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Up to 9% of LSD and psilocybin users report reoccurring effects after drug has worn off

New research published in the journal Psychopharmacology explored the occurrence of flashback phenomena — drug-like effects that spontaneously recur following hallucinogen exposure. The results from six placebo-controlled studies revealed that flashback phenomena occurred for up to 9.2% of participants after LSD or psilocybin exposure.

In recent years, psychedelic drugs like LSD and psilocybin have garnered increased attention for their potential therapeutic effects. These drugs are considered relatively safe and non-addictive. But one notable side effect that has received limited attention is the occurrence of drug-like experiences after the initial effects of the drugs have worn off.

These recurring drug-like effects are called flashbacks, and symptoms include vision changes, mood changes, and derealization/depersonalization. If these flashbacks persist and cause significant distress or impairment, they might be referred to as hallucinogen-persisting perception disorder (HPPD), a condition named by the Diagnostic and Statistical Manual of Mental Disorders (DSM-V).

Study author Felix Müller and his team say that the scientific understanding of flashbacks is limited, and existing data has relied on case reports and naturalistic studies. The researchers sought to better describe flashback phenomena and HPPD by analyzing data pulled from multiple clinical trials.

The researchers collected data from six double-blinded, placebo-controlled studies that included a total of 142 participants between the ages of 25 and 65. During the trials, 90 participants received LSD, 24 received psilocybin, and 28 received both drugs. Doses varied by trial, with participants receiving between 1–5 doses of LSD that ranged from 0.025 and 0.2 mg, and/or between 1–2 doses of psilocybin that ranged from 15 and 30 mg.

Throughout the study sessions, subjects were asked to report any adverse experiences, including the occurrence of flashbacks. After the final study session, an end-of-study visit took place where subjects were again asked to describe any flashback phenomena that had occurred over the course of the study.

On the final study visit, 13 participants (9.2%) described some type of flashback experience. Of these cases, seven had occurred after taking LSD, two after psilocybin, and four after taking both substances. For 11 of the 13 participants, these flashbacks consisted of visual alterations. Three participants experienced other phenomena along with these visual effects, such as auditory or cognitive symptoms, and two subjects experienced emotional alterations only. All flashback experiences lasted from seconds to minutes, except in one case when the flashbacks lasted for hours.

The majority of participants (76.9%) reported that these flashbacks were neutral or positive experiences. Two subjects deemed them unpleasant, one of whom described a single distressing episode occurring 17 days after taking 25 mg of psilocybin.

Notably, this participant was also on day two of taking a serotonin reuptake inhibitor, which has been found to induce flashbacks in hallucinogen users. The other participant who reported unpleasant flashbacks said the experiences occurred across four days after taking 0.2 mg of LSD. In both cases, the flashback episodes did not impair participants’ daily lives and resolved spontaneously.

Müller and his colleagues also conducted a follow-up study after the final study visit. Since the studies were completed at different times, the time from the last study visit to the follow-up varied from months to years. Only one participant described the occurrence of further flashbacks, which consisted of visual alterations that occurred about 30 times in seven months. These flashbacks lasted seconds long and did not impair daily life. The researchers determined that none of the participants met criteria for HPPD at any time point, although they note that HPPD is rare and that their sample size was small.

In general, these findings suggest that flashback experiences are relatively common in LSD and psilocybin trials, with about 9% of participants reporting such effects. However, only 1.4% of participants experienced distressing flashbacks, and no treatment was needed in these cases. “Overall,” the authors conclude, “our data suggests that flashbacks are not a clinically relevant problem in controlled studies with healthy participants.”

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