Last year was truly exceptional for the marijuana movement. In fact, it may go down as the best year on record. In total, five new states legalized medical cannabis, two of which (Ohio and Pennsylvania) did so by passing up ballot initiatives and legalized medical pot through the legislative process. There are now 28 legal medical marijuana states. Furthermore, the number of recreational-legal states doubled to eight from four.

Yet, in spite of its phenomenal state-level growth, marijuana remains firmly entrenched as a Schedule 1 substance by the U.S. Drug Enforcement Agency. Schedule 1 substances are deemed to have no medical benefits and are illegal. This status makes researching marijuana's possible medical benefits very difficult, and it constrains the ability of the industry to rapidly expand. Remember, since marijuana is a federally illegal substance, pot businesses are unable to take normal business tax deductions, and they're often unable to receive basic banking services.

Lawmakers on Capitol Hill have opined for years that they would consider rescheduling marijuana if more positive evidence of its benefits were available. The problem is there are literally thousands of reports on the effects of marijuana available in printed and online journals, and until recently, there was little in the way of a centralized thesis. Thankfully, that's changed.

Everything you've ever wanted to know about cannabis

Last week, a 395-page report was released by the National Academies of Science, Engineering, and Medicine (NASEM) that examined approximately 24,000 scientific papers from the nation's top universities. In total, the team of a dozen researchers from NASEM reached nearly 100 conclusions. Though we're not going to cover every single one of those here today, we can list some of the most important correlative conclusions drawn from the report.

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Studies demonstrating strong evidence

Marijuana treats chronic pain very well: Researchers found pretty strong evidence that marijuana helps patients treat their chronic pain symptoms. This might come as a bit of a shock to GW Pharmaceuticals '(NASDAQ: GWPH) shareholders given that cannabinoid-based drug Sativex failed to demonstrate a statistically significant benefit to cancer patients in its phase 3 trial. Even more impressively, the researchers found that the availability of medical marijuana can reduce opioid dependence and opioid-related overdoses. In 2015, the American Society of Addition Medicine notes that 20,101 people died in the U.S. from opioid overdoses. The hope here is that more widespread use of medical cannabis could result in fewer opioid-related overdose deaths.

Researchers found pretty strong evidence that marijuana helps patients treat their chronic pain symptoms. This might come as a bit of a shock to '(NASDAQ: GWPH) shareholders given that cannabinoid-based drug Sativex failed to demonstrate a statistically significant benefit to cancer patients in its phase 3 trial. Even more impressively, the researchers found that the availability of medical marijuana can reduce opioid dependence and opioid-related overdoses. In 2015, the American Society of Addition Medicine notes that 20,101 people died in the U.S. from opioid overdoses. The hope here is that more widespread use of medical cannabis could result in fewer opioid-related overdose deaths. Marijuana use increases traffic accidents: Sorry, pro-legalization enthusiasts, but the aggregated data demonstrated a strong link between marijuana use and increase motor vehicle traffic accidents. Nonetheless, the risk associated with using marijuana while behind the wheel appears to be substantially lower than the risks associated with driving while under the influence of alcohol.

Sorry, pro-legalization enthusiasts, but the aggregated data demonstrated a strong link between marijuana use and increase motor vehicle traffic accidents. Nonetheless, the risk associated with using marijuana while behind the wheel appears to be substantially lower than the risks associated with driving while under the influence of alcohol. Long - term pot use increases the risk of psychotic or schizophrenic symptoms: The evidence found a strong correlation between long-term or heavy cannabis use and the likelihood of developing schizophrenia, social anxiety disorders, and depression (albeit to a lesser extent). The report also notes that pot users are more likely to have suicidal thoughts than non-users.

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Studies demonstrating moderate or mixed evidence

Marijuana's long-term effects on the lungs and heart are mixed: One of the bigger issues that's held marijuana back from being rescheduled by Congress is the concern of what sort of health issues it brings to the table over the long term. Researchers at NASEM found a modest link between long-term marijuana use and chronic health conditions. For example, there was substantive evidence that long-term pot use leads to chronic bronchitis, and limited evidence that weed use can increase acute heart attack risk and stroke risk. However, there was essentially no link between marijuana use and other cardiopulmonary issues.

