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Is weed getting stronger?
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Is pot stronger these days? Some folks state as fact that parents should realize "pot these days is stronger than when they smoked." Any evidence? -- Dinsdale

Yes, pot is stronger than in the old days. This is bad?

Reliably determining marijuana potency has its challenges, starting with the fact that we're talking about a generally illegal substance. The Drug Enforcement Administration draws its samples almost exclusively from seized imported herb and sees little domestic product, which is markedly different.

Cannabis potency is typically measured by tetrahydrocannabinol (THC) content, though that's just one of several pharmacologically active compounds. Defining terms is key, as cannabis can refer to the herb itself, resin produced from it, or a pharmaceutical extract of the resin. Cannabis herb potency can vary widely depending on plant variety and production method-samples from more than a dozen European countries in 2003 showed THC content from less than 1 percent to almost 14 percent.

Given these variables, it's not difficult to find backing for alarmist claims. But here are the facts.

• Testing before the mid-1970s is suspect due to sampling problems and poor storage, but one study found average THC levels for all forms of marijuana, including garden-variety marijuana, high-powered sinsemilla, and barely-beats-oregano ditchweed, were well under 3 percent until about 1982, with samples collected in the 1975-1976 time frame having under 1 percent THC. From 1975-2009 the potency of imported cannabis seized by the DEA rose, eventually reaching 6 or 7 percent. Domestic herb showed more fluctuation, peaking at around 4 percent in the late 1990s but dropping to 2 percent a decade later.

• Data collected by the Marijuana Potency Monitoring Project (and how's that for a dream job?) found average THC content of basic marijuana increased from 1 percent in 1980 to 4 percent by 1997, while the average THC potency of all types of cannabis increased from 2 percent to 4.5 percent over the same period.

• A later study by the same group, examining more than 46,000 cannabis samples seized between 1993 and 2008, found cannabis potency increased over that period from 3 percent to 6 percent. The potency of sinsemilla, extracted from seedless female plants, rose till the late ‘90s and since then has bounced around 11 to 12 percent.

To summarize, all these studies show THC potency doubling or tripling since the late 1970s.

The overall numbers mask a lot of regional variation. The mean THC value of European cannabis increased only slightly from 1999-2003, hovering around 5 to 6 percent, but this may be a false result as it lumps in locally-cultivated herb with imported products. Cannabis potency is affected by oxidation-store your pot in the open air at room temperature and more than a sixth of its potency can evaporate annually. Given that imports could be months old and exposed to high temperatures during shipment, it's easy to see why they might be less potent.

Comparing locally grown cannabis to imports, we can see some sharp increases in potency over a short period of time. The UK saw a nearly 100 percent increase in locally-produced sinsemilla strength from 1995-2002, presumably the result of hydroponic cultivation, fine-tuned grow lighting, and propagation of female plants via cuttings.

In European studies we found, imported product was of poorer quality than domestic stuff, less than half as potent in some years. The situation is reversed in the U.S. The 1993-2008 study cited above found the potency of imported weed surpassed domestic in 2000 and has pulled away ever since.

So potency on average has risen significantly, though not to the extent some claim. That said, averages don't tell the whole story-there's some truly devastating smoke out there. One variety of Dutch cannabis, nederwiet, has tested at THC levels as high as 40 percent. We can say with reasonable confidence that shifting from the 1.2 percent marijuana typical of 1980 to the five or even ten times more potent stuff available now won't blow the cortical fuses.

But 33 times? Gotta level with you, man. There I'm not so sure.