Investors have, in general, been treated to phenomenal stock returns since the market bounced from its March 2009 lows. All three of the major U.S. indexes have more than tripled in value, with the Nasdaq Composite approaching a near quintupling in value from its 2009 lows.

Marijuana stocks leave regular stocks in the dust

Nevertheless, these eight-plus-year returns pale in comparison with what marijuana stocks have been able to do in just one year's time. Over the trailing-12-month period through July 15, the average marijuana stock with a market cap in excess of $200 million -- a dozen stocks in total, which excluded MedReleaf because it just went public a month ago -- was up by 332%.

Overall, just a single pot stock out of the 12 examined was down on a year-over-year basis, although a single company, Axim Biotechnologies (OTC:AXIM) skewed the results with a 2,363% 12-month gain. And no, that's not a typo. Even if we were to exclude Axim from the calculations, the average gain of the remaining 11 marijuana stocks would be 147% over the past year. By comparison, the broad-based S&P 500 is up just 13.5% over the same timeframe. Marijuana stocks have averaged anywhere from a tenfold to 23-fold better return than the S&P 500, depending on whether you want to include Axim Biotechnologies in the calculation.

Here's a quick rundown of the dazzling dozen, based on performance:

Axim Biotechnologies: up 2,363%.

Aurora Cannabis NYSE:ACB)

Aphria : up 209%.

: up 209%. Cara Therapeutics : up 207%.

: up 207%. Canopy Growth Corp. NYSE:CGC)

Medical Marijuana, Inc. OTC:MJNA)

Zynerba Pharmaceuticals : up 157%.

: up 157%. Corbus Pharmaceuticals : up 146%.

: up 146%. Arena Pharmaceuticals : up 43%.

: up 43%. Scotts Miracle-Gro : up 25%.

: up 25%. GW Pharmaceuticals : up 17%.

: up 17%. Insys Therapeutics: down 18%.

Here's why marijuana stocks have been unstoppable

What the heck is going on here? Look no further than changing public perceptions toward marijuana and possible expansion opportunities as reasonsthese stocks are headed to the stratosphere.

In terms of favorability, marijuana's popularity is higher now than it's ever been. Both the 2016 poll from Gallup (60%) and the April 2017 CBS News survey (61%) showed that an all-time high number of respondents wants to see cannabis legalized across the country. A separate study conducted by Quinnipiac University in April of this year also showed that 94% of respondents want medical cannabis legalized at the federal level. As the percentage of Americans who want to see pot legalized rises, the pressure put on lawmakers in Washington to seek change grows as well.

Recent legalizations, and the potential for some very notable expansions in the near term, also has the industry excited.

For example, in June, Mexican President Enrique Pena Nieto signed into law a bill that legalized medical cannabis in Mexico. Even though Mexico has long had an issue with drug cartels, this legalization is a shot in the arm for shareholders in Medical Marijuana, Inc. This is a company with a distribution subsidiary in Mexico for cannabidiol-based products, and the Mexican medical cannabis legalization, which covers THC as well, should provide a boost to sales.

In terms of expectations, Canadian pot stocks have raced higher on the hope that introduced legislation from Prime Minister Justin Trudeau will legalize recreational marijuana by July 2018. This is why companies such as Aurora Cannabis and Canopy Growth Corp. have exploded higher over the trailing year. Then again, it also doesn't hurt that Canadian pot stocks such as Aurora and Canopy Growth are benefiting from significant medical cannabis user growth, according to regulatory agency Health Canada.

Here's why you should be worried

Though marijuana stocks have been top performers over the past year, there's also a lot to be worried about if you're an investor, or if you've considering dipping your toes into the water.

Obviously, there are ongoing legal concerns. In the U.S., marijuana is still entirely illegal at the federal level, and it doesn't look as if that view is going to change anytime soon. Matters are made worse when Attorney General Jeff Sessions would just as soon trample states' rights and prosecute medical-weed dispensaries, if he could gain approval from Congress. Marijuana's Schedule I status at the federal level means businesses will continue to struggle in securing basic banking services, including a checking account. It also means they'll be unable to take normal corporate income-tax deductions.

But there's more to be worried about than just legalese. Investors should be seriously concerned about how much they're willing to pay for future growth, especially when that future growth is far from guaranteed. Axim Biotechnologies, up 2,363% over the past year, hardly has anything in its pipeline past the dose-finding stage. Most of its dozen-plus trials are either preclinical or in the phase 1 planning stage. At the same time, the company ended its most recent quarter with just $632,424 in cash. How the company expects to run more than dozen studies with such little capital on hand is a head-scratcher.

In other words, marijuana stock valuations should be heavily scrutinized after their recent run.

While it's not out of the question that pot stocks could head higher, we as investors are going to need to see a discernible move toward recurring profits from these companies if these moves are to be sustainable over the long term.