HPPD, or Hallucinogen Persisting Perception Disorder, is essentially the transient recurrence of disturbances in perception experienced by people who have been under the influence of a hallucinogenic drug. It is the medical term for the more commonly used name, flashback.

The disturbances commonly include visual hallucinations such as geometric shapes, peripheral-field images, flashes or intensification of colors, "trails" (images that remain in the path of a moving object) and halos. The symptoms can also include a changed mind set, appearing to be similar to the effect of the drug when originally ingested. While the effects can come suddenly and unexpectedly, some people find them pleasant, while others find them more disturbing and frightening (possibly because their unknown origin).

Symptoms may abate after a few months, but people have reportedly experienced them after as much as 5 years.

The origin of these symptoms is a bit of a mystery. Many tend to consider the disorder psychological in nature, as no traces of the psychedelic drugs can be found in the victim's body. Another possible explanation could be alternations of neurochemical processes in the brain, although the latter seems unlikely.

These effects have been reported after use of most of the psychedelics (e.g. LSD, MDMA / Ecstasy, Mescaline, DMT et al.), but LSD seems to be the most common culprit.