Over the long run, legal marijuana could prove to be a big-time moneymaker for investors. After logging $10.9 billion in legal global sales in 2018, according to Arcview Market Research and BDS Analytics, the legal pot industry could see annual worldwide sales soar to anywhere between $50 billion and $200 billion in a decade's time. That's plenty of growth to get Wall Street and investors excited.

Of course, the past couple of months have been anything but exciting. The Horizons Marijuana Life Sciences ETF has shed more than a third of its value, and pretty much every major marijuana stock has face-planted. Everything from supply chain issues in Canada to resilient black markets in the U.S. are to blame for this persistent retracement in pot stocks.

While this pullback has created a buying opportunity in some cannabis stocks, others should remain cordoned off from investors. Consider the following three pot stocks as ones you'd be best off avoiding like the plague in September.

A businessman putting his hands up, as if to say no thanks.

Image source: Getty Images.

Cronos Group

Perhaps it comes as little surprise that what could arguably be my least favorite cannabis stock, Cronos Group (NASDAQ:CRON), makes the list as a company worth avoiding in September.

On one hand, I do understand why investors are attracted to Cronos. Even after its acquisition of Redwood Holdings, it'll likely have close to $1.5 billion in cash and cash equivalents, which gives Cronos Group a reasonable valuation floor. The company also has a major equity investor in tobacco giant Altria (NYSE:MO), which owns a 45% non-diluted stake in Cronos. Presumably, Altria will use its industry expertise and ties to vaping device Juul -- Altria is a 35% stakeholder in Juul -- to aid Cronos in its push to launch high-margin vape products by mid-December.

While this is all well and good, there are three pretty sizable problems. First of all, supply issues affecting the Canadian marijuana landscape are unlikely to be resolved by the time derivative products (e.g., edibles and vapes) launch in mid-December. This means the ramp-up in expected sales will take longer than expected.

Secondly, Cronos Group's operating results haven't been that impressive. Strip out the revaluation of derivative liabilities tied to its Altria warrants, as well as fair-value adjustments, and Cronos is decidedly losing money on an operating basis. Since earnings actually matter now, Cronos has shown little to inspire confidence that it's nearing operating profitability.

Third and finally, with Cronos Group heavily focused on high-margin vape products, it could be negatively impacted by growing concerns of mysterious lung diseases that may be associated with vaping. More than 200 people have reported vaping-related lung problems, and that could have a direct impact on vape-focused stocks. 

In short, Cronos may have a lot of cash and a well-known investor, but it's not worth buying.

A small pyramid of tobacco cigarettes lying atop a bed of dried tobacco.

Image source: Getty Images.

22nd Century Group

Next up is small-cap plant biotechnology company 22nd Century Group (NASDAQ:XXII), which happened to be the top-performing marijuana stock in August. Shares of 22nd Century Group gained 23% last month after the company's second-quarter operating results featured a narrower operating loss and a reduction in equity-based compensation. The latter not only reduces profitability, but it can balloon a company's outstanding share count, leading to dilution. 

Though 22nd Century Group gets its association with the cannabis industry from its potential to control tetrahydrocannabinol (THC) or cannabidiol (CBD) content in a plant, the company is arguably best known for its attempt to bring Very Low Nicotine Content (VLNC) cigarettes to market. These are cigarettes with at least 95% less nicotine than the top products on store shelves today, with the goal being to provide users with a nonaddictive nicotine product. While this probably sounds like a novel idea, I foresee two drawbacks.

The first is that tobacco and vape users purposely seek nicotine products for their "buzz." It's unclear if these users would consider switching to a nonaddictive VLNC product that doesn't provide the legal high they're looking for.

The other problem is that former Food and Drug Administration Commissioner Scott Gottlieb was really heralding the charge against big tobacco, and it's unclear, following his resignation earlier this year, if his successors will fight those same battles. In other words, while 22nd Century Group appears to have a novel idea on paper, it's still a big risk as to whether it'll translate into real-world sales. With any shot at profitability still a long way off, I'd consider taking August's run-up as a gift and head for the sidelines.

A businessman holding a stack of cash behind his back, with his fingers crossed.

Image source: Getty Images.

CannTrust Holdings

Lastly, for a second month in a row, investors would be wise to keep their distance from CannTrust Holdings (OTC:CNTTQ) until the company, and Health Canada, provide better clarity on its current "predicament."

For those who may not follow CannTrust too closely, in early July, the company disclosed that it had been growing cannabis in five unlicensed rooms at its flagship Niagara campus for a period of six months. Subsequent to this admission, it was revealed that select top-level executives were aware of this purposeful deception, leading to the firing of now-former CEO Peter Aceto. Now, to make matters worse, CannTrust's product sales have been halted, and the company's other facility, Vaughan, was found to be noncompliant by Health Canada. Pretty much everything that could go wrong has gone wrong.

At this point, Wall Street and investors are awaiting word on what CannTrust's punishment might be. The company could get off light and perhaps lose 5,000 kilos in inventory. Then again, CannTrust might see its cultivation and/or sales license suspended or revoked completely. It certainly seems to be taking Health Canada longer than expected to come to its decision, which is clearly worrying investors.

Although CannTrust has the potential to be a top five producer in Canada -- and there's no denying how inexpensive the company has become relative to its growing potential -- there remain far too many unknowns to buy in right now.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.