Participants were randomly assigned to receive either gradually increasing doses or gradually decreasing doses, to test a hypothesis that it might be more effective to start with a high initial dose.

Neither the study participants nor researchers knew what group the participants were in or what dose of the drug they were going to get when they showed up for their sessions.

Sessions were conducted in a lab furnished to look like a living room. Study volunteers were encouraged to wear an eye mask to control visual stimulation and to lie down on a couch. They listened to music through headphones. They were encouraged to inwardly focus their attention. Two trained monitors stayed in the room throughout the test session, which lasted about eight hours.

About 40% of study participants, or seven out of 18, reported feeling extreme anxiety and fear while they were on the two highest doses of the drug. Six of the seven, however, experienced the fear while on the highest dose of the drug. Only one person reported negative fear effects on the 20 mg dose.

Examples of the delusions experienced by study participants included the belief that a child or loved one had died while the session was ongoing or that that the monitors were being cruel or manipulative.

Some experts say those negative feelings shouldn’t necessarily be avoided, especially if people are trying to work through addictions or end-of-life issues.

“When you work with people that have emotional issues, they are going to have difficult experiences,” says Rick Doblin, PhD, director of the Multidisciplinary Association for Psychedelic Studies in Santa Cruz, Calf. “Those are emotions that we anticipate, and to a certain degree, people need to work through them.”

Indeed, many people who went through so-called “bad trips” also reported that the feelings were eventually replaced by more positive thoughts during the same session, and none reported that the fear or anxiety they experienced caused any long-term harm.

In contrast, nearly three out of four people on the highest psilocybin doses rated their experiences as mystical, transformative, and highly beneficial.