Marijuana, a mixture of the leaves, flowering tops, and other parts of the plant Cannabis sativa, is a chemically complex substance. It contains over 400 chemical constituents including simple acids, alcohols, more complex hydrocarbons, terpenes, and some 61 compounds unique to the cannabis plant called cannabinoids. The cannabinoid Δ-9-tetrahydrocannabinol (THC) is the principal psychoactive ingredient, although other constituents may modify its action or have psychoactive effects of their own. This compound is present in varying amounts. High-grade seedless varieties of cannabis called sinsemilla may have percentages (by weight) of THC as high as 11%. Street marijuana is believed to have increased in average THC content—from less than 1% in the 1960s and 1970s to as high as 5% in the mid-1980s. Variability in marijuana’s potency and a failure to adequately specify the material used has often made the results of earlier marijuana research (before 1960) hard to interpret. Cannabis cigarettes supplied by the National Institute on Drug Abuse for research use typically contain 1–2%.
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