Psychedelics, such as LSD and magic mushrooms, do not increase risk of developing mental health problems, according to the new study.
Previously, the researchers behind the study - from the Norwegian University of Science and Technology in Trondheim - had conducted a population study investigating associations between mental health and psychedelic use. However, that study, which looked at data from 2001-04, was unable to find a link between use of these drugs and mental health problems.
"Over 30 million US adults have tried psychedelics and there just is not much evidence of health problems," says author and clinical psychologist Pål-Ørjan Johansen.
"Drug experts consistently rank LSD and psilocybin mushrooms as much less harmful to the individual user and to society compared to alcohol and other controlled substances," concurs co-author and neuroscientist Teri Krebs.
For their study, they analyzed a data set from the US National Health Survey (2008-2011) consisting of 135,095 randomly selected adults from the US, including 19,299 users of psychedelic drugs.
Krebs and Johansen report that they found no evidence for a link between use of psychedelic drugs and psychological distress, depression, anxiety or suicidal thoughts, plans and attempts.
In fact, on a number of factors, the study found a correlation between use of psychedelic drugs and decreased risk for mental health problems.
"Many people report deeply meaningful experiences and lasting beneficial effects from using psychedelics," says Krebs.
Study cannot exclude instances of adverse effects in individuals
However, Johansen acknowledges that - given the design of the study - the researchers cannot "exclude the possibility that use of psychedelics might have a negative effect on mental health for some individuals or groups, perhaps counterbalanced at a population level by a positive effect on mental health in others."
Despite this, Johansen believes that the findings of the study are robust enough to draw the conclusion that prohibition of psychedelic drugs cannot be justified as a public health measure.
"Concerns have been raised that the ban on use of psychedelics is a violation of the human rights to belief and spiritual practice, full development of the personality, and free-time and play."
Commenting on the research in a piece for the journal Nature, Charles Grob, a paediatric psychiatrist at the University of California-Los Angeles, says the study "assures us that there were not widespread 'acid casualties' in the 1960s." However, he urges caution when interpreting the results, as individual cases of adverse effects can and do occur as a consequence of psychedelic use.
For instance, Grob describes hallucinogen persisting perception disorder, sometimes referred to as "a never-ending trip." Patients with this disorder experience "incessant distortions" in their vision, such as shimmering lights and colored dots. "I've seen a number of people with these symptoms following a psychedelic experience, and it can be a very serious condition," says Grob.
In 2012, Medical News Today reported on previous research by Krebs and Johansen, which investigated LSD as a treatment for alcoholism.