One possible explanation, says Alex Stevens, a professor of criminal justice at the University of Kent, is the rise in Britain of high-strength “skunk”, which is grown domestically [ed. and more expensive], and the decline of cannabis resin, which is milder, and mostly brought in from north Africa.
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The extra potency also cuts down the ways cannabis can be used. The drug is often taken alongside others, such as ecstasy; such mixing is riskier when weed’s effects are heightened.
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The trend coincides with a decline in smoking. The most common way to take cannabis in Britain is still to mix it with tobacco, so non-smokers are less likely to take up the drug. In America, where cannabis tends to be smoked by itself, there seems to be no such link: the number of smokers has been falling since 2005, while that of cannabis users has risen.
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Young Britons are becoming generally better-behaved than their peers: nearly all intoxicants, from alcohol to cocaine, are falling out of fashion, even as they become more popular elsewhere. British cannabis has become dearer, too. A quarter-ounce (7 grams) of skunk has risen in price from £30 in 2006 (then $55) to £50 ($77). That is about two-thirds more expensive than in Spain, for instance.
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Some young people are turning to other substances. Since 2013 there has been a 25% rise in the number of new synthetic “legal highs” reported; about a third were cannabis imitators.
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Indeed, it is unlikely that the decline in cannabis use is down to anything much the government has done, or could do. Jane Mounteney at the EMCDDA notices a pattern among some large European countries: cannabis use peaks when around 20% of young adults report use in the past year, then declines. It could be that Britain got into cannabis early, and now has simply got bored with the habit.