Pot today is "as much as seven times stronger than the 'grass' available four years ago," warned an article in Newsweek. That was in 1980. It was already a well-worn theme. And it continues to be a media standard, appearing several times in this newspaper over just the last two weeks. Marijuana strength has "skyrocketed from the giddy low potency highs experienced by happy sixties hippies," Margret Kopala wrote, a claim that has been repeated so often by so many sources that journalists and politicians take it as an accepted fact, something "everybody knows." But this claim, although widely accepted, is substantially inaccurate. In 2004, the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA), a European Union agency, conducted a comprehensive review of available evidence in Europe, the United States, Australia and New Zealand. "Statements in the popular media that the potency of cannabis has increased by 10 times or more in recent decades are not supported by the limited data that are available from either the USA or Europe," the report concluded. In New Zealand, the report added, researchers found no increase in potency between 1976 and 1996. In Australia, there was only a "modest" rise. The EMCDDA report also cautioned that increases in potency should be kept in context. "The natural variation in the THC content between and within samples of herbal cannabis or cannabis resin at one any one time or place far exceeds any long-term changes that may have occurred either in Europe or the USA. This natural variation is even greater when material from different geographic locations is examined." One reason why the myth of exploding pot potency started is that measurement of potency prior to the 1980s is very poor. There was no established protocol and improperly stored marijuana -- such as bags of weed sitting in police evidence rooms for weeks -- rapidly loses THC content. As a result, samples tested during the era of lava lamps and bell bottoms were commonly found to have THC content of less than one per cent -- a patently absurd result because marijuana with less than one per cent is incapable of producing any psychoactive effect. Claims today by U.S. drug czar John Walters and others that pot potency has exploded to such an extent that marijuana is no longer the drug baby boomers remember are partly the result of using these faulty baselines in potency calculations. To untangle the reality, Mitch Earleywine, a professor of psychology at the University of Southern California, carefully assessed the credible evidence on potency in his book Understanding Marijuana (Oxford University Press 2002). He concluded average pot in the early 1970s was likely 1.5 per cent THC. By the late 1990s, that had risen to 4.5 per cent. According to the U.S. government's Marijuana Potency Monitoring Project, the average potency in 2001 was 5.2 per cent. According to an RCMP report, seized samples of Canadian pot in the late 1990s had an average THC content of 5.5 to six per cent -- marginally above the U.S. average. Almost one-third of the pot tested at the time was below three per cent THC. In 2003, the average potency of the seized samples tested by Health Canada was 9.7 per cent. However, that average is misleading because it is skewed high by a small number of high-potency samples. Slightly more than one-quarter of the 2003 samples had a THC content between zero and five per cent. One-third came in between six and 10 per cent, while another one-third were in the range of 11 to 15 per cent. Just seven per cent of marijuana tested had a potency 15 per cent or greater. Clearly, growers have successfully used traditional selective breeding principles and intensive cultivation to produce strains of marijuana that are far more potent than average pot 30 years ago. But these strains are still relatively rare. Most marijuana today is only modestly more potent than that remembered wistfully by baby boomers, and there are no grounds for claiming, as many politicians and police officers have, that marijuana today is so much stronger as to be a wholly different drug. The evidence also shows there is no major difference in potency between Canadian and U.S. marijuana. Some strains of Canadian marijuana are very potent but, as a report of the National Drug Intelligence Center of the U.S. Department of Justice noted, "growers in both Canada and the United States have access to the same strains of cannabis seeds and the same cultivation techniques." There is simply no solid evidence to show that Canadian marijuana is uniquely potent. It is an urban legend repeated by politicians, police officers, journalists, gullible customers and dealers happy to pocket a premium for a "unique" product. (Ms. Kopala's claim that "90 per cent" of Canadian-grown marijuana is smuggled into the United States is dubious. Numbers such as these are simply guesses made by police officers who have no evidence to support them. RCMP reports frankly admit they don't know how much pot is being smuggled. And a report from the National Drug Intelligence Center of the U.S. Department of Justice noted that although "a number of international publications have reported that approximately 50 to 60 per cent of the marijuana produced in Canada is smuggled into the United States annually ... in-depth analysis and consultations between officials of both countries have concluded these estimates cannot be substantiated through current reporting.") Is More Potent Pot More Dangerous? This is the tacit assumption behind fears of rising marijuana potency, but few commentators have attempted to provide any evidence to support it. One who recently did was the U.S. drug czar. "The number of Americans admitted to hospital emergency wards because of marijuana use has doubled to 120,000 annually in the past five years," said a recent news story in this newspaper, paraphrasing Mr. Walters at a Washington news conference. Mr. Walters also noted the number of U.S. teens seeking treatment for marijuana dependency has increased so greatly that it now exceeds the treatment demand for all other drugs combined. The "big new factor" accounting for this alarming shift, Mr. Walters said, is "the enormous growth of very-high-potency marijuana coming from Canada." U.S. government reports have repeatedly confirmed that Canadian marijuana -- high-potency or not -- only accounts for a tiny fraction of the total marijuana available in the U.S., so it's difficult to accept Mr. Walters' claim that Canadian pot is responsible for such dramatic trends. It's also telling that Canadian hospitals and treatment programs are not being overwhelmed by desperate Canadian pot smokers. However, in other statements, Mr. Walters made the same claims while leaving Canada out of the equation. The surge in emergency-room visits and treatment admissions is proof of the damage done by high-potency marijuana, he argued. Is it? As it turns out, Mr. Walters was playing fast and loose with the statistics. The emergency-room data Mr. Walters' cites are not a tally of people whose marijuana use caused them to rush to hospital. Rather, the data come from a system that records emergency-room "mentions." To be counted, a person checking into emergency merely has to mention marijuana, in any context. A woman who says she was smoking a joint when she tripped over a footstool and sprained her ankle would be added to the tally. So would someone who tells a doctor he was quietly smoking marijuana in his living room when he was hit by a stray bullet from a gunfight outside. Obviously, emergency "mentions" will be heavily influenced by patients' beliefs about acceptable conduct. If they believe they will be looked down on, or suffer legal consequences, for mentioning marijuana use, they will be reluctant to do so. If they think it's no big deal, they will be more likely to mention it. As these beliefs rise and fall, so will the data. Similarly, the beliefs of doctors and nurses will influence their decision to record a marijuana mention. Mr. Walters' use of treatment data is even more misleading. It is true that there has been a surge in the numbers of U.S. teens entering marijuana treatment. But Mr. Walters neglected to mention that almost 60 per cent of those admissions were ordered by judges. Nor did he mention that the proportion of people entering treatment as a result of a court order has risen substantially. This reflects a sea-change in U.S. criminal justice. Since 1990, there has been an explosion in the number of U.S. "drug courts" which allow individuals charged with some drug offences to avoid jail if they follow a carefully supervised treatment program. At the same time, the idea of "therapeutic justice," as it is sometimes called, has been widely adopted by regular U.S. courts. As a result, Americans charged with pot offences today are routinely given a choice between punishment or treatment. Not surprisingly, most choose treatment. Mr. Walters' dubious statistics aside, is there any evidence that more potent marijuana is more dangerous? Experience with hashish suggests not. Hashish is a typically very potent derivative product of marijuana that is typically between 15 and 20 per cent THC but can be as potent as 50 per cent THC. Consumption of hashish varies greatly from place to place but the EMCDDA report notes that hashish is more common than herbal marijuana in several European countries. If higher potency meant greater danger, the damage should be obvious in countries where hashish predominates. But that's not the case. In fact, there doesn't seem to be any evidence that hash consumers suffer worse harms than marijuana smokers. Another substance that casts doubt on the idea that greater potency means greater risk is Marinol, a pill developed with the strong backing of the U.S. government and officially approved for the control of nausea in cancer patients and appetite stimulation in AIDS patients. Marinol is synthetic THC. Its potency is 100 per cent. The 2004 EMCDDA report found there is "an absence of direct evidence of any clear additional health risk" posed by more-potent marijuana, but the report also added that despite all the talk about higher potency there hasn't been a great deal of systematic research on the subject. The report cited a recent Dutch government study that concluded "higher-potency cannabis products did not pose any additional risk than those for present cannabis products as a whole, either to the individual, to society, to public order or criminality." The medical marijuana provided to Dutch patients, the report added, is a highly potent 18 per cent THC. It may seem counter-intuitive that more potent pot may not be more dangerous, but the explanation is actually straightforward. First, the assumption that higher potency is more dangerous rests on another assumption: that THC is the source of whatever harms marijuana can cause. But THC is only one of hundreds of components of marijuana smoke. Unless THC is shown to be the specific cause of a specific harm, higher THC content is irrelevant. But there's an even more basic explanation: Most cannabis users, like most ordinary users of alcohol and other drugs, are not seeking to become as inebriated as possible for as long as possible. "Most people on a six-point scale say they want to get to a three or a four and they don't want to spend more than about three or four hours high," says Mitch Earleywine, citing surveys he and another researcher have conducted among pot smokers. Users of a drug -- any drug -- regulate their intake according to potency and desired effect. A person sipping vodka with a potency of 40 per cent alcohol will consume a much smaller volume of liquid than he would if he were drinking wine with a potency of 16 per cent alcohol or beer at five per cent. Similarly, someone smoking pot with 15 per cent potency will inhale far less smoke than he would if he were smoking pot with two per cent potency. (The parallel with alcohol breaks down in one respect, however: Someone who drinks an entire bottle of vodka can overdose and die whereas it is physically impossible for a person to consume enough marijuana to cause a fatal overdose.) Although this simple equation is rarely mentioned in debates about marijuana, it is common sense among pot smokers. And it has been borne out by research, including a University of Michigan study of teen pot smokers which concluded that rising THC levels caused users to inhale "less marijuana as measured by volume." Mr. Earleywine notes that surveys asking users how high they get show no change since the 1970s, despite the increase in marijuana potency. "It's just that they're smoking less of it, rather than getting higher." Oddly enough, this suggests that rising marijuana potency may produce a modest health benefit. "When smoking stronger pot, you smoke less and you have less exposure to tars and respiratory irritants," Mr. Earleywine says, adding with a laugh, "so in some ways it's worth smoking the best pot you can afford."