Marijuana, whose botanical name is cannabis, has been used by humans for thousands of years. It was classified as an illegal drug by many countries in the 20th century. But over the past two decades, there has been a growing movement to legalize it, primarily for medical purposes.

Medical marijuana use has surged in the 15 states and the District of Columbia that allow its use. But states and cities are also wrestling with the question of what medical marijuana is, or should be.

The Montana House of Representatives voted in February 2011 to repeal the state's six-year-old medical marijuana law. The 63-to-37 vote, largely along party lines in the Republican-controlled chamber, pushed Montana to the front lines of a national debate about social policy, economics and health regarding medical marijuana use

Montana's House speaker, Mike Milburn, a Republican and sponsor of the repeal bill, who said he thought that the arguments about medical use had been a pretext for encouraging recreational use and creating a path to full legalization. He said he feared gang drug wars in Montana's cities and debilitation of its youth. If the legislation is passed by the Montana Senate, it would face an uncertain fate on the desk of Gov. Brian Schweitzer, a Democrat, who has said he believes the laws need to be tightened, but he has not taken a position on repeal.

New Mexico's Republican governor, Susana Martinez, has also expressed interest in repeal in 2011. Colorado was formulating some of the most detailed rules in the nation for growing and selling. Lawmakers in New Jersey have jousted with the governor over regulation.

In November 2010, Californians defeated Proposition 19, a ballot measure that would have legalized possession and growing of marijuana outright, and taxed and regulated its use. California already reduced its penalty for possession, putting those caught with small amounts of the drug on the same level as those caught speeding on the freeway.

Advocates for Proposition 19 had said that if legalized California could raise $1.4 billion in taxes and save precious law enforcement and prison resources. Attorney General Eric Holder had insisted that the federal government would continue to enforce its laws against marijuana in California even if they conflict with state law.

Currently 15 states allow the use of marijuana for pain relief, nausea and loss of appetite by people with AIDS, cancer and other debilitating diseases. Those laws, however, are at odds with federal law. The federal government continues to oppose any decriminalization of the drug. And while the Obama administration has signaled some leeway when it comes to medical marijuana, raids on dispensaries and growers by law enforcement agencies are still common -- even in California, where the industry effectively began in 1996, with the passage of the landmark Proposition 215, which legalized medical marijuana.

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Rules vary widely in the states that permit medical marijuana. Some states require sellers to prove nonprofit status -- often as a collective or cooperative -- and all states require that patients have a recommendation from a physician. But even those in favor of medical marijuana believe that the system is ripe for abuse or even unintentional lawbreaking.

Although party line positions defined the issue in Montana, with Republicans mostly lined up in favor of restriction or repeal, there is widespread agreement among legislators and residents that medical marijuana has become something very different than it was originally envisioned to be.

Sixty-two percent of voters approved the use of medical marijuana in a Montana referendum in 2004. But the real explosion of growth came only in 2010, after the federal Department of Justice said in late 2009 that medical marijuana would not be a law enforcement priority. Since then, the numbers of patients have quadrupled to more than 27,000 -- in a state of only about 975,000 people -- and millions of dollars have been invested in businesses that grow or supply the product.

With a growing number of Americans favoring legalization -- a Gallup poll released in October 2010 found a record 46 percent approving of legalization -- perhaps no ballot measure in the country was more closely watched than Proposition 19 in California.

The California ballot measure would have allowed anyone over 21 to buy, possess, use or cultivate marijuana. It would have barred personal possession of more than one ounce as well as smoking the drug in public or around minors.

Some civil rights activists favored the legalization of the drug on the grounds that marijuana arrests are wildly disproportionate in their racial impact and adversely affect minorities.

But the measure was strongly opposed by law enforcement, which said it would actually end up costing the state in increased public health and safety expenses.

As more and more states allow medical use of the drug, marijuana's supporters are pushing hard to burnish the image of marijuana by franchising dispensaries and building brands; establishing consulting, lobbying and law firms; setting up trade shows and a seminar circuit; and constructing a range of other marijuana-related businesses.

In July 2010, the Department of Veterans Affairs announced that it will formally allow patients treated at its hospitals and clinics to use medical marijuana in states where it is legal, a policy clarification that veterans had sought for several years.

The department directive resolves the conflict in veterans facilities between federal law, which outlaws marijuana, and the 14 states that allow medicinal use of the drug, effectively deferring to the states.

Marijuana is the only major drug for which the federal government controls the only legal research supply and for which the government requires a special scientific review. The University of Mississippi has the nation's only federally approved marijuana plantation. If researchers wish to investigate marijuana, they must apply to the National Institute on Drug Abuse to use the Mississippi marijuana and must get approvals from a special Public Health Service panel, the Drug Enforcement Administration and the Food and Drug Administration