The other main division, WeedMaps Media, operates the WeedMaps.com website. The site helps people find medicinal-cannabis dispensaries, which pay a fee to be listed. WeedMaps already covers 1,800 of the country's roughly 2,000 dispensaries. So growth here will depend in part on more states approving medical marijuana.

Another way to get pure-play exposure to medical marijuana is through ArcView Angel Network. It's a startup that helps people with at least $50,000 find and invest in medical-marijuana-related business in areas like insurance, packaging, consumption devices and growing supplies. It doesn't raise money for dispensaries or growers.

ArcView Group CEO Troy Dayton says a few hundred investors and entrepreneurs have already contacted him. Dayton's partner, Steve DeAngelo, runs a large dispensary called Harborside Health Center in Oakland, Calif., so they have some direct experience with the business.

A real drug company

GW Pharmaceuticals (GWPRF, news) of Britain is one of the safer pot-related investment plays, at least in terms of legal risk. It develops and sells legal extracts from pot plants, as opposed to the plants and buds you'd find at medical marijuana dispensaries. Pot-based drugs are already mainstream, but this is the only pharmaceutical company that's a pure play.

The potential here could be big, because there are more than 70 separate cannabinoids, identifiable substances, in the marijuana plant. Any of them might have medical applications. "In a sense, the cannabis plant is a little pharmaceuticals factory," says GW's Mark Rogerson.

His company has already brought two such substances to market in a drug called Sativex, used to treat spasticity, a symptom of multiple sclerosis. The active ingredients alter electrical signals in the central nervous system, Rogerson says.

The key thing, he says, is that when using Sativex, "people don't just get high and not care about the symptoms anymore. They are taking a much lower dose of the active ingredient than recreational users."

Sativex, taken as a spray inside the mouth, is approved for use in Britain, Spain, Canada and New Zealand. Sales jumped 64% last year to $1.7 million, helping to drive overall GW revenue up 27%, to $18.8 million.

GW has partnerships with Novartis (NVS, news), Bayer (BAYRY, news) and Japan's Otsuka Pharmaceutical to either sell Sativex or develop pot-based drugs. The company is also researching the use of such substances to treat pain, epilepsy, diabetes, obesity and some mental disorders. GW is conducting trials in the U.S. on using pot extracts to treat cancer-related pain.

The other suppliers

To get exposure to the marijuana business with no legal risk, I'd suggest home gardening and farming supply retailers like Home Depot (HD, news) and Tractor Supply (TSCO, news), as well as fertilizer suppliers such as Mosaic (MOS, news), Potash of Saskatchewan (POT, news), Intrepid Potash (IPI, news) and CF Industries (CF, news).

These companies are doing a lot of business with pot growers, even if they don't know it. After all, marijuana is the biggest agricultural crop in California, worth about $14 billion a year, according to Drugscience.org. There are no official statistics, but that would mean that growers spend a huge amount of money on supplies.

How much? Industry insiders familiar with the economics of growing pot tell me the cost can range anywhere from about $50 a pound, for large-scale outdoor operations, to $1,000 a pound for indoor farms. As a rough average, I'll take the $400-a-pound cost cited by a grower in a 2008 CNBC special on the business of marijuana. I'll assume that about three-fourths of those costs are for labor, which leaves $100 a pound for supplies like equipment and fertilizers.

Roughly 5 million pounds of pot, or $20 billion worth, is purchased a year in the U.S., according to Jeffrey Miron, who teaches economics at Harvard University. This suggests around a half a billion dollars a year goes to buy the supplies used to grow pot.

But what's the future?

These sorts of suppliers are safe, mainstream stocks, and companies working on marijuana pills and medical sprays seem relatively safe as well. But there's clearly a big risk in putting your money into General Cannabis or other companies linked to selling the plant form of marijuana -- the shaky legal ground.

As popular as medical-pot dispensaries are in some circles, a lot of doctors and drug researchers haven't bought into the idea. "It does not make sense," says Ryan Vandrey, an assistant professor in the Behavioral Pharmacology Research Unit at Johns Hopkins University School of Medicine.

Doctors like Vandrey oppose smokable pot being dispensed as medicine because it can cause lung damage; it's difficult to prescribe dosage amounts since usage and potency vary; and there are no manufacturing controls to check for risky impurities.

Also, while the feds may choose not to enforce the law right now, the law has not changed. The marijuana plant -- as opposed to synthetic derivatives or plant extracts -- remains a "schedule 1" drug, the toughest classification from a law-enforcement perspective.

Pot is unlikely to get knocked down to the more lenient "schedule 2" category, which embraces potentially harmful drugs that have legitimate medical purposes -- including synthetic pot like Marinol. The reason: There have been no large, statistically valid medical studies performed in the U.S. on the use of the plant for medical purposes, says Vandrey.

And these studies are unlikely to ever happen for a simple reason, he says. Pharmaceutical companies can't effectively patent the use of the plant for medical use, unlike its spray or pill forms. So they are unlikely to spend the millions of dollars it would take to conduct such research.

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Legalization would clean all this up, of course. But Miron, of Harvard, doubts the mood of the country is shifting that far. He cites the failure of a 2010 California referendum on legalization.

But others still hope. Allen St. Pierre of the National Organization for the Reform of Marijuana Laws (NORML) believes the ascent of more baby boomers into key leadership roles in government and the media will help pave the way for legalization. These people have more lenient attitudes towards pot, he says, which may actually translate into legalized pot for everyone -- not just those who want the drug for medicinal purposes. "Prohibition," he says, "is wearing a little thin."

At the time of publication, Michael Brush did not own any stocks mentioned in this column.

Michael Brush is the editor of Brush Up on Stocks, an investment newsletter. Click here to find Brush's most recent articles and blog posts.