In 2004, 14.6 million Americans age 12 and older used marijuana at least once in the month prior to being surveyed. About 6,000 people a day in 2004 used marijuana for the first time—2.1 million Americans. Of these, 63.8 percent were under age 18. In the last half of 2003, marijuana was the third most commonly abused drug mentioned in drug-related hospital emergency department (ED) visits in the continental United States, at 12.6 percent, following cocaine (20 percent) and alcohol (48.7 percent).

Some of marijuana's adverse health effects may occur because THC impairs the immune system's ability to fight disease. In laboratory experiments that exposed animal and human cells to THC or other marijuana ingredients, the normal disease-preventing reactions of many of the key types of immune cells were inhibited. Research clearly demonstrates that marijuana has the potential to cause problems in daily life or make a person's existing problems worse. Depression, anxiety, and personality disturbances have been associated with chronic marijuana use. Because marijuana compromises the ability to learn and remember information, the more a person uses marijuana the more he or she is likely to fall behind in accumulating intellectual, job, or social skills. Moreover, research has shown that marijuana’s adverse impact on memory and learning can last for days or weeks after the acute effects of the drug wear off.