Eric Risberg / AP Packets of marijuana buds are shown for sale at the San Francisco Medical Cannabis Clinic in San Francisco, Oct. 19, 2009

On Oct. 19, the U.S. Justice Department announced that federal prosecutors would not pursue medical-marijuana users and distributors who comply with state laws, formalizing a policy at which the Obama Administration hinted earlier this year. Currently, 13 states allow doctors to prescribe medical marijuana to patients suffering from ailments ranging from AIDS to glaucoma, and in Maryland a prescription can soften punishment if a user faces prosecution. But until now those laws didn't provide any protection from federal authorities. (Read "Can Marijuana Help Rescue California's Economy?")

Should Professors Cheech and Chong ever receive university tenure teaching the medical history of their favorite subject, the course pack would be surprisingly thick. As early as 2737 B.C., the mystical Emperor Shen Neng of China was prescribing marijuana tea for the treatment of gout, rheumatism, malaria and, oddly enough, poor memory. The drug's popularity as a medicine spread throughout Asia, the Middle East and down the eastern coast of Africa, and certain Hindu sects in India used marijuana for religious purposes and stress relief. Ancient physicians prescribed marijuana for everything from pain relief to earache to childbirth. Doctors also warned against overuse of marijuana, believing that too much consumption caused impotence, blindness and "seeing devils."

By the late 18th century, early editions of American medical journals recommend hemp seeds and roots for the treatment of inflamed skin, incontinence and venereal disease. Irish doctor William O'Shaughnessy first popularized marijuana's medical use in England and America. As a physician with the British East India Company, he found marijuana eased the pain of rheumatism and was helpful against discomfort and nausea in cases of rabies, cholera and tetanus.

The sea change in American attitudes toward pot came at the end of the 19th century, when between 2% and 5% of the U.S. population was unknowingly addicted to morphine, a popular secret ingredient in patent medicines with colorful names like "The People's Healing Liniment for Man or Beast" and "Dr. Fenner's Golden Relief." To prevent more of the country from being washed over with a morphine-induced golden relief, the government introduced the Pure Food and Drug Act in 1906, creating the Food and Drug Administration. While it didn't apply to marijuana and merely brought the distribution of opium and morphine under doctors' control, the regulation of chemical substances was a major shift in American drug policy. (See pictures of cannabis culture.)

It wasn't until 1914 that drug use was defined as a crime, under the Harrison Act. To get around states' rights issues, the act used a tax to regulate opium- and coca-derived drugs: it levied a tax on nonmedical uses of the drugs that was much higher than the cost of the drugs themselves, and punished anyone using the drugs without paying the tax. By 1937, 23 states had outlawed marijuana: some to stop former morphine addicts from taking up a new drug, and some as a backlash against newly arrived Mexican immigrants, some of whom brought the drug with them. Also in 1937, the Federal Government passed the Marihuana Tax Act, which made nonmedical use of marijuana illegal. Only the birdseed industry, which argued that hemp seeds gave birds' feathers a particularly shiny gloss, was exempted, and to this day birdseed producers are allowed to use imported hemp seeds treated so they don't sprout.

With an exception during World War II, when the government planted huge hemp crops to supply naval rope needs and make up for Asian hemp supplies controlled by the Japanese, marijuana was criminalized and harsher penalties were applied. In the 1950s Congress passed the Boggs Act and the Narcotics Control Act, which laid down mandatory sentences for drug offenders, including marijuana possessors and distributors.

Despite an easing of marijuana laws in the 1970s, the Reagan Administration's get-tough drug policies the following decade applied to marijuana as well. Still, the long-term trend has been toward relaxation. Since California became the first state to legalize medical marijuana in 1996, a dozen states have followed. Critics say the legalization of medical marijuana has sparked an underground pot culture in states that sanction its use  Los Angeles County district attorney Steve Cooley has estimated that there are about 1,000 illegally operated marijuana shops in that city alone. And although the Justice Department's newly unveiled policy will keep authorities from cracking down on those with legitimate marijuana prescriptions, all other smokers still run the risk of prosecution.