Reexperiencing, after ceasing the use of a hallucinogen, one or more of the perceptual symptoms that had been part of the hallucinatory experience while using the drug. It also commonly occurs as a symptom of acute stress disorder or posttraumatic stress disorder (PTSD).

The unpredictable phenomenon of undergoing again the effects of LSD weeks or even months after the last use of the drug.

A repeated and very vivid memory of a traumatic event.

Unexpected return to a bad trip, even though the drug has not been used.

The return of imagery and/or hallucinations after the immediate effects of a traumatic or hallucinogenic experience.

Flashbacks are recurrences of certain aspects of a person’s drug use without the user having repeated the use of the drug. They are experienced by many LSD users.

The resurgence of a traumatic memory as a vivid recall of sounds, images, and sensations linked to the traumatic event. Individuals experiencing such a memory often perceive themselves as reliving the event itself.