Whether you realize it or not, the legal weed industry is among the fastest growing in North America. ArcView, a leading cannabis research company, suggests that legal weed sales can grow at a pace of 26% annually through 2021. If this estimate proves accurate, North American annual marijuana sales could be close to $22 billion in just a few years. This could be a big-money opportunity for investors willing to place their bet(s) on a drug that's still widely considered to be illegal around the world.

But we're also witnessing major advances on the legalization front. Mexico legalized medicinal cannabis in June 2017, while Canada appears to be on the verge of making weed legal for adult purchase by July 2018. Not to mention, 29 U.S. states have legalized medical marijuana, and nine others have OK'd recreational pot. This expansion, which is being fueled by a steady change in the public's perception of cannabis, is what's expected to fuel triple-digit sales growth for weed stocks in the years ahead.

Rows of weed plants in a commercial grow farm.

Image source: Getty Images.

Fast-growing weed stocks to consider buying

However, not all weed stocks are created equally. Throwing a dart at the newspaper and hoping you land on a winner probably isn't going to produce the result you desire. Instead, those stocks with the fastest projected sales growth in the following fiscal year (2019, for most companies) might actually offer the best returns for investors.

The following three weed stocks are all expected to grow their sales by more than 400% next year, and may be worth investors' consideration.

1. GW Pharmaceuticals

The company with the highest expected sales growth in 2019 (1,072%) is cannabinoid-based drug developer GW Pharmaceuticals (NASDAQ:GWPH). Following an expected $25.5 million in sales in 2018, the company's total revenue is expected to catapult to nearly $300 million by 2019, all thanks to Epidiolex.

Epidiolex is the company's lead drug, and it's currently being reviewed by the Food and Drug Administration for approval. It's a cannabidiol (CBD)-based therapy -- meaning it uses the non-psychoactive component of cannabis -- designed to treat two rare types of childhood-onset epilepsy, Dravet syndrome and Lennox-Gastaut syndrome. In two phase 3 studies for each indication, Epidiolex handily met its primary endpoint of a statistically significant decline in seizure frequency relative to the placebo. Since there are no FDA-approved treatments for either indication, Epidiolex could quickly launch out of the gate, if approved. 

Cannabis leaves next to biotech lab equipment.

Image source: Getty Images.

There are, however, two considerations that investors should make. First of all, no cannabinoid-based drug has ever been approved by the FDA before, so there could be extra steps in Epidiolex's path to pharmacy shelves. Even if it is approved, there could be a delay in getting the drugs' label correct and in potentially scheduling it prior to launch.

Secondly, investors should also understand that Dravet syndrome and Lennox-Gastaut syndrome are highly competitive indications. This probably won't be Epidiolex's sole playground for long, which could cap its peak potential.

Nevertheless, GW Pharmaceuticals could be generating recurring profits by as soon as 2020, making it worth a closer look for weed stock investors.

2. Aphria

Among Canadian cannabis growers, few if any can hold a candle to the revenue increase that Aphria (NASDAQOTH:APHQF) is expected to bring to the table. Following an expected 110% increase in forecasted sales in 2018, its revenue is projected to rise by 421% in 2019 to around $180 million.

Aphria has benefited from rapid patient growth and strong medical cannabis demand in Canada, where medical weed was legalized all the way back in 2001. It's one of just two marijuana stocks to have reported a full-year profit in each of the past two years solely on account of medical pot sales. Of course, the real allure is the expected legalization of recreational marijuana by July.

An outdoor cannabis grow farm.

Image source: Getty Images.

In anticipation of this event, and a general increase in medical cannabis demand, Aphria is working on two ambitious projects. First, it has its organic four-phase expansion that, when finished in late January 2019, will span 1 million square feet. Costing well in excess of $100 million, it should yield around 100,000 kilograms of dried cannabis a year once complete. The other "project" is its recently announced strategic relationship with Double Diamond Farms that'll add an estimated 120,000 kilograms of production by January 2019. Aphria is on track to among the top three annual producers come 2019. 

Of course, Aphria has also had a major run-up in its share price in anticipation of this expected legalization. Despite strong sales growth, it could be valued at nearly 100 times its 2019 profit projections, which isn't cheap. Then again, Aphria's management team focuses on profitability, much more so than its weed industry peers. That's a positive for shareholders, and all the more reason to consider adding Aphria to your portfolio.

3. Cronos Group

A third intriguing weed stock that could help you get in on the green rush with strong sales growth potential is Cronos Group (NASDAQ:CRON). Cronos is neither a grower nor drug developer. Instead, it aims to work smarter as an investment company in cannabis projects. In fiscal 2018, its upcoming year, sales are expected to catapult higher by almost 750%.

A cannabis bud lying atop a messy pile of hundred dollar bills.

Image source: Getty Images.

Cronos has invested in about a half-dozen companies, with its core portfolio consisting of PeaceNaturals, a producer of medical cannabis and oils; Original BC, a grower and retailer of medical pot in British Columbia; and Whistler Medical Marijuana Company, which also cultivates and sells medical marijuana and oils. It has full ownership in PeaceNaturals and Original BC, and owns 21.5% of Whistler Medical Marijuana. It also has smaller investments in Canopy Growth Corp. and AbCann

The beauty of Cronos Group's investment portfolio is that shareholders get instant diversity by buying into this weed stock. If one company struggles, there are other investments that can step in and fill the gap. It may not be as diverse as an exchange-traded fund, but Cronos has done a good job of partially diversifying its operations.

The company should also continue to see an uptick in demand. In May 2017, Health Canada noted that medical marijuana patient enrollment was growing by roughly 10% a month. Though Cronos will likely only be marginally profitable in its upcoming fiscal year, it offers high-margin growth potential for the future.

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