A MARTÍNEZ, HOST:
One of the top reasons people say they use marijuana for medical purposes is for mental health. Now the largest scientific review yet concludes there is little to no high-quality data on whether it works. Here's NPR health correspondent Will Stone.
WILL STONE, BYLINE: Jack Wilson studies cannabis and mental health at the University of Sydney in Australia. So when he undertook this effort to round up all the data from cannabis trials over the past 40 years, he already had a sense of the evidence base.
JACK WILSON: But it was quite surprising just how weak the evidence was.
STONE: His review in the medical journal Lancet Psychiatry found no evidence that cannabis was effective for anxiety, post-traumatic stress disorder or depression. Some areas had more supporting data, like insomnia, autism and tic or Tourette syndrome. However, even that was deemed of low quality.
WILSON: We clearly need to do more research on cannabis medicines, but in the absence of evidence at this time, the routine use of medical cannabis products really should be rarely justified.
STONE: Doing well-controlled trials on cannabis is notoriously difficult. More than 50 years ago, the federal government placed it in a category reserved for drugs with no accepted medical use. Ryan Vandrey is at Johns Hopkins University School of Medicine and studies cannabis.
RYAN VANDREY: It's embarrassing how little we have done, given how widely this is available as a therapeutic.
STONE: Because the Lancet paper had strict criteria for which studies could be included, the final analysis only had data from close to 2,500 patients. There wasn't a single study on depression. Vandrey says this kind of review, although well done, has real limits.
VANDREY: When you're trying to collapse data across multiple studies that use different products, different doses and sometimes different routes of administration, it can be challenging to draw a firm conclusion.
STONE: He says if you consider other studies not included in the Lancet analysis, you see a subset of patients with anxiety, depression or PTSD who can benefit from cannabis.
VANDREY: What I can't say right now is that we know - if we see a given patient, we know exactly what product they should use, how they should use it and what dose.
STONE: The Lancet paper comes on the heels of another review. That had a similar conclusion on the lack of evidence of cannabis and mental health, and it warns of substantial risks, especially for some with mental illness like schizophrenia or bipolar disorder. Dr. Devan Kansagara, who led that study, is at Oregon Health and Science University and the Portland VA.
DEVAN KANSAGARA: Part of what we wanted to do is convey, you know, cannabis is not one thing because it's such a complex substance, and then the dosing part of it is so variable.
STONE: He says most concerning are products with high levels of THC. That has a very different effect on, say, anxiety than CBD, another compound which has shown promise. Kansagara says doctors need to talk with their patients who feel cannabis is helping them and give them the best advice possible with what evidence there is.
Will Stone, NPR News.
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