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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. Over the past two decades, there has been a growing movement to legalize marijuana, primarily for medical purposes.

Medical marijuana use has surged in the 16 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.

Its use has particularly increased among teenagers. According to a December 2011 government report, one out of every 15 high school students smokes marijuana on a near daily basis, a figure that has reached a 30-year peak even as use of alcohol, cigarettes and cocaine among teenagers continued a slow decline.

The popularity of marijuana, which is now more prevalent among 10th graders than cigarette smoking, reflects what researchers and drug officials say is a growing perception among teenagers that habitual marijuana use carries little risk of harm. That perception, experts say, is fueled in part by wider familiarity with medical marijuana and greater ease in obtaining it.

In late November 2011, the governors of Washington and Rhode Island petitioned the federal government to reclassify marijuana as a drug with accepted medical uses, saying the change was needed so states like theirs, which have decriminalized marijuana for medical purposes, can regulate the safe distribution of the drug without risking federal prosecution.

The move by the governors — Christine Gregoire of Washington, a Democrat, and Lincoln Chafee of Rhode Island, an independent who used to be a Republican — injected new political muscle into the long-running debate on the status of marijuana. Their states are among the 16 that allow medical marijuana, but which have seen efforts to grow and distribute the drug targeted by federal prosecutors.

In April, Gov. Brian Schweitzer of Montana, a Democrat, vetoed a bill that would have repealed the state’s voter-approved medical marijuana law. Even so, Mr. Schweitzer made it clear that he would like to see reform of the law.

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