Editor's Note: The original article, published on May 4, 2018, incorrectly stated that there were no FDA-approved therapies for Lennox-Gastaut syndrome. This has been corrected. The Fool apologizes for the error.

This has truly been a year of big changes in the cannabis industry.

Earlier this year, marijuana investment firm Cronos Group became the first pot stock to uplist from the over-the-counter exchange to the Nasdaq. Not only could this move wind up paving the way for other large marijuana stocks to uplist to reputable exchanges, but it also validates just how far the legal cannabis industry has come in such a short amount of time.

A cannabis plant in the sunlight.

Image source: Getty Images.

We also witnessed history in January, when Vermont became the very first state to legalize the use of recreational marijuana entirely through the legislative process -- i.e., without residents voting on an amendment or initiative. Though the sale of adult-use weed still isn't allowed in Vermont, its effective decriminalization pushed the number of states where adult-use weed is legal to nine. Overall, 29 states have Ok'd broad-sweeping medical cannabis laws. 

In June, Canada is looking to become the first developed country in the world to legalize recreational marijuana. A vote is currently set for June 7, with a positive outcome likely leading to legalization shortly thereafter. The current timeline suggests that Canadian adults could be purchasing legal cannabis by as soon as August or September. In the process, Canadian recreational weed is expected to generate $5 billion or more in added annual sales on top of what it's already generating from medical weed sales and exports.

Arguably the biggest marijuana milestone of the year is approaching

But for all of these monumental events that have, or are expected, to occur in 2018, perhaps none stands out as more game-changing for the marijuana industry -- at least in the U.S. -- than the PDUFA decision date for GW Pharmaceuticals' (NASDAQ:GWPH) lead drug Epidiolex, which is currently slated for June 27.

Epidiolex is a cannabidiol (CBD)-based therapeutic designed to treat two rare forms of childhood-onset epilepsy: Dravet syndrome and Lennox Gastaut syndrome. Cannabidiol is one of the two most popular cannabinoids founds in the cannabis plant. In particular, CBD is the non-psychoactive component best known for its medicinal properties, while tetrahydrocannabinol, or THC, is the cannabinoid that gets you "high."

Cannabis leaves next to biotech lab equipment.

Image source: GW Pharmaceuticals.

GW Pharmaceuticals wound up running two pivotal-stage clinical trials in each indication (four total trials), with each and every phase 3 study demonstrating a statistically significant reduction in seizure frequency relative to baseline when compared to the placebo. In fact, one Dravet syndrome study demonstrated a 39% reduction in seizure frequency relative to baseline, which was three times higher than the 13% reduction provided by the placebo. 

Furthermore, on April 19, the Food and Drug Administration's (FDA) advisory panel unanimously recommended Epidiolex for approval. Understand that while the FDA is under no obligation to follow the recommendation of its panel, it does more often than not.

Why this decision is so exciting

You're probably wondering what the big deal is regarding the FDA's decision on whether or not to green-light a cannabinoid-based drug. The intrigue can be broken down into three parts.

First, there are no FDA-approved therapies to treat Dravet syndrome, meaning the FDA has an opportunity to introduce a medicine that could truly make a difference for these folks. Dravet syndrome affects an estimated 1 in every 40,000 people in the U.S. Meanwhile, an estimated 14,000 to 18,500 patients with Lennox-Gastaut syndrome would potentially benefit from GW Pharmaceuticals' drug in the U.S., if approved. 

A female doctor giving a high-five to a child sitting on her mother's lap.

Image source: Getty Images.

Secondly, it would be the very first cannabinoid ever approved by the FDA. There are synthetic versions of cannabis-like medicines that have been given the green light by the FDA, but there aren't any medicines derived from the cannabis plant that have been approved for use by the nation's regulatory drug agency.

Finally, and perhaps most importantly, the potential approval of Epidiolex could reshape the federal scheduling of cannabidiol, as well as coerce lawmakersin to taking a closer look at other aspects of the cannabis plant, potentially including THC. If the FDA is in favor of approval, it's possible that CBD could be rescheduled or descheduled completely, paving the way for states to use CBD to treat a variety of ailments. At the moment, 46 out of 50 states do allow the use of CBD, low-THC, or THC in some form, with Idaho, South Dakota, Kansas, and Nebraska completely banning all three variations.

GW Pharmaceuticals' shareholders may not be so lucky

On the other hand, while this could prove to be a game-changing approval for Dravet syndrome and Lennox-Gastaut syndrome patients, as well as open the door for federal CBD rescheduling or descheduling, it's not necessarily a big win for the shareholders of GW Pharmaceuticals for two reasons.

To begin with, the expectation of approval has been factored into GW's share price for quite some time. Investment firm Cowen Group, which has been especially optimistic on the cannabis industry and cannabinoid-based drugmakers, sees annual sales of Epidiolex topping out at $1.3 billion by 2022. It would be up to the company to execute flawlessly at this point if it wants to maintain its already lofty valuation.

A biotech lab researcher using a dropper.

Image source: Getty Images.

The other concern is that while it may be the first to market in treating Dravet syndrome and Lennox-Gastaut syndrome, another experimental therapy is nipping at its heels. In September, Zogenix (NASDAQ:ZGNX) announced surprisingly strong phase 3 data for ZX008, a low-dose fenfluramine that absolutely blew away the placebo in Dravet syndrome patient testing. Zogenix's lead compound led to a 72.4% reduction in convulsive seizure frequency over a 14-week period, compared to the 17.4% reduction for placebo patients.

Keep in mind that even though GW Pharmaceuticals' Epidiolex and Zogenix's ZX008 haven't gone head to head in a trial, and the parameters of their phase 3 studies weren't the same, the overwhelming success of ZX008 in phase 3 studies could clearly pressure Epidiolex, if approved by the FDA. With visions of expanding ZX008 to Lennox-Gastaut syndrome as well, GW Pharmaceuticals likely has quite the fight on its hands in the years to come.

For the time being, though, we simply watch and wait for the FDA's all-important and potentially historic decision come June 27.

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