Reports of drug-induced flashbacks have existed since the 1950s. Though the term "flashback" wasn't used specifically until 1969, as early as 1954 scientists noticed LSD users complaining of a reoccurrence of an LSD-like state long after the drug's effects should have worn off.
The risk of a flashback has become a standard line in anti-drug messaging. Months or years after taking LSD, suddenly, the trip could come back without warning, and suddenly you would be floridly hallucinating again. An urban legend holds that molecules of LSD stay in the body, hiding in fat or in the spine, and can later be re-released.
There are plenty of myths floating around about LSD use, though. So are flashbacks even real, or just another example of public health messaging exaggerating the risks of illicit drugs?
A recent study published in PLOS ONE by Norwegian University of Science and Technology neuroscientist Teri Krebs found no association between using psychedelics, including using LSD in the past year, and seeing things other people don't. But psychiatrists who work with psychedelic users say that the phenomenon, though very rare, is also very real.
"I don't think there's any question that HPPD is a genuine clinical phenomenon."In 1986, the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders included diagnostic criteria for what later became known as Hallucinogen Persisting Perception Disorder (HPPD), the clinical term for seeing weird, trippy things like geometric hallucinations, flashes of color, afterimages and false perceptions of movement after taking psychedelics. To be considered a candidate for HPPD, the type of visual phenomena that occurs during an acid trip has to spontaneously reappear "long after the use of hallucinogens has stopped," cause significant distress, and not be explainable by any other mental disorder or medical condition. For many, it's less of a sudden "flashback" and more of a continuous disturbance of vision.
"I don't think there's any question that HPPD is a genuine clinical phenomenon," says Charles Grob, a professor of psychiatry at UCLA's medical school who does research with hallucinogens.
However, how many people are truly affected remains unknown. In a 2002 review of the scientific literature on HPPD over the last 50 years, psychiatrist John Halpern and his co-authors found most studies provided too little information to estimate the prevalence "even crudely" of HPPD in the population. "It is often unclear whether symptoms occurred exclusively following hallucinogen intoxication," they write. "It is also difficult to rule out other medical or psychiatric conditions that might cause 'flashbacks', including current intoxication with another drug, neurological conditions, current psychotic or affective disorders, malingering, hypochondriasis, or even other anxiety disorders such as posttraumatic stress disorder (PTSD)."
One study found that 60 percent of 2,455 hallucinogen users who took a self-reported survey on the drug information site erowid.org reported experiencing some kind of visual phenomena, similar to those induced by hallucinogens, when they had been drug-free for at least three days. A lesser number, 4 percent of respondents, found this significantly distressing. The study found LSD to be the most statistically robust predicator of unusual visual experiences, compared to other drugs the survey asked about, like pot, mushrooms and salvia ( via popsci.com ).