Overview: Marijuana is a common name for the drug contained in the leaves and flowering tops of the hemp plant, known scientifically as cannabis sativa. The plant has been cultivated for both its fiber content and its medicinal and psychoactive effects for at least 4,000 years, but has generated heated controversy and emotional debate in this country since use was outlawed in 1937. Slang names include pot, reefer, chronic, grass, and weed.

Actions/Effects: The main psychoactive drug in marijuana is delta-9 tetrahydrocannabinol, or THC. It activates receptors in the brain called anandamides, which trigger the drug's effects. Subjective effects include mild sensory distortions, feelings of euphoria, and increased appetite. Objective effects include reddening of the eyes and increased heart rate.

Risks/Side Effects: The most common adverse reactions linked to marijuana are psychological and include feelings of unease, anxiety, or restlessness, which usually pass without outside intervention. Other potential risks include smoking-related respiratory damage, temporary impairment of short-term memory, and psychological dependence.

Medical Uses: Marijuana's medical status remains controversial-despite approval by voters or lawmakers in eleven states, and legislation enacted in New Mexico in 2007, to authorize its medical use. Although marijuana has no federally-sanctioned medical uses, it's been used through the ages to treat a variety of ills. Currently (and unofficially), it's used by sufferers of glaucoma to reduce pressure inside the eye, by cancer patients (to reduce the vomiting caused by chemotherapy), and by people with AIDS to combat the appetite loss and "wasting syndrome" associated with that disease.

Duration: 3-4 hours, although subtle effects may linger for several more hours.

Trends: Marijuana use appears on the decline, as reflected in a 2006 nationwide survey of high school seniors. The number of students reporting any lifetime use dropped for the fifth year in a row, falling from 49.0 percent in 2001 to 42.3 percent in 2006. Past-month use over the same period showed a similar drop, falling from 22.4 to 18.3 percent.

Demographics: Although marijuana use extends across all demographic groups, it continues to be most prevalent among the young. More than 97 million Americans have tried pot, with 2.1 million trying it for the first time last year. An estimated 14.5 million Americans smoke it on a regular basis.