Reported by Drs. Tiffany Chao and Shari Barnett:

Teenagers lighting joints may end up less bright, according to new research released Monday.

In a study of more than 1,000 adolescents in New Zealand, those who began habitually smoking marijuana before age 18 showed an eight-point drop in IQ between the ages of 13 and 38, a considerable decline.

The average IQ is 100 points. A drop of eight points represents a fall from the 50th percentile to the 29th percentile in terms of intelligence.

The research, published in the Proceedings of the National Academy of Sciences, charted the IQ changes in participants over two decades.

Researchers tested the IQs of all of the study subjects at age 13 before any habitual marijuana use. Researchers then split the study into five "waves" during which time they assessed cannabis use - ages 18, 21, 26, 32, and 38. They again tested IQ at age 38. The authors also controlled for alcohol use, other drug use and education level.

The eight-point drop in IQ was found in subjects who started smoking in adolescence and persisted in "habitual smoking" - that is, using cannabis at least four days per week - in three or more of the five study waves.

People who started smoking in adolescence but used marijuana less persistently still had a hit to their IQ's, but it was less pronounced than the group that used it early and persistently.

In contrast, those who never used marijuana at all gained nearly one IQ point on average.

Madeline Meier, lead researcher and a post-doctoral associate at Duke University, said that persistent use of marijuana in adolescence appeared to blunt intelligence, attention and memory. More persistent marijuana use was associated with greater cognitive decline.

"Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects," Meier writes in the study.

Of particular worry is the permanence of these effects among people who began smoking marijuana in adolescence. Even after these subjects stopped using marijuana for a year, its adverse effects persisted and some neurological deficits remained. People who did not engage in marijuana smoking until after adolescence showed no adverse effects on intelligence.

Experts in child development said the reasons adolescents may be more susceptible to the harmful effects of marijuana may have to do with a substance called myelin. Myelin can be thought of as a kind of insulation for nerve cells in the brain that also helps speed brain signals along - and in adolescent brains, the protective coating it forms is not yet complete.

"Frontal lobe myelination is not fully completed until age 25 years or so, and the pre-myelinated brain is more susceptible to damage from neurotoxins," says Dr. Richard Wahl, director of adolescent Medicine at the University of Arizona. "Cannabis, most likely, is a neurotoxin in high and continuous doses."

The study appears to lend credence to "stoner" stereotypes in popular media. However, no previous studies can provide data for this phenomenon, since establishing whether a drop in IQ has actually occurred requires that a baseline IQ be obtained before a person ever started using marijuana. This study did just that.

"[The findings] provide evidence for the actual - rather than ideological and legal - basis for concerns regarding cannabis use," said Dessa Bergen-Cico, a assistant professor of public health, food studies and nutrition at Syracuse University. "These findings reinforce recommendations on the importance of primary prevention, evidence based drug education and policy efforts targeting not only adolescents, but elementary age children before they start."

Though the study was conducted among New Zealand young people, the findings could be extended to adolescents in the United States as well. According to statistics released in June by the U.S. Centers for Disease Control and Prevention, American teenagers today are more likely to be using marijuana than tobacco products. Of particular worry is the attitude that marijuana is one of the more harmless drugs.

"Increasing efforts should be directed toward delaying the onset of cannabis use by young people," writes Meier, "particularly given the recent trend of younger ages of cannabis-use initiation in the United States and evidence that fewer adolescents believe that cannabis use is associated with serious health risk."