WASHINGTON - Marijuana potency increased last year to the highest level in more than 30 years, posing greater risks to people who may view the drug as harmless, according to a report released today by the White House.

The latest analysis from the University of Mississippi's Potency Monitoring Project tracked the average amount of THC, the psychoactive ingredient in marijuana, in samples seized by law-enforcement agencies from 1975 through 2007. It found that the average amount of THC reached 9.6 percent in 2007, compared with 8.75 percent the previous year.

The 9.6 percent level represents more than a doubling of marijuana potency since 1983, when it averaged just under 4 percent.

"Today's report makes it more important than ever that we get past outdated, anachronistic views of marijuana," said John Walters, director of the White House Office of National Drug Control Policy. He cited baby-boomer parents who might have misguided notions that the drug contains the weaker potency levels of the 1970s.

"Marijuana potency has grown steeply over the past decade, with serious implications in particular for young people," Walters said. He cited the risk of psychological, cognitive and respiratory problems, and the potential for users to become dependent on drugs such as cocaine and heroin.

The White House office attributed the increases in marijuana potency to sophisticated growing techniques in use at sites in the United States and Canada.

"Particularly worrisome is the possibility that the more potent THC might be more effective at triggering the changes in the brain that can lead to addiction," said Dr. Nora Volkow, director of the National Institute on Drug Abuse, which funded the University of Mississippi study.

The project analyzed data on 62,797 cannabis samples obtained primarily from seizures by law-enforcement agencies in 48 states since 1975. *