For years now, experiments with LSD and other psychedelic drugs have suggested that 'microdoses' of the substances can offer a range of psychological benefits to people, with the potential to help treat depression and other mental health conditions.

The notion that these long-controversial drugs might actually improve people's mental wellbeing, cognition, and creativity was hailed as an exciting new paradigm in medical research – but according to a new study, the supposed benefits of these substances may not be what they seem.

In what's being described as the largest placebo-controlled trial on psychedelics to date, researchers found that the positive psychological effects linked with psychedelic microdosing may just be a manifestation of the placebo effect.

The placebo effect is a strange phenomenon where people appear to experience a medical benefit even when they've only taken a placebo, such as a sugar pill that contains no active medical substance.

While the mechanisms that empower the placebo effect remain debated, researchers suggest the phenomenon is tied to people's expectations: if people believe they might be affected by something, that belief in itself can trigger varying physiological effects that may alter their experience.

In the new LSD study, the same phenomenon could well have been taking place.

Researchers from Imperial College London recruited 191 volunteers: people who were already experienced psychedelics microdosers.

In an online 'self-blinded' experiment – in which the participants did not know what was inside the capsules they were taking – half the group ingested LSD microdoses, and the other half acted as controls, taking capsules that looked the same but were in fact placebos.

Over the course of four weeks, the participants took their regimen of mystery capsules (either LSD or placebo), while filling out surveys on how they were feeling, and performing cognitive tests online.

The results ultimately showed that those taking LSD microdoses felt better after taking their pills, significant improvements in psychological measures of well-being, mindfulness, life satisfaction, and paranoia.

However, the same benefits were seen in the people taking the placebo pills, with no significant differences evident between the two groups.

"Our results are mixed: on the one hand, we observed microdosing's benefits in a wide range of psychological measures; on the other hand, equal benefits were seen among participants taking placebos," says first author of the study Balázs Sziget, a research associate with Imperial College London's Centre for Psychedelic Research.

"These findings suggest that the benefits are not due to the drug, but rather due to the placebo-like expectation effects."

"Many participants who reported that they experienced positive effects while taking the placebo were shocked to learn after the study that they hadn't been taking the real drug."

It's not the first time that researchers have probed the strange nexus between psychedelics and placebos, but the researchers say theirs is the first to conduct a placebo-controlled investigation of the accumulative effects of repeated microdosing.

Based on the results, the experiment seems to confirm the commonly reported anecdotal reports that the act of microdosing LSD confers positive psychological benefits – only it suggests those improvements "are not due to the pharmacological action of microdosing, but are rather explained by the placebo effect".

That said, minor differences in some psychological measures were evident between the two groups, although the researchers say the effect sizes were small, with debatable clinical and practical value.

"In summary," the authors conclude, "these results strongly suggest that the actual content of capsules did not determine differences between the conditions, but beliefs about their content did." (Original emphasis.)

The researchers emphasise that their self-blinding experiment – in which the participants mixed up their own capsules at home to take part in the trial – comes with certain limitations, acknowledging that the results are less rigorous than data from a conventional clinical trial.

But there's no denying the potential impact of these findings, which show that we can't discount the possibility that the placebo effect may influence the results of contemporary psychedelics research – and in a way that is quite mind-blowing.

"An empty pill with strong belief/intentions makes nearly everything," one astonished participant, who took only placebos in the trial, told the researchers.

"You put spirituality into an empty pill here…wow!"

The findings are reported in eLife.