Marijuana stocks are sizzling hot as the new year begins. However, a study just released by the National Academies of Sciences, Engineering, and Medicine slams many of the purported benefits of marijuana. In one of the most comprehensive studies of the effects of marijuana ever, a heavy-hitting lineup of health and medical experts punched holes in several key arguments that marijuana proponents make. Could this study be disastrous for marijuana stocks?

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The good

The study did reach some positive conclusions about marijuana use. Perhaps most important for the growing number of companies that sell medical marijuana is that "conclusive or substantial" evidence was found for use of cannabis in treating chronic pain in adults.

It also wasn't surprising that the study concluded that use of oral cannabinoids is beneficial for treatment of chemotherapy-induced nausea and vomiting. In July 2016, the U.S. Food and Drug Administration (FDA) approved Insys Therapeutics' (NASDAQ:INSY) oral cannabinoid Syndros for this indication based on clinical study data. It would certainly have seemed odd if the study's findings conflicted with this FDA approval.

There was also good news about use of oral cannabinoids in improving patient-reported multiple sclerosis spasticity symptoms. The researchers stated that conclusive or substantial evidence supported this use of marijuana. In addition, the study found moderate evidence that cannabinoids could "improve short-term sleep outcomes in individuals with sleep disturbance associated with obstructive sleep apnea syndrome, fibromyalgia, chronic pain, and multiple sclerosis."

The study also found that there was moderate evidence of no statistical link between marijuana use and lung cancer and head and neck cancer. Only limited evidence was uncovered for a statistical link between use of marijuana and testicular tumors or for several cardiometabolic risk factors, including heart attack and stroke. 

The bad

However, there was plenty of bad news in the report. Only weak evidence was found by the research team supporting use of marijuana in a long list of indications. The study concluded that there was limited evidence that marijuana is effective for increasing appetite and decreasing weight loss associated with HIV/AIDS, for treating Tourette syndrome, for helping anxiety, and for treating post-traumatic stress disorder.

The study also appeared to diminish the importance of one of its positive conclusions about marijuana use. Although substantial evidence was found that oral cannabinoids improved patient-reported MS spasticity symptoms, only weak evidence was found that improvement occurred when measured by clinicians.

There was also limited evidence of a statistical link between use of marijuana and better outcomes after a traumatic brain injury or intracranial hemorrhage. The research team looked at patient outcomes including mortality and disability but didn't find much in support of use of marijuana. 

... And the ugly

For those who think that using marijuana leads to all kinds of problems, the recent study gives them plenty of ammunition to support their views. Substantial evidence was found for a statistical link between smoking marijuana and worse respiratory symptoms, as well as more frequent chronic bronchitis episodes.

Substantial evidence was found for a link between marijuana use and increased risk of motor vehicle crashes. The study also cited significant evidence for a statistical link between mothers who smoke marijuana during pregnancy and lower birth weights of their babies.

One of the most concerning conclusions of the study was that substantial evidence was found linking marijuana use to the development of schizophrenia or other psychoses. The most frequent users had the highest risk levels.

Marijuana stocks up in smoke?

What will the impact of this study be for marijuana stocks? For companies that have already won FDA approval, such as Insys Therapeutics, there won't be any negative effects.

The study's findings also won't have any bearing for other companies that use clinical study data to support regulatory approval for cannabinoid drugs. GW Pharmaceuticals (NASDAQ:GWPH) is a great example. The company has conducted its own clinical studies that show improvement in patients with rare forms of epilepsy after taking Epidiolex. Regulatory approval will depend entirely on GW Pharmaceuticals' evidence -- not findings from an external report.

The real question will be how this report influences government decision-makers and the public. It's possible, for example, that these conclusions could spur the federal government to more rigidly enforce laws prohibiting the sale and use of marijuana. Some states that don't allow use of marijuana might be less likely to change their laws. Such actions could have a trickle-down effect on the stocks of companies that depend on sales of medical marijuana that doesn't go through the FDA approval process.

For now, though, all we have are conclusions in black and white in a report. Words have power, though. It will probably take months and perhaps even years before we know the full impact of this National Academies of Sciences, Engineering, and Medicine study.

Keith Speights has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.