<P> An oft - repeated legend is that today's cannabis is at least an order of magnitude stronger than in the past (and by implication much more dangerous). THC levels are allegedly 10, 20 or even 30 times higher than in the 1960s or 1970s . Although potency levels have risen in several countries (such as the US and UK), the actual increases have been much more modest (almost threefold from 1982 to 2007 in the US) and high - potency strains have always existed, as have various concentrated forms of cannabis . Furthermore, potency of seized samples was not tested before 1971, leaves were not distinguished from buds by testers at first, and samples from before the early 1980s (when testing and storage procedures were changed) were often degraded, making comparisons going that far back inaccurate . Non-representative sampling was also an issue . Since most of the increase happened after 2000, this legend can be considered an example of ostension (people have been making such claims as far back as the 1970s). </P> <P> A related claim, especially in the UK, is that the cannabidiol / THC ratio has decreased over the past few decades, resulting in a new and presumably more dangerous form of cannabis that never existed before (since CBD is thought to attenuate some of the negative side effects of THC). While there is little to no reliable data before 2005 on such ratios in the UK, making comparisons to the past impossible, the US data going back to the 1970s shows little to no clear trend, and there have always been strains with extremely low ratios . Ratios are also known to vary widely between strains and growing / harvesting methods . </P> <P> Some versions of this legend claim the potency change is due to "genetic modification," a term which often evokes fear in the popular consciousness, but there is no hard evidence that anything other than selective breeding and enhanced growing techniques are behind the change . "Genetic modification" insofar as attempting to emphasize desirable traits by the practice of selective breeding is standard practice across many areas of farming, including the production of cannabis . It is likely that the term "genetic modification" is used by people who do not understand that selective breeding is not the same as genetic engineering . </P> <P> Another claim about cannabis is that impairment due to smoking it lasts many days after using it, since THC is stored in the fat cells, leading to a low - grade haze long after the primary high is gone . This myth is based primarily on anecdotal evidence and the known fact that urine drug tests remain positive for at least several days after using, and longer for regular users . The tests however measure non-psychoactive metabolites, not active THC . And the blood levels of THC generally fall well below the psychoactive threshold within 2--4 hours of smoking (4--8 hours after oral use). A cannabis equivalent of a hangover may occur the morning after taking high doses, but even that ends much sooner than the legend suggests . Although smoking cannabis on a Friday night would most likely test positive in a urinalysis Monday, no amount of the psychoactive effect would be retained by that time . </P>

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