Marijuana is one of the most studied substances in scientific literature, but the answers are scattered across tens of thousands of scientific papers. The quality of evidence these papers offer can vary wildly and the results sometimes contradict each other.

Fortunately, the National Academies of Sciences, Medicine and Engineering have brought a great deal of clarity to the situation with an encyclopedic report summarizing pretty much everything researchers know (and don't know) about the health effects of marijuana use.

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For the 395-page report, a team of dozens of drug policy experts at some of the nation's most prestigious universities analyzed 24,000 scientific papers to arrive at more than 100 conclusions regarding the effects of marijuana use. In addition, they also published a number of recommendations for policymakers and researchers going forward. The work was sponsored by a number of public and private health agencies, including the Centers for Disease Control and Prevention, the Food and Drug Administration and the National Institute on Drug Abuse.

Here's what they found:

Therapeutic effects

Can it treat chronic pain? The committee found strong evidence showing marijuana is effective at treating chronic pain in adults. Given the current public health crisis involving tens of thousands of deaths annually because of painkiller overdoses, this is a potentially significant finding. Numerous studies have found that medical marijuana availability may decrease rates of opioid dependence and overdoses.

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How about nausea and vomiting? The report also turned up strong evidence that marijuana is effective at treating nausea and vomiting in cancer patients, and muscle spasticity in multiple sclerosis patients.

What about glaucoma or epilepsy? The research done to date doesn't provide solid evidence that marijuana is useful for a number of conditions for which it's often used — particularly glaucoma and epilepsy.

Physical health risks

Does marijuana cause cancer? The report found little to no evidence linking marijuana use to any number of cancers, including a number of cancers often associated with cigarette smoking. In the case of lung, head and neck cancers, there is moderate evidence of no association between marijuana use and those cancers whatsoever. For a number of other cancers, there may be an association with marijuana use, but the research to date has not demonstrated that.

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Does marijuana use cause heart attacks or lung issues? There is some limited evidence linking marijuana use to acute heart attack risk and strokes. And there is substantial evidence linking long-term marijuana use to chronic bronchitis. But evidence on links between marijuana use and other cardiopulmonary issues is either scant or nonexistent.

How about driving while stoned? There's strong evidence linking marijuana use with increased risk of motor vehicle accidents. Most research indicates that the marijuana-induced risk of motor vehicle accidents is significantly less than the risk associated with alcohol.

What about kids who get exposed to weed? There's also moderate evidence linking legal marijuana availability to increases in pediatric marijuana exposure — those calls to poison control centers involving children eating pot brownies that often pop up in the news.

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Is there any risk of fatal marijuana overdose? There remains no evidence linking marijuana use to overdose deaths of any kind. Nor is there any good evidence linking marijuana use to workplace injuries or general mortality.

Should pregnant mothers use marijuana? There's substantial evidence linking maternal marijuana use to low birth weight of children. There's also limited evidence linking maternal pot use to pregnancy complications for the mother and admission of the child to the neonatal intensive care unit.

Mental health risks

What kind of cognitive effects does marijuana have? There's moderate evidence that marijuana use impairs learning, memory and attention for a short while thereafter. The question for long-term users, in particular, of whether these impairments persist after an individual has stopped using is trickier to answer.

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Reviewing the literature, the committee found limited evidence that these impairments continue even after sustained abstinence from marijuana use. There's also limited evidence of links between long-term marijuana use and impaired academic achievement and job outcomes, although the authors stress that the myriad social and economic factors that also affect these outcomes make it difficult to draw firm conclusions.

Any other mental health connections? The report did find very clear evidence of a link between heavy marijuana use and the development of psychotic of schizophrenic symptoms. “The association between cannabis use and the development of a psychotic disorder is supported by data synthesized in several good-quality systematic reviews,” the authors write. “The magnitude of this association is moderate to large and appears to be dose-dependent" -- that is the more you smoke or the longer you smoke, the greater the risk.

There's also moderate evidence linking marijuana use with a small increased risk of depressive disorders and social anxiety disorders. Perhaps of most concern, there is also moderate evidence linking marijuana use with suicidal ideation.

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Drug dependency

Who is at most risk for becoming dependent on, or addicted to, marijuana? The research is generally in agreement that being male and smoking cigarettes are two significant risk factors “for the progression of cannabis use to problem cannabis use." (Men generally have higher rates of drug use and abuse than women.) Also, the younger you start smoking marijuana, the greater risk you have for eventually becoming dependent on it.

Does marijuana cause people to go on to harder drugs? The literature shows limited evidence that marijuana use is linked to the use of other substances — primarily tobacco. But interestingly, there appears to be better evidence that marijuana use is linked to becoming dependent on other substances. That is, marijuana may not be more likely to make you move on to alcohol or heroin. But if you do, there may be a greater chance you become dependent on those drugs.

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The report does not address the implications of these findings for current legalization debates. The researchers do, however, state emphatically that the current designation of marijuana as a Schedule 1 controlled substance — recently reaffirmed by the Obama administration — is one of the chief barriers to conducting more badly needed research that could provide more definitive answers to some of the questions above.

Opponents and supporters of marijuana legalization are commending the report for its rigor, although the two sides are drawing considerably different conclusions from the findings.

“This report completely vindicates the scientific community that has been saying for years marijuana is a serious health issue,” said Kevin Sabet of the anti-legalization group Smart Approaches to Marijuana. “We should be doing everything we can to stop the marijuana industry from continuing to deceive the public the same way Big Tobacco did for a century.”

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