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Marijuana may have limited medical value. That's okay.

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Medical marijuana may help alleviate muscle stiffness from multiple sclerosis and chronic pain, but there's no good evidence for its ability to treat other conditions, according to the largest review of the research on pot yet.

The study, published in the Journal of the American Medical Association, disputes many of the claims of medicinal pot advocates, who frequently characterize marijuana as an untapped wonder drug that could help relieve many medical conditions that currently don't have good treatment options. The Associated Press's Lindsey Tanner reported that several states and Washington, DC, allow medical marijuana for the treatment of Alzheimer's disease, epilepsy, glaucoma, kidney disease, lupus, and Parkinson's disease — but the JAMA review's findings show there's not much evidence that pot works against these conditions.

But the study only shows that there's currently no evidence for marijuana's ability to treat these conditions, not that pot can't help treat them. And even if marijuana is only good for chronic pain and muscle stiffness in multiple sclerosis, that could be a promising finding — not least because it could provide an alternative to opioid-based prescription painkillers that are killing thousands of Americans each year.

The study found limited evidence of effectiveness for conditions besides chronic pain and muscle stiffness

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The JAMA review evaluated 79 studies that tested marijuana's medicinal effectiveness among nearly 6,500 patients. It concluded that there's "moderate-quality evidence" for medical marijuana treating chronic pain and muscle stiffness among multiple sclerosis patients, and "low-quality evidence" for pot improving nausea and vomiting due to chemotherapy, weight gain in HIV infection, sleep disorders, and Tourette syndrome. And weed was linked to short-term adverse effects such as dizziness, dry mouth, nausea, fatigue, drowsiness, and confusion.

The evidence, then, only really supports pot's use for chronic pain and muscle stiffness, not the wide range of conditions typically covered by the 23 states (and Washington, DC) that allow medical marijuana. And a surprising amount of research on marijuana is still very bad, according to the JAMA review.

But part of the reason there might be a lack of good research and evidence for medical marijuana is because the federal government has made it so difficult to study the drug through strict regulatory barriers on pot research. Earlier this week, the feds took a big step to make research easier to conduct, but that's only after decades of strict regulations by the Department of Health and Human Services, the Food and Drug Administration, and the Drug Enforcement Administration. So it's possible that the evidence is lacking simply because the federal government made it so hard to conduct better research.

Still, the review shows a lot of states allow medical marijuana to treat conditions that aren't supported by the medical research.

In an accompanying editorial, Yale School of Medicine professors Deepak Cyril D'Souza and Mohini Ranganathan called for better standards and studies as states move forward with medical marijuana laws. "Evidence justifying marijuana use for various medical conditions will require the conduct of adequately powered, double-blind, randomized, placebo/active controlled clinical trials to test its short- and long-term efficacy and safety," they wrote. "The federal government and states should support medical marijuana research."

But finding an alternative to prescription painkillers would be great

The JAMA review isn't all bad news for medical marijuana advocates. The findings that pot can help treat chronic pain in particular validate one of the major cases for the drug: it might be able to, at least in some cases, substitute much deadlier, more addictive opioid-based prescription painkillers.

Since the late 1990s, the number of people dying from opioid painkiller overdoses has steadily risen — with more than 16,000 deaths reported in 2013. And one study in JAMA Psychiatry found opioid painkiller use has contributed to the rising use of heroin, another opioid, which is even deadlier and more addictive than painkillers.

One of the reasons medical marijuana may be promising is it could act as a substitute to painkillers. And the JAMA review's findings might put pot ahead of opioids for chronic pain — the research on prescription painkillers suggests they can treat short-term, acute pain, but there's no good evidence to support their use for chronic pain.

Other evidence already suggests medical marijuana can reduce prescription painkiller deaths. One study published in JAMA found that states that allow pot for medicinal purposes have fewer prescription painkiller deaths than one would otherwise expect. That suggests pot really might be substituting painkillers in some cases. But experts caution this field of research needs more study to see how much of the relationship between medical pot and prescription painkillers is causation and not just correlation.

So marijuana may not be the miracle drug some advocates say it is. But even if it's only good at treating some conditions, that could still make its use a fairly important medical breakthrough.