Merck Manual

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Gerald F. O’Malley

, DO, Grand Strand Regional Medical Center;

Rika O’Malley

, MD, Albert Einstein Medical Center

Last full review/revision Jun 2020| Content last modified Jun 2020
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NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
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Hallucinogens are a class of drugs that cause profound distortions in people's perceptions.

  • Hallucinogens distort and intensify sensations, but the actual effects are variable and highly unpredictable.

  • The chief dangers are the psychologic effects and impaired judgment they cause. Most people are aware they are hallucinating and can be reasoned with.

  • Diagnosis is made based on a doctor's evaluation.

  • A dark, quiet room and calm, nonthreatening talk can help users who are intoxicated. Sometimes, antianxiety drugs and psychiatric care are necessary.

Hallucinogens may be obtained from plants or be man-made chemicals (synthetic). Some typical examples include

  • LSD, or lysergic acid diethylamide (from a fungus)

  • Psilocybin (from several types of mushrooms)

  • Mescaline (from the peyote cactus)

  • N,N-dimethyltryptamine (DMT)

  • 5-methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT)

Many new compounds are being synthesized, and the list of hallucinogens is growing.

These drugs may be taken various ways. LSD is taken by mouth using tablets or blotter paper. Psilocybin and mescaline are taken by mouth. DMT can be smoked.

People may become psychologically dependent on hallucinogens, but physical dependence is not typical.

(See also Drug Use and Abuse.)


Hallucinogens cause immediate and sometimes long-term symptoms.

Immediate effects

Physical effects of hallucinogen use often include nausea and vomiting. LSD can also cause enlarged pupils, blurred vision, sweating, palpitations, and impaired coordination.

Hallucinogens distort and intensify auditory and visual sensations. For example, people may feel as if they are seeing sounds and hearing colors (called synesthesias). People feel as if they are not real (called depersonalization) or are detached from their environment (called dissociation). People experience changes in moods (most often euphoria but sometimes depression), and their judgment becomes impaired. Users refer to the combination of these effects as "a trip."

The actual effect can depend on the user’s mood and expectations of the "trip," ability to cope with hallucinations, and the setting in which the drug is taken. Visual hallucinations are more common with the use of psilocybin and mescaline than with LSD. For example, users who were depressed before the drug was taken are likely to feel sadder when the drug takes effect. The chief dangers of using these drugs are the psychologic effects and impaired judgment they cause, which can lead to dangerous decisions or accidents. For example, users may think they can fly and may even jump out a window to prove it.

The user’s ability to cope with the visual and auditory distortions also affects the experience—or “trip.” Inexperienced, frightened users are less able to cope than someone who is more experienced and not afraid of the trip. Users under the influence of a hallucinogen, usually LSD, can become extremely anxious and begin to panic, resulting in a bad trip. They may want to stop the trip, which is not possible.


High doses (overdose, greater than 0.5 mg of LSD) increase blood pressure and heart rate. Some people can develop extremely high body temperature (hyperthermia) because the drug impairs the ability to control body temperature. Hyperthermia can cause high fever, nausea and vomiting.

Overdose is rare but can sometimes result in death. Lethal doses of LSD are estimated to be 10 mg, or approximately 200 units, of street blotter. Most cases of hallucinogen death are attributed to the synthetic hallucinogens 25I-NBOMe and 25C-NBOMe.

Long-term effects

Some users remain out of touch with reality (psychotic) for many days or longer after the drug’s effects have worn off. It is unclear if the drug use causes psychosis or simply uncovers an underlying mental health disorder.


Some people—especially long-term or repeated users of hallucinogens, particularly LSD—may experience flashbacks after they stop using the drugs. Flashbacks are similar to but generally less intense than the original experience. Typically, flashbacks disappear over 6 to 12 months but can recur for years after the last use of LSD, especially in users who have an anxiety disorder or another mental health disorder or after the use of marijuana, alcohol, or barbiturates.


  • A doctor's evaluation

Doctors usually base the diagnosis on symptoms in people known to have used a hallucinogen. Tests are not available to confirm the use of many of these drugs.


  • Calming environment

  • Sometimes antianxiety drugs such as benzodiazepines

  • Sometimes psychiatric care

Most users do not seek treatment for the effects of hallucinogens. A quiet, dark room and calm, nonthreatening talk can help users who are having a bad trip. They need reassurance that the effects are caused by the drug and will end. If anxiety is severe, benzodiazepines (sedatives), such as lorazepam, may help. People who experience a prolonged psychosis may need mental health treatment.

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Drugs Mentioned In This Article

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