Marijuana is the most commonly used illicit drug in America today. The term marijuana, as commonly used, refers to the leaves and flowering tops of the cannabis plant.

A tobacco-like substance produced by drying the leaves and flowering tops of the cannabis plant, marijuana varies significantly in its potency, depending on the source and selection of plant materials used. The form of marijuana known as sinsemilla (Spanish, sin semilla: without seed), derived from the unpollinated female cannabis plant, is preferred for its high THC content.

Marijuana is usually smoked in the form of loosely rolled cigarettes called joints or hollowed out commercial cigars called blunts. Joints and blunts may be laced with a number of adulterants including phencyclidine (PCP), substantially altering the effects and toxicity of these products. Street names for marijuana include pot, grass, weed, Mary Jane, Acupulco Gold, and reefer.

Although marijuana grown in the United States was once considered inferior because of a low concentration of THC, advancements in plant selection and cultivation have resulted in highly potent domestic marijuana. In 1974, the average THC content of illicit marijuana was less than one percent; in early 1994, potency averged 5 percent. The THC of today's sinsemilla ranges up to 17 percent.

Marijuana contains known toxins and cancer-causing chemicals which are stored in fat cells for as long as several months. Marijuana users experience the same health problems as tobacco smokers, such as bronchitis, emphysema and bronchial asthma. Some of the effects of marijuana use also include increased heart rate, dryness of the mouth, reddening of the eyes, impaired motor skills and concentration, and frequently hunger and an increased desire for sweets. Extended use increases risk to the lungs and reproductive system, as well as suppression of the immune system. Occasionally, hallucinations, fantasies and paranoia are reported.