Source: Flickr user Cannabis Pictures. 

Three national polling agencies all recently showed that respondents had a slim but favorable view of marijuana. When considering just the medicinal uses of marijuana, respondents had an even more favorable view of the still-illicit drug.

It's not hard to see why marijuana has garnered so much favor in recent years, and why 23 states have allowed marijuana to be legalized for medicinal use. For states, marijuana offers a source of new revenue via taxes that could help fund education, transportation, and infrastructure, or even create jobs. For medical marijuana users, it represents a potential new pathway to help alleviate or even cure symptoms of various chronic or terminal diseases.

Over the past few years we've seen numerous positive studies emerge on the effects of marijuana. Some of these studies have shown that marijuana may have a positive effect on treating aggressive brain cancers, Parkinson's disease, Alzheimer's disease, epilepsy, and even type 2 diabetes.

But could millions of Americans be wrong about marijuana's perceived safety profile?

A new study suggests marijuana could worsen this disease
According to a study conducted by Ph.D. student Mike Bancks from the University of Minnesota Public Health, marijuana could actually have an adverse effect on users' blood sugar by middle-age, leading to a condition known as prediabetes (i.e., blood sugar levels that are above normal, but not fully diabetic). To be clear, Bancks' study did not show evidence that regular adolescent marijuana use led to full-blown diabetes.

Source: National Cancer Institute.

The study, which was published in the journal Diabetologia earlier this week, showed that individuals who used marijuana a lot during their adolescence were 40% more likely to develop prediabetes when they became middle-age adults. This finding contradicts a 4,657-person study from 2013 that was published in The American Journal of Medicine, which demonstrated marijuana use led to a 16% reduction in fasting insulin levels and a 17% reduction in HOMA-IR (a measure of insulin resistance and beta-cell function).

One of the controls that Bancks used in his study that wasn't present in the previous studies from other researchers was a diabetes diagnosis prior to beginning the study. In fact, all participants in Bancks' study needed to be free of diabetes before it began. According to Bancks, previous studies didn't define whether marijuana use began before or after a diabetes diagnosis, which can definitely affect the results.

Despite Bancks' findings, the study still raised a lot of questions and left very little settled. For instance, Bancks couldn't say why adolescent marijuana users exhibited higher rates of prediabetes but not diabetes, although suggestions were certainly offered. The study author implied that because study participants had to be free of diabetes prior to beginning the study, it could have left people more likely to develop diabetes out of the study. It was also suggested that marijuana may simply have a greater impact on blood sugar levels when they're in the prediabetic range rather than in patients that have full-blown diabetes.

Bancks' summation of the study essentially concludes that more research is needed, but opines that physicians should closely monitor the blood sugar level of heavy marijuana users as a precaution.

Source: National Institutes of Health. 

Many challenges lie ahead
If this latest study proves anything, it's that there are still countless hurdles the drug, and the industry, have yet to overcome.

Arguably at the top of that list is the safety of marijuana. Although we've witnessed plenty of studies that have shown positive or negligible health effects of marijuana usage on people, we also have decades worth of clinical research that suggests otherwise. Until researchers can cull more data with as many controls as possible to narrow down the actual effects marijuana has on the body, it could be difficult, if not impossible, to get lawmakers to change their stance on marijuana.

But there are more problems facing the industry than just ongoing safety concerns.

For starters, legal marijuana businesses are struggling to find adequate financing to expand their business, or in some cases even buy equipment or products to continue running their business. Although some states have set up regulations to allow financial institutions to offer basic banking functions to the marijuana industry, most banks have chosen to stay on the sidelines for fear of prosecution for money laundering. Remember, even though 23 states have legalized medical marijuana, the plant is still illegal with the federal government. Without a receptive financial system the marijuana industry's supply chain is a constant question mark.

Source: Pixabay.

Lawmakers could prove to be another sticking point. Not only is Congress waiting for mature data on marijuana's safety profile before jumping the gun on reform, but we're nearing a pivotal election in November 2016 that could change the controlling interests in Congress and the presidency. If the political party in charge, or new Commander-in-Chief, doesn't have a favorable view of marijuana's possible legalization or decriminalization, it could be even longer before the industry is given any chance for change.

Lastly, federal tax law isn't working in the industry's favor. Although the federal government has taken a hands-off approach to states regulating their own marijuana industry, federal tax law still supersedes state tax law. Because of this, legal marijuana businesses aren't able to take simple deductions that normal businesses can take on their taxes. In effect, they're being overtaxed and winding up with a much smaller figure in their bottom line.

With an abundance of challenges and question marks lying ahead for marijuana, my suggestion remains the same for investors: avoid the industry like the plague. If we witness a rescheduling of marijuana by Congress, then it would be time to consider marijuana stocks as possibly investment worthy. Until that happens, marijuana stocks are extremely risky investments with no guarantee of long-term success.