The U.S. has been a global leader in marijuana reform, but other countries are increasingly taking their cues from Canada when setting up their medical cannabis programs.

"The vast majority of [medical marijuana programs] look completely different from the U.S. You can't go in and buy a gummy bear and have that be considered medicine," said Gomez. "The Canadian approach to regulation and product differentiation is much more reflective of what we're likely to see internationally."

At the same time, taking a more pharmaceutical approach limits the size and scope of these programs. Medical cannabis can be so cost-prohibitive that patients turn to home grows or the black market for access. "If [cannabis] oil costs $330 in a country where the median income is $10,000 a year, the ability for the program to scale is significantly lower," said Gomez.

Most international medical marijuana markets are taking that approach. But like U.S. states, Gomez expects these markets to incrementally increase access to medical cannabis. "Every country is going to be cautious about legalizing a drug that has been demonized for so many years," she said.

The financial projections rely on assumptions about how marijuana policy reform will play out. Gomez says the report is "more conservative" in estimating the prospects of marijuana legalization: "We certainly strive not to create overblown numbers for the sake of having headlines."

Cannabis usage rates in adults often serve as a good proxy for the likelihood of a legalization measure passing, she explained. Spain has one of the highest rates of past-month cannabis use among adults in Europe, according to a report from the United Nations Office on Drugs and Crime. Brightfield projects that a Spanish recreational marijuana market will come online in 2019 and will be worth nearly $206 million by 2021.

While there's great potential for growth, investors and entrepreneurs should tread carefully when it comes to the international marijuana market: "There's lot of opportunity, but be cautious. Most of the hype is overblown," said Gomez.