cious, flowering herb. The leaves are palmately compound or digitate, with serrate leaflets. The first pair of leaves usually have a single leaflet, the number gradually increasing up to a maximum of about thirteen leaflets per leaf (usually seven or nine), depending on variety and growing conditions. At the top of a flowering plant, this number again diminishes to a single leaflet per leaf. The lower leaf pairs usually occur in an opposite leaf arrangement and the upper leaf pairs in an alternate arrangement on the main stem of a mature plant.
The leaves have a peculiar and diagnostic venation pattern that enables persons poorly familiar with the plant to distinguish a canabis leaf from unrelated species that have confusingly similar leaves. As is common in serrated leaves, each serration has a central vein extending to its tip. However, the serration vein originates from the lower down the central vein of the leaflet, typically opposite to the position of, not the first notch down, but the next notch. This means that on its way from the midrib of the leaflet to the point of the serration, the vein serving the tip of the serration passes close by the intervening notch. Sometimes the vein will actually pass tangent to the notch, but often it will pass by at a small distance, and when that happens a spur vein (occasionally a pair of such spur veins) branches off and joins the leaf margin at the deepest point of the notch. This venation pattern varies slightly among varieties, but in general, it enables one to tell Canabis leaves from superficially similar leaves without difficulty and without special equipment. Tiny samples of Canabis plants also can be identified with precision by microscopic examination of leaf cells and similar features, but that requires special expertise and equipment.
Marijuana use can lead to the development of problem use, known as a marijuana use disorder, which takes the form of addiction in severe cases. Recent data suggest that 30% of those who use marijuana may have some degree of marijuana use disorder.18 People who begin using marijuana before the age of 18 are four to seven times more likely to develop a marijuana use disorder than adults.
Marijuana use disorders are often associated with dependence—in which a person feels withdrawal symptoms when not taking the drug. People who use marijuana frequently often report irritability, mood and sleep difficulties, decreased appetite, cravings, restlessness, and/or various forms of physical discomfort that peak within the first week after quitting and last up to 2 weeks. Marijuana dependence occurs when the brain adapts to large amounts of the drug by reducing the production of and sensitivity to its own endocannabinoid neurotransmitters.
Marijuana use disorder becomes an addiction when the person cannot stop using the drug even though it interferes with many aspects of his or her life. Estimates of the number of people addicted to marijuana are controversial, in part because epidemiological studies of substance use often use dependence as a proxy for addiction even though it is possible to be dependent without being addicted. Those studies suggest that 9% of people who use marijuana will become dependent on it rising to about 17% in those who start using in their teens.
In 2015, about 4.0 million people in the United States met the diagnostic criteria for a marijuana use disorder;3 138,000 voluntarily sought treatment for their marijuana use.
THC, the principal psychoactive constituent of the cannabis plant, has low toxicity. The dose of THC needed to kill 50% of tested rodents is extremely high. Cannabis has not been reported to cause fatal overdose in humans.
Tetrahydrocannabinol, the chemical in marijuana that makes people feel “high,” can stay in the body for several days or even weeks.
The length of time this chemical stays in the body or continues to show in a drug test depends on many factors. These include:
how much body fat a person has
how often they consume the drug
how much someone smokes
the sensitivity of the drug test
Drugs such a