Endogenous cannabinoids assist in regulation of hunger, pain perception, inflammation, and stress responses. Tetrahydrocannabinol (THC), a component of cannabis, activates cannabinoid receptors, producing effects that are often emotionally pleasing and cognitively interesting. THC effects impair complex tasks, such as driving. Addiction develops in 8--10 percent of all cannabis users, and in about 25 percent of daily users. Regular adolescent users are especially vulnerable. Adverse outcomes of cannabis addiction include too much time spent intoxicated, important activities given up, worsening of psychological problems, and failed attempts to stop use. The withdrawal syndrome includes irritability, anxiety, depression, and sleep difficulties. Long-term heavy use of cannabis is associated with academic failure and subtle cognitive impairment. Medical uses of cannabis include relief of nausea, appetite improvement, and lessened neuropathic pain. Medical use may increase cannabis addiction, a risk somewhat similar to that of other, more traditional medications for pain, anxiety, and attention disorders. Because cannabis is now a commercial product, its potency has increased in recent years.