One of the bigger issues that's held marijuana back from being rescheduled by Congress is the concern of what sort of health issues it brings to the table over the long term. Researchers at NASEM found a modest link between long-term marijuana use and chronic health conditions. For example, there was substantive evidence that long-term pot use leads to chronic bronchitis, and limited evidence that weed use can increase acute heart attack risk and stroke risk. However, there was essentially no link between marijuana use and other cardiopulmonary issues. Memory and learning are somewhat negatively affected in the short term: According to the aggregated reports, there was moderate evidence to link memory and learning impairments with marijuana use, at least when talking about a short-term time frame. Over the long-term, the evidence of impairment was far more limited. It was also difficult to draw long-term conclusions on learning and academic success due to a myriad of external factors that could have affected academic performance and job outcomes.

According to the aggregated reports, there was moderate evidence to link memory and learning impairments with marijuana use, at least when talking about a short-term time frame. Over the long-term, the evidence of impairment was far more limited. It was also difficult to draw long-term conclusions on learning and academic success due to a myriad of external factors that could have affected academic performance and job outcomes. Legalizing marijuana increases adolescent exposure: One of lawmakers' greatest fears is that legalizing cannabis will ease access to the drug for minors. Even if a few recent reports have suggested otherwise, the cumulative data from NASEM found moderate evidence that legal pot states are allowing adolescents easier access to marijuana and/or pot edibles.

Studies that demonstrated low correlative evidence

Marijuana doesn't cause cancer: One of the most promising findings was that there was little evidence linking marijuana use to the development of cancer. Among the reports studied, head and neck cancers, along with lung cancer, demonstrated moderate evidence of no link to marijuana. Meanwhile, other cancer types may have an association with marijuana use, but none of the research papers studied indicated a correlation linking pot to cancer with any concreteness.

One of the most promising findings was that there was little evidence linking marijuana use to the development of cancer. Among the reports studied, head and neck cancers, along with lung cancer, demonstrated moderate evidence of no link to marijuana. Meanwhile, other cancer types may have an association with marijuana use, but none of the research papers studied indicated a correlation linking pot to cancer with any concreteness. Surprisingly, cannabis may not treat glaucoma or epilepsy: Interestingly enough, despite some clinical reports suggesting that marijuana is useful as a treatment for glaucoma and epilepsy, the culled data suggests little evidence that this is the case. And, once again, this works contrary to the phase 3 data presented by GW Pharmaceuticals' cannabinoid-based medication Epidiolex, which met its primary endpoint of a reduction in seizures in two types of childhood-onset epilepsies, Dravet syndrome and Lennox-Gastaut syndrome.

Interestingly enough, despite some clinical reports suggesting that marijuana is useful as a treatment for glaucoma and epilepsy, the culled data suggests little evidence that this is the case. And, once again, this works contrary to the phase 3 data presented by GW Pharmaceuticals' cannabinoid-based medication Epidiolex, which met its primary endpoint of a reduction in seizures in two types of childhood-onset epilepsies, Dravet syndrome and Lennox-Gastaut syndrome. No link to overdose deaths: Perhaps the strongest lack of evidence came from research into whether overdosing on marijuana can lead to death. Though it does remain possible to overdose on cannabis, there haven't been any reported deaths from marijuana overdoses in recent years.

Perhaps the strongest lack of evidence came from research into whether overdosing on marijuana can lead to death. Though it does remain possible to overdose on cannabis, there haven't been any reported deaths from marijuana overdoses in recent years. Marijuana isn't a gateway drug: Another strongly positive finding from researchers is there was limited evidence that marijuana turns users onto harder drugs, such as cocaine or heroin. However, there was a modest correlation that if users do wind up moving onto other drugs, marijuana use may make the user more dependent on those drugs.

Expect the status quo to continue

With this new report, lawmakers on Capitol Hill have practically every data point they could ever imagine. Despite this, I'd fully expect the status quo to continue in Washington under President Trump. Republican lawmakers hold a majority of seats in both houses of Congress, and historically Republican lawmakers are far less likely to consider the legalization of marijuana than their Democratic counterparts.

One thing the NASEM report can't address is how well or poorly the states that have legalized recreational cannabis can regulate their industry. I believe it's this point (and the need for time to assess the success of legal states in regulating their industries), as well as the concerns of adolescent access, that'll keep marijuana firmly as a Schedule 1 substance for at least the next couple of years.

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Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen nameTMFUltraLong, and check him out on Twitter, where he goes by the handle@TMFUltraLong.The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has adisclosure policy .

The views and opinions expressed herein are the views and opinions of the author and do not necessarily reflect those of Nasdaq, Inc.

The views and opinions expressed herein are the views and opinions of the author and do not necessarily reflect those of Nasdaq, Inc.