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"Today's report makes it more important than ever that we get past outdated, anachronistic views of marijuana," said John Walters, director of the White House Office of National Drug Control Policy.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.
While the drug's potency may be rising, marijuana users generally adjust to the level of potency and smoke it accordingly, said Dr. Mitch Earleywine, who teaches psychology at the State University of New York in Albany and serves as an adviser for marijuana advocacy groups. "Stronger cannabis leads to less inhaled smoke," he said.
The White House office attributed the increases in marijuana potency to sophisticated growing techniques that drug traffickers are using at sites in the United States and Canada.
A report from the office last month found that a teenager who has been depressed in the past year was more than twice as likely to have used marijuana than teenagers who have not reported being depressed - 25 percent compared with 12 percent. The study said marijuana use increased the risk of developing mental disorders by 40 percent.
"The increases in marijuana potency are of concern since they increase the likelihood of acute toxicity, including mental impairment," said Dr. Nora Volkow, director of the National Institute on Drug Abuse, which funded the University of Mississippi study.
"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," Volkow said.
But there's no data showing that a higher potency in marijuana leads to more addiction, Earleywine said, and marijuana's withdrawal symptoms are mild at best. "Mild irritability, craving for marijuana and decreased appetite - I mean those are laughable when you talk about withdrawal from a drug. Caffeine is worse."
The project analyzed data on 62,797 cannabis samples, 1,302 hashish samples, and 468 hash oil samples obtained primarily from seizures by law enforcement agencies in 48 states since 1975.