You can't kill a zombie with a pen. Jab it in the eye. Spear it in the chest. It will just keep shuffling along, moaning and snarling and trying to eat your brain.

Here comes one now.

"To think that marijuana today is the same benign thing, or, you know, the drug that the baby boomers used to take in the 1960s, '70s, '80s, it's totally false." That's Sgt. Pat Poitevin talking. He's a Mountie who works in "drug awareness" and he made that comment in an interview with CTV News, which aired a series of alarming stories about "potent pot" last week.

The reporter then summarized the facts, as told to her by Sgt. Poitevin. "He says marijuana today is genetically engineered for a bigger hit. Grow-ops have managed to boost the THC, that's the active chemical in marijuana, from one or three per cent in the '70s to 10 or 11, even 32 per cent today. Add to that the gallons of pesticides, fungicides, and fertilizers growers are using to boost production of the plants."

Cut to Sgt. Poitevin. "You don't have the same drug on the street any more."

There it is. A classic zombie.

It was first spotted staggering around, eating people's brains, in the mid-1970s. By 1980, when Newsweek urgently reported that today's pot is "as much as seven times stronger than 'grass' available four years ago," it was familiar to anyone who followed the news. In fact, through most of the years when pot was "benign," to use Sgt. Poitevin's description, police officers like Sgt. Poitevin were warning the public that marijuana was no longer the benign stuff they knew back in the day.

But to be fair, maybe, after decades saying the same thing over and over, it's finally true.

Is it? Let's break the claim down into its components, both explicit and implicit: One, the potency of marijuana is dramatically higher today than in the past; two, more potent pot is more dangerous pot; three, the difference in potency and danger between today's marijuana and the old stuff is so extreme they are effectively two different drugs.

The last component is the easiest to dispose of. Higher THC content in marijuana no more makes it a different drug than the higher alcohol content of vodka (40 per cent) makes it a different drug than beer (five per cent). That is nothing but rhetoric. It has no basis in pharmacological reality.

Whether pot is dramatically more potent is a trickier question. THC analyses done in the 1960s and 1970s are notoriously unreliable but in his 2002 book Understanding Marijuana, Mitch Earleywine, a professor of psychology at the University of Southern California, carefully examined the available American data and estimated that average pot potency was around 1.5 per cent in the disco era. By the late 1990s, it had risen to 4.5 per cent.

In Canada, the data are even sketchier. An RCMP report claimed the average potency in the late 1990s was 5.5 to six per cent. Health Canada data for samples seized in 2003 showed an average potency of 9.7 per cent.

So does that show a dramatic rise? Not really. The 2003 average is skewed high by a few very potent samples. Slightly more than one-quarter of the samples had a potency between zero and five per cent; one-third was between six and 10 per cent; another third was between 11 to 15 per cent. Only seven per cent of samples had a potency higher than 15 per cent.

On balance, the evidence suggests there has been an increase in average marijuana potency over the last several decades. But it's modest. And even that exaggerates the reality.

A 2004 report by the European Union's European Monitoring Centre for Drugs and Drug Addiction noted that "the natural variation in the THC content between and within samples of herbal cannabis or cannabis resin at one time or place far exceeds any long-term changes that may have occurred either in Europe or the USA." In other words: There has always been pot that wouldn't get a mouse high, pot that can get you buzzed, and pot that delivers the full Timothy Leary experience. There still is. That range dwarfs any modest change in average potency that may have occurred. And it makes a hash - pardon the pun - of claims that baby boomers' "benign" pot has been replaced by a dangerous new substance.

Lastly, there's the underlying assumption that more potent pot is more dangerous pot.

We know that's not true for many reasons. For one thing, marijuana is typically consumed in the form of hashish, not herbal marijuana, in several European countries. That's significant because hashish usually has a potency of 15 to 20 per cent. It can even be as much as 50 per cent THC. If "more potent" equals "more dangerous," that should be evident in cross-national comparisons. But as the EMCDDA report noted, it's not.

The main reason why potency isn't correlated with risk is simple. People aren't idiots. They vary their consumption to account for potency. Someone who says "enough" after drinking 18 ounces of beer will not, if given vodka instead, drink 18 ounces. Similarly, marijuana users will smoke less if the pot they are consuming is of a higher potency. (And since lung irritation from inhaling smoke is a clear harm of pot-smoking, that is a good thing.)

But still the zombie stumbles on. This column won't stop it. I know that from personal experience. In fact, all the data I used here came from a long essay I wrote about the "potent pot" myth six years ago. I didn't slay the beast then. I won't now.

I should note that I called Health Canada to get more recent figures. Health Canada said it no longer reports the THC content of seized marijuana. I also called the RCMP. I asked them to identify the source of Sgt. Poitevin's figures and to provide me with whatever THC content data they have. The RCMP informed me they do not analyse THC content and advised me to call Health Canada.

I believe this demonstrates the extent of the government's interest in knowing the truth. And why this zombie has been able to keep going all these years.