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Study Finds up to 9% of Psilocybin, LSD Experiences Result in Flashbacks

Those who have used psilocybin or LSD may be familiar with the experience: The day following your trip, it’s back to reality, yet there are still glimmers of your previous day’s journey, a fleeting visual cue borrowing from the more intense hallucinogenic effects you just experienced.

These spontaneously recurring, drug-like effects following hallucinogenic exposure are referred to as flashback phenomena; symptoms can include vision changes, mood changes and derealization/depersonalization. People who experience persistent flashbacks that cause significant impairment or distress may be suffering from hallucinogen persisting percept disorder (HPPD). This is extremely rare.

Published new research in the journal PsychopharmacologyThe phenomenon of psychedelic flashbacks is further explored with six placebo-controlled trials revealing that it occurred in as high as 9.2% for participants exposed to LSD or Psilocybin.

The authors note that data and current knowledge on both flashbacks and HPPD is “very limited,” even though they are assumed to be among “the most relevant side effects of hallucinogenic drugs.” For the study, researchers analyzed data pulled from multiple clinical trials in order to better describe flashback phenomena and HPPD.

Data from six placebo-controlled, double-blinded studies were used by researchers to analyze data. The total number of participants was 142. There were 90 people who received LSD; 24 took psilocybin, and 28 each of both. Participants received LSD in varying doses depending on their trial. They were given one to five doses (0.025 to 0.2mg) and one to two doses (15 to 30mg) of psilocybin.

Each study session was filled with questions asking subjects to describe any negative events that occurred since they last spoke with the team. Flashbacks and flashbacks were also recorded. Each study included an end of-study visit, in which all subjects were asked to report any flashbacks that occurred during the entire study. All subjects who experienced flashbacks were required to provide details about the phenomena. This included the frequency, quality, level of impairment, and the date.

The study ended with flashbacks being reported by participants. They were followed up via email to see if they had continued flashbacks. The end-of-study visits were used to ask participants to provide details about flashbacks, including any triggers.

The final study visit revealed that 13 of the participants (or 9.2%) experienced flashbacks. Seven instances were after they had taken LSD; two occurred after they took psilocybin, and four happened after both. For 11 out of 13 participants, flashbacks consisted mainly of visual changes. However, three other participants reported experiences such as hearing/cognitive effects (or a feeling like disintegration). One participant only reported emotional alterations. 

Research also indicated that flashbacks are limited to one week following the administration of drugs in most cases, except for two.

The flashbacks last for most subjects for between seconds and minutes (69.2% to 23.1%), though one subject (7.7%) experienced persistent changes for several hours. This was described by the subject as an intensified sense of color and slower thinking after three sessions. 

The phenomenon occurred in most cases (53.8%), and it was only once. Two cases (14.4%) had symptoms that lasted more than five days. The first subject reported 20 visual flashbacks in a matter of 24 hours following drug administration. Within seven months of drug administration, the other subject suffered approximately 30 visual flashbacks. After the final study visit, this patient was the only one to report flashbacks. However, both patients reported flashbacks that lasted only seconds and didn’t affect their daily lives. 

The flashbacks were experienced by more than half of those who participated (or 1.4%) of all the 142 participants. Two participants reported flashbacks as being unpleasant. Ten others said that they were positive or neutral. Undocumented was the case of one other participant.  All in all, the symptoms did not cause any impairment to their day.

Researchers found that the HPPD criteria was not met by any of the participants at any given time. They also noted the rarity and small sample size of HPPD.

“Drug-like experiences after the administration of LSD and psilocybin seem to be a relatively common phenomenon in clinical trials with healthy participants,” the authors concluded, clarifying that those flashbacks that occurred were mostly benign and didn’t impair daily life. “Overall, our data suggests that flashbacks are not a clinically relevant problem in controlled studies with healthy participants.”

Is it possible to use flashback symptoms in therapeutic settings? While the study reveals new insights that could help to inform further research on the topic, especially as the West continues to embrace psychedelic medicine, there’s still a lot more to uncover.

The study, “Flashback phenomena after administration of LSD and psilocybin in controlled studies with healthy participants,” was authored by Felix Müller, Elias Kraus, Friederike Holze, Anna Becker, Laura Ley, Yasmin Schmid, Patrick Vizeli, Matthias E. Liechti and Stefan Borgwardt.