As they approach their 40th birthdays, adults who smoked marijuana early and often in life face a higher likelihood of shearing off IQ points and performing more poorly on tests of reasoning, attention and memory than those who smoked pot less often, says a new study.

The latest research, published in the journal Proceedings of the National Academies of Science, underscores what the authors call the “toxic effects of cannabis on the brain” -- especially the developing brain. Against the backdrop of resurgent marijuana use among U.S. high school students, they recommend “increasing efforts” to “delay the onset of cannabis use” among teens.

“Collectively, these findings are consistent with speculations that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” the authors concluded.

The study establishes marijuana’s mounting neuropsychological effects over time by tracking a group of 1,037 children born in New Zealand for 38 years from the times of their births in 1972 and 1973. The children underwent extensive testing from their earliest days, and were tested for cognitive performance at 13 years of age -- before any had smoked pot -- and then again at 38.


In between, they were asked at 18, 21, 26 and 32 years of age about their use of illicit drugs, including marijuana. Participants were graded along a continuum according to whether they had a pattern of regular (four times per week or more), ongoing use of cannabis, or were classified as cannabis-dependent. The neuropsychological functioