Leary was a clinical psychologist and professor at Harvard. After taking mushrooms in 1960, he “declared that he learned more in the following five hours than he had done in 15 years of study and research in psychology,” wrote psychologist and consciousness researcher Sue Blackmore in The Guardian .

From 1960 to 1962, the Harvard Psilocybin Project conducted unconventional experiments, like giving psilocybin to prison inmates to see if it would reduce recidivism, or doling it out to theology students to provoke a religious experience. Led by Timothy Leary and Richard Alpert, their goal was to test the potential applications of the active ingredient in magic mushrooms.

But a sticky question remains: how much first-hand knowledge should a researcher have with hallucinogens? Leary's scientific rigor was doubted because of his personal experiences (and exuberance) with these drugs. Unlike other medications, these compounds alter the mind and consciousness. Is it possible that they create a kind of positive bias, or are the insights from taking them important for scientists to experience?

Today, the study of psychedelics and their potential application in medicine and research is undergoing a revival and reclaiming its legitimacy. Psychedelics are promising treatment options for depression and other mental health disorders , and tools for scientists to probe the inner workings of the brain and perception .

Researchers like Leary were, perhaps, irresponsible with their drug use and promotion. But they were also used as scapegoats to illustrate the potential danger of these medications, said Dominic Sisti, an associate professor at the Department of Medical Ethics & Health Policy at Perelman School of Medicine at the University of Pennsylvania. In 1966 , the federal government banned the manufacturing and possession of hallucinogens.

Leary began to regularly consume hallucinogenic drugs, leading to his dismissal from Harvard and a questioning of the validity of his research. His reputation morphed from respected professor to drug-addled hippie—he allegedly gave LSD to hundreds of students and stopped showing up to the classes he was teaching. Leary's name became associated with the counterculture movement he advised to “turn on, tune in, and drop out," and the drugs he once studied became linked to that notoriety as well. (He eventually went to prison, only to escape and run away abroad, until he was arrested in Afghanistan.)

Luke thinks that some of the early studies in psychedelics, where people had traumatic experiences , were a result of a lack of insight from the clinicians. He told me his own psychedelic experiences have been useful for guiding research questions.

David Luke, a psychologist at the University Of Greenwich, said that if you are a psychologist who wants to understand the psychedelic experience, there is something to learn from taking them yourself. "It really depends on your field," he said. "If you are a biochemist working in translational medicine with rats, then maybe you don't need to take psychedelic particularly. But if you a psychologist wanting to understand the psychedelic experience then, there's something perhaps to be gained."

I talked to about a dozen psychedelic researchers, and found that opinions about first-hand drug experiences varied widely. One of the common effects of psychedelic drugs is that they create meaningful and transcendent experiences, and some felt that was a source of positive bias that could influence research. Others thought the opposite: that clinicians who prescribe these drugs should intimately know what the experience is like in order to properly design their studies and provide informed consent.

In this next wave of psychedelic research, should the researchers be taking these drugs themselves? And if they are, will we allow them to be honest about it?

These aren’t necessarily negative effects, but they don’t have much to do with smoking. Testing these drugs as medication goes beyond just their physical effects, and informed consent should include all potential outcomes of a trial. “Many of these drugs do have a profound impact on one’s identity,” Sisti said. “And to be able to really articulate that might mean trying it.”

Sisti, at UPenn, said that informed consent might only be possible if researchers really know what the experience of psychedelic drugs is like. For example, last year a study conducted at Johns Hopkins gave people psilocybin to help them quit smoking. By the end of the study, quitting smoking was reported as “one of the least important effects of the study.” Instead, the participants said they had “long-term changes in thoughts, affects, deciders or behaviors,” and “increased aesthetic appreciation, and heightened altruism and pro-social attitudes.”

“You need to be able to negotiate the landscapes of the drugs as a guide for your patient,” he said. “You cannot do this effectively—or be able to reassure the patient—if you have not also been there yourself under controlled clinical conditions.”

One medical doctor and psychedelic researcher in the UK, who asked not to be named, told me he’d tried LSD, DMT, MDMA, ketamine, and psilocybin—all legally—in approved research studies. He has also administered most of these drugs legally to people in medical and research contexts.

Thomas Metzinger, a professor of theoretical philosophy at the Johannes Gutenberg University of Mainz, called this an illusion of insight or profundity. When someone takes psychedelic drugs, they often feel like they’ve seen or experienced something immensely relevant. “Some of these people have become researchers, just for that reason,” Metzinger said. “But just having the feeling of knowing doesn't justify that that is knowing.”

Doss expressed concern that because these drugs create such meaningful experiences, some might be over-interpreting or misinterpreting those feelings. “What's worse,” Doss added, “is that the experience these drugs produce seems to give the impression that one is gaining insight into the architecture of the brain/mind, perhaps leading to nearly every psychedelic researcher claiming that these drugs will teach us something about consciousness or cognition more broadly. Instead, what all the research has taught us is about the effects of drugs on the mind, not some underlying principle we did not already know about the mind.”

Manoj Doss, a postdoctoral research fellow Johns Hopkins University studying the cognitive, emotional, and neural mechanisms of psychedelic drugs, told me that he isn't convinced taking the drugs is necessary, and that he thinks subjective experiences have biased work in this field.

“Someone in the team has to know something about the impact of mind-altering substances in order to be able to understand the profound changes one can go through," Wittmann said.

Marc Wittmann, a neuropsychologist at the Institute for Frontier Areas of Psychology and Mental Health in Germany, agreed that researchers should know what an experience is like if they intend to study it. He doesn’t think a team that hasn’t taken these drugs can't do good research, but he thinks there's power in knowing what altered states of consciousness are like. He said this can also be achieved without drugs through meditation or floating tanks.

Sisti said that the use of randomized controls, placebo groups, and Institutional Review Boards (IRBs) eased his mind. “We didn't have that in the 50s and 60s," he said. "So, we've evolved, both technologically, clinically, and ethically, and now we're in a place where this research is being done in a way that's ethically and scientifically legitimate."

Others I spoke to echoed that despite their personal views, the way science is practiced has improved since the 1950s and 1960s, so they're not worried. “If you can conduct methodologically sound and well-controlled research, it shouldn't make any difference whether or not you had taken psychedelic in the past or not," Luke said.

It's important to remember that bias exists in all science, not just psychedelic fields, Metzinger said. Is it a concern? Yes. But he trusts the scientific method to filter it out. “I see many possibilities where problems can creep in," he said. "For instance, in the selection of subjects, or in the way they treat subjects when they come across as somebody already experienced and the subjects try to please the experimenter. All these things have to be neutralized—but there are methods to do so.”

Rick Strassman, a clinical associate professor of psychiatry at the University of New Mexico School of Medicine, known for his research on DMT, said that while he thinks there are pros and cons to taking psychedelics, some people can become "messianic"— "seriously blurring their objectivity, perspective, and common sense," he said. "They become activists and zealots rather than impartial researchers. While it is fine to have deeply held convictions about what these drugs do, it is very important to not let those beliefs bleed into either the research design, execution, or interpretation."

Several scientists told me that they had taken psychedelic drugs, but would not say so on the record since they are still illegal. Doss said that there has been a precedent set by most drug researchers not to disclose their drug use. Some fear that it would create a negative perception that would ultimately affect how much funding they receive.

“Criminalization has brought a degree of paranoia to the narrative about personal use,” said James Rucker, a clinician scientist at King's College London. “This is unfortunate. It means that, yes, I feel a certain degree of personal and professional anxiety in talking about it.”

Do people in the field know where their peers stand? “I would say researchers have a sense of how much others researchers have tripped,” Luke said. “I think it's fairly known you know, between the lines. Obviously, it's not the kind of thing that's reported in the research papers but people get a sense of it.” Luke suggested that one day, perhaps a person's personal experience with a drug could be disclosed on a paper, similar to the way researchers declare financial conflicts of interest.