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. Those with persistent recurring flashbacks causing significant distress or impairment may have hallucinogen persisting perception disorder (HPPD), which is considered extremely rare.

New research published in the journal Psychopharmacology further explores psychedelic flashbacks, with results from six placebo-controlled studies revealing that the phenomena occurred for up to 9.2% of participants after LSD or psilocybin exposure.

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.

Researchers used data from six double-blinded, placebo-controlled studies with a total of 142 participants aged 25 to 65. In total, 90 participants received LSD, 24 received psilocybin and 28 received both substances. The doses varied depending on the trial; participants received one to five LSD doses ranging from 0.025 and 0.2mg, and/or between one to two doses of psilocybin ranging from 15 to 30mg.

Subjects were asked at each study session to report any adverse events since their last contact with the study team, and any event, including flashbacks, was recorded. All studies also included an end-of-study visit after the last study session, where all subjects were asked for the occurrence of flashback phenomena throughout the entire course of the study. Those who reported flashbacks were asked to describe the phenomenon, specifically the quality, quantity, impairment level and time of occurrence.

Those who reported flashbacks at the end of the study were then conducted via email for follow-up, specifically to assess the occurrence of continued flashbacks of HPPD. Those who reported further flashbacks were again asked to describe them (with the terms of the end-of-study visit) along with any potential triggers to the flashbacks.

During the final study visit, 13 participants (9.2%) described a flashback experience; seven instances occurred after taking LSD, two after psilocybin and four after taking both substances. Most of the flashbacks were visual alterations (for 11 of the 13 participants), and three participants experienced other phenomena (such as auditory/cognitive effects or a feeling of disintegration). Two participants exclusively reported emotional alterations. 

Researchers also noted that flashbacks were limited to the week after the last drug administration in all but two cases.

For most subjects, the flashbacks lasted for seconds (69.2%) to minutes (23.1%), though one case (7.7%) reported alterations persisting for hours. The subject specified that this involved intensified perception of colors and slowed thinking the day following three study sessions. 

For most of these cases (53.8%), the phenomena only occurred once. In two cases (15.4%), symptoms persisted more than five times. One of these subjects reported around 20 visual flashbacks within a short period roughly 24 hours after drug administration. The other subject experienced approximately 30 visual flashbacks within a seven-month period after drug administration. This was the only patient who clearly reported flashbacks after the end-of-study visit. Though, researchers noted that flashbacks lasted just seconds, were experienced as benign and did not impair daily life in both cases. 

More than 50% of participants said the flashbacks occurred while relaxing or shortly before sleep (meaning 1.4% of all 142 subjects reported distressing flashback experiences). While two participants said the flashbacks were experienced as unpleasant, 10 cases said they were neutral or positive. The remaining case was not sufficiently documented.  In all, none of the subjects reported impairment in their daily life due to these symptoms.

Researchers also determined that none of the participants met the criteria for HPPD at any time point, though they noted the rarity of the disorder and the small sample size as factors.

“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.”

Could honing in on flashback symptoms hold greater potential in a therapeutic setting? 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.

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