One of the strongest known strains of marijuana in the world is called Bruce Banner #3, a reference to the comic-book scientist whose alter ego is the Hulk. This is probably an appropriate nickname. With a THC concentration of 28 percent—THC is one of the key chemicals in marijuana—Bruce Banner #3 packs a punch. It's something like five times as potent as what federal researchers consider to be the norm, according to a 2010 Journal of Forensic Sciences paper. High Times marveled in a review: "Who knows what you’ll turn into after getting down with Bruce?"

As marijuana goes increasingly mainstream—and, crucially, develops into big (and legal) business—more super-potent novelty strains are likely to crop up. Bruce Banner #3 is the marijuana industry's answer to The End of History, an ultra-strong Belgian-style ale that the Scottish beer-maker Brewdog made in a specialty batch—which was then served in bottles inside taxidermied squirrels—in 2010. Its alcohol by volume was 55 percent. That's way, way stronger than most beers. "It’s the end of beer, no other beer we don’t think will be able to get that high," James Watt, one of the founders of Brewdog, told me when I visited the Brewdog headquarters in Scotland in 2010.

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Yet three years later, another Scottish brewery had whipped up a batch of barley wine called Snake Venom that boasted higher than 67 percent alcohol by volume.

This is human nature. Or maybe it's just capitalism. One person makes a superlative product, which prompts the next person to best them. Given the opportunity to try something extreme—the biggest, the strongest, the best, the craziest—plenty of people will go for it. But most people don't pick Snake Venom as their typical pint. And Bruce Banner #3 probably is not representative of the average joint.

But what is?

For years, people have talked about increasing marijuana potency. The idea that pot is getting stronger—much stronger than the stuff that got passed around at Woodstock, for instance—is treated like conventional wisdom these days. Maybe it shouldn't be.

"It's fair to be skeptical," said Michael Kahn, the president of Massachusetts Cannabis Research, a marijuana testing and research lab in New England. "Back then the predominant method for quantitation was gas chromatography, which is not quite appropriate for cannabinoid quantitation. This is because [it] heats up the test material before analysis, which also alters the chemical profile—including breaking down the THC molecule."

Kahn's lab uses a technique called liquid chromatography instead. Another potency tester, Denver-based CannLabs, uses a similar method. "Depending on what the sample is—flower, hash oil, hundreds of edibles ranging from ice cream to pasta sauce to seeds—you use different solvents to do the extraction," said Gennifer Murray, the CEO of CannLabs. "You mix it with a special solvent, basically shake it around, centrifuge it, and then it goes onto the instrument... That's the liquid chromatograph."

The federal government has been testing marijuana potency for more than 40 years, and has long acknowledged the limitations to its methodologies. Along with some of the issues with gas chromatography—which it was still using at least as recently as 2008—the National Institute on Drug Abuse potency testing has always depended on what researchers have been able to get their hands on. Since 1972, tens of thousands of test samples for the Potency Monitoring Program have come from law enforcement seizures, which have varied dramatically in scope and type. A drop in THC concentration in the early 1980s, for instance, was attributed to the fact that most of the marijuana researchers analyzed came from weaker domestic crops.

In National Institute on Drug Abuse studies over the past several decades, the age of samples has varied from a few weeks old to a few years old—and researchers made no attempt to compensate for the loss of THC during prolonged storage, according to a 1984 paper. They also get different results when taking into account how the potency of a particularly large seizure could skew the overall sample. For example, measured one way, researchers found what looked like a continuous and significant increase in potency in the late 1970s. But normalizing those findings showed there was "an increase up to 1977 with slight decline in 1978 and a significant decline in 1979," according to a 1984 paper in the Journal of Forensic Science.

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