Tomorrow, a Food and Drug Administration advisory committee will meet to discuss whether the United States should approve its first psychedelic drug. The fate of the treatment—MDMA-assisted therapy for post-traumatic stress disorder—will turn on how the FDA interprets data from two clinical trials that, on their face, are promising. Long-suffering patients who took the drug while undergoing intensive talk therapy were about twice as likely to recover from PTSD as patients who got the placebo with therapy.

If the treatment is approved this summer, it could bring relief to some of the approximately 13 million Americans with PTSD. It could also serve as a model for other psychedelics to meet the FDA’s regulatory bar. But there’s a conundrum at the core of these two clinical trials, one that has plagued virtually all efforts to study psychedelics.

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In clinical trials, participants (and the researchers studying them) generally aren’t supposed to know whether they’re getting the actual drug or a placebo, to avoid allowing people’s expectations about a treatment to shape their response to it. Blinding, as this practice is called, is a key component of a randomized controlled clinical trial, or RCT—medicine’s gold standard for demonstrating that a drug actually works. But virtually no one can take a psychedelic drug and not know it.