It's one of the most convenient arguments parents can use to discourage their children from doing precisely what they did in their teenage heyday: Sure, I smoked pot, but it's so much stronger now than it was in our day that you can't even compare them. Smoking pot today is too dangerous.

D.A.R.E. (the Drug Abuse Resistance Education program) and the Partnership for a Drug-Free America offer this line to parents. And it seems like an excuse made to order for former pot-smoking baby boomers seeking to reconcile their own reckless past with a useful anti-drug message.

But in this case -- and we know you hate it when it turns out this way -- the claim appears to be partly true, according to the National Institute on Drug Abuse (NIDA). A key study published in the Journal of Forensic Sciences in 2000 reported that marijuana seized in drug arrests during the late 1990s was twice as potent as that seized in the late 1980s -- and nearly four times as potent as that seized in the 1970s. Data from the U.S. Drug Enforcement Administration support this conclusion. Medical and policy experts are divided, however, over whether stronger pot is more dangerous.

The main active ingredient that produces the high in marijuana is delta-9-tetrahydrocannabinol, or THC. On average, the pot on the streets 30 years ago was 1 percent to 2 percent THC; today the average is about 4 percent.

"There's no question marijuana is more potent today," said Mahmoud El Sohly, director of the Marijuana Project, a NIDA-funded program at the University of Mississippi that grows marijuana for study. "There has been a constant increase in potency" from the mid-1970s through the mid-1980s, he said. The levels fluctuated until around 1990 and then consistently increased again.

Yet smokers in the 1960s and 1970s certainly had access to more potent varieties of marijuana -- what Mom and Dad may recall as "killer weed." Sinsemilla, made from just the buds and flowering tops of female plants, averages 7.5 percent THC. Hashish, the sticky resin from the female plant flowers, can be as high as 28 percent THC. Hashish oil can contain up to 43 percent THC.

Still, most pot sold on the streets in the 1970s was low-quality cannabis from Mexico -- what Mom and Dad may remember as "bunk weed," according to Steven Hager, editor-in-chief of High Times magazine. The three main species of marijuana, cultivated for thousands of years, haven't changed much in the past 30 years, Hager said. What has changed is the quality and variety available to U.S. teenagers.

A generation ago, the typical bag of pot contained mostly leaves, which don't have as much THC as the buds, along with stems and seeds, which have very little potency. A typical bag of pot today contains more buds, Hager said. Also, most marijuana is now grown domestically, often hydroponically, where controlled growing methods result in a plant with consistently higher THC levels, which ultimately fetch a higher price on street.

Where's the Danger?

Here's the problem for boomer parents who inhaled: Just because today's pot is stronger doesn't necessarily mean it's more dangerous.

Comparing marijuana strength through the years is "absurd," according to Lester Grinspoon, an emeritus professor at Harvard Medical School, who consults patients, many of them elderly, on using marijuana to relieve pain and nausea. "The whole issue on potency is a red herring," he said. "The more potent the pot, the less you use."

Grinspoon said that studies have shown -- and his patients' experiences confirm -- that marijuana users smoke until they feel high -- or, as he prefers to say, "achieve symptom relief," -- and then stop, whether it took two hits or an entire joint. In this regard, today's higher-potency pot is no more "dangerous" than the bunk weed of yesteryear, he said.

NIDA data from an ongoing project called Monitoring the Future support the notion that smokers aren't getting higher from more potent joints. Surveys show that joint sizes have dropped over the years from half a gram to about a quarter of a gram because more experienced smokers know that a smaller amount of pot can go a longer way toward making the smoker high.

"Even as the cannabis has gotten stronger, students [in the NIDA surveys] don't report that they're getting any higher," said Mark Kleiman, a professor of policy studies at the School of Public Policy and Social Research at the University of California, Los Angeles Kleiman.

El Sohly disagrees. "This is a drug that produces tolerance," he said. "The smoker has to increase the amount he uses . . . just like alcohol" to get the same high over time. Further, he argues, smoking more pot is dangerous because the drug is bi-phasic: a "reasonable dose will produce euphoria," El Sohly said, but a higher dose will produce unpleasant and potentially harmful effects, such as paranoia and violence.

High-potency pot ups the ante, producing a higher tolerance more quickly; and such higher levels of THC, quickly pumped into the body, can be great enough to induce the drug's more negative effects, El Sohly said.

To Tell the Truth

So what are concerned parents who smoked pot to do when discussing drugs with their children? Lying, you may have guessed, should not be an option, according to Howard Simon, a spokesman for the Partnership for a Drug-Free America, a private-sector coalition that collaborates with the federal Office of National Drug Control Policy.

"Honesty makes a lot of sense," he said. "If you smoked marijuana, you don't want to lie about it with your children, because that will only destroy your credibility in the long run."

Simon said this doesn't mean you have to divulge your life history with your kids. "Don't answer the questions you haven't been asked," he said (perhaps inadvertently suggesting to teenagers a tactic their parents would rather not face). Instead, tell your kids what you learned from your experience. Perhaps you have seen lives ruined by marijuana use.

"Every boomer parent knows, or used to know, people who smoke too much pot and don't seem to do much else," said Kleiman. "They can always serve as a bad example."

Kleiman is queasy about half-truths, but he is willing to agree with part of the "more dangerous" argument. He is concerned, along with NIDA and groups such as D.A.R.E. America, about children's beginning to smoke marijuana at younger and younger ages. Here is where parents who smoked pot, perhaps in college, can caution their children.

Kleiman said he knew of no one who smoked in his high school in 1968, but many people smoked at his college. A few years later, pot was creeping into the high schools. NIDA data indicate that by 1979 more than 50 percent of high school seniors had tried marijuana. Through the 1980s, junior high school students were smoking pot. By 1990, pot had made its way into grade schools. Since 1996, yearly figures show that more than 20 percent of eighth-graders report having smoked pot at least once.

Marijuana use at such a young age, the experts agree, is a clear problem. A child is not responsible enough to use the drug, as is also true of drinking alcohol or having sex. The drug itself can interfere with cognitive development during adolescence. And being high can interfere with schoolwork and social life. An adolescent is less likely to know when it might be appropriate to use marijuana than a college student is.

"A teenager who spends the 10th and 11th grades stoned all the time will miss some growing up that can't be accomplished later," said Kleiman. (Data suggest that most people stop smoking pot regularly by the time they enter the workforce, regardless of when they started.) Nevertheless, there is still too little data on what exactly happens to the brain from years of smoking pot.

"I'd be pretty tough on a 14-year-old," Kleiman said. "I'd say, 'You're going to have to make a living with that mind.' "

Christopher Wanjek is a frequent contributor to the Health section.