"Many investors have made a quick buck betting on legalization via marijuana stocks over the past year," said Alyssa Oursler, assistant editor at InvestorPlace.com, in a recent report. "Still others, though, have watched their investment fall flat following a big-time February spike."
Fraud alert: The scams are coming
Like many other scams, pitches for bogus marijuana investment opportunities will be made in a variety of ways, Finra said. These include email, tweets, blog posts, faxes, webinars, infomercials and invitations to attend a presentation in your area.
"People get burned when they act solely on some stock tip they hear about from a source they don't know," said Finra President Gerri Walsh. "Such wide solicitations, where a large number of people are being invited to get in on the ground floor, that's the hallmark of a scam."
Her advice: Stop and ask yourself, "Why me?" Why is a total stranger telling you about this really great investment opportunity?
"Con artists sometimes use false and misleading information to create unwarranted demand for shares of small, thinly traded companies that often have little or no history of financial success," Finra warns.
Once the stock rises, those behind these "pump-and-dump" schemes sell off their shares, and those who bought later in the game are left with worthless shares.
(Read more: Obamacare is coming, and so are the con artists)
One company highlighted in Finra's Marijuana Stock Scams alert issued more than 30 press releases during the first half of this year. It also used sponsored links and spam email to promote its entry into the medical marijuana business. The company said its stock was "poised to light up the chart!" and that it "could double its price SOON."
Finra found that the company had just started to formulate a business plan and had nothing but losses on its balance sheet.
"Beware of any sort of guaranteed yield or return on investment, because no one knows how successful these companies are going to be. And beware of anyone who promises there's no risk," said Colorado Securities Commissioner Fred Joseph.
The bottom line
Be cautious. Don't be rushed or high-pressured into something, and don't get greedy. Investigate before you invest. Check with your state's regulatory agencies, and make sure the promoter is licensed in your state using Finra's BrokerCheck. You can check on the investment using the SEC's EDGAR database.
Remember, any investment in the marijuana business, no matter how legitimate, is risky because cannabis is still viewed as illegal by the federal government, which could decide to shut down a company at any time.
—By CNBC contributor Herb Weisbaum. Follow him on Facebook and Twitter @TheConsumerman or visit The ConsumerMan website.