline. All are derived from natural products:

  • LSD from a fungus that often contaminates wheat and rye flour

  • Psilocybin from several types of mushrooms

  • Mescaline from the peyote cactus

Many of the newer synthetic compounds (“designer drugs”) have been produced, usually based on tryptamine or phenylethylamine molecules. Tryptamines include N,N-dimethyltryptamine (DMT) and 5-methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT).

Some other drugs, including marijuana Marijuana (Cannabis) Marijuana is a euphoriant that can cause sedation or dysphoria in some users. Psychologic dependence can develop with chronic use, but very little physical dependence is clinically apparent... read more , also have hallucinogenic properties. The term hallucinogen persists, although use of these drugs may not cause hallucinations. Alternative terms, such as psychedelic and psychotomimetic, are even less appropriate.


LSD, psilocybin, and many designer hallucinogens are serotonin receptor agonists. For mescaline, a phenylethylamine similar to amphetamines Amphetamines Amphetamines are sympathomimetic drugs with central nervous system stimulant and euphoriant properties whose toxic adverse effects include delirium, hypertension, seizures, and hyperthermia... read more , the exact mechanism has not been determined.

Mode of use and effects vary:

  • LSD is taken orally from drug-impregnated blotter paper or as tablets. Onset of action is usually 30 to 60 minutes after ingestion; duration of effects can be 12 to 24 hours.

  • Psilocybin is taken orally; effects usually last about 4 to 6 hours.

  • Mescaline is taken orally as peyote buttons. Onset of effects is usually 30 to 90 minutes after ingestion; duration of effects is about 12 hours.

  • DMT, when smoked, has onset in 2 to 5 minutes; duration of effects is 20 to 60 minutes (accounting for one of its street names, “businessman’s lunch”).

A high degree of tolerance to LSD develops and disappears rapidly. Users tolerant to any of these drugs are cross-tolerant to the other drugs. Psychologic dependence varies greatly; there is no evidence of physical dependence or a withdrawal syndrome.

Symptoms and Signs

Intoxication results in altered perceptions, including synesthesias (eg, seeing sounds, hearing colors), intensification of sensations, enhanced empathy, depersonalization (feeling the self is not real), a distorted sense of the environment’s reality, and changes in mood (usually euphoric, sometimes depressive). Users often refer to the combination of these effects as a trip. Periods of intense psychologic effects may alternate with periods of lucidity.

LSD may also have several physical effects, including mydriasis, blurred vision, sweating, palpitations, and impaired coordination. Many other hallucinogens cause nausea and vomiting. With all, judgment is impaired.

Responses to hallucinogens depend on several factors, including the user’s expectations, ability to cope with perceptual distortions, and the setting. With LSD, delusions and true hallucinations occur but are rare, as are anxiety attacks, extreme apprehensiveness, and panic states.

Psilocybin and mescaline are more likely to cause hallucinations. When hallucinogenic reactions occur, they usually subside quickly if treated appropriately in a secure setting. However, some people (especially after using LSD) remain disturbed and may have a persistent psychotic state. Whether drug use has precipitated or uncovered preexisting psychotic potential or can cause this state in previously stable people is unclear.

Chronic effects

Some people, especially long-term or repeat users (particularly of LSD), experience apparent drug effects long after they have stopped drug use. These recurrent episodes (flashbacks, hallucinogen persisting perception disorder) are usually visual illusions but can include distortions of virtually any sensation (including self-image or perceptions of time or space) and hallucinations.

Flashbacks can be precipitated by use of marijuana, alcohol, or barbiturates or by stress or fatigue or can occur without apparent reason. Mechanisms are not known. Flashbacks tend to subside within 6 to 12 months, but can recur for years.


  • Clinical evaluation

Diagnosis of hallucinogens use is usually made clinically. Drug levels are not measured. Except for PCP, most hallucinogens are not included in routine urine drug screens Drug Testing Drug testing is done primarily to screen people systematically or randomly for evidence of use of one or more substances with potential for abuse. Testing is done in the following: Certain groups... read more .


  • For acute intoxication, supportive measures and relief of anxiety and agitation

  • For persistent psychosis, psychiatric care

For acute intoxication from hallucinogens, a quiet, calming environment with reassurance that the bizarre thoughts, visions, and sounds are due to the drug and will go away soon usually suffices. Anxiolytics (eg, lorazepam, diazepam) may help reduce severe anxiety.

Persistent psychotic states or other mental disorders require appropriate psychiatric care. Flashbacks that are transient or not unduly distressing to the patient require no special treatment. However, flashbacks associated with anxiety and depression may require anxiolytics as for acute adverse reactions.

More Information

  • Listing of licensed US providers of treatment for substance use disorders

Click here for Patient Education
NOTE: This is the Professional Version. CONSUMERS: Click here for the Consumer Version
quiz link

Test your knowledge

Take a Quiz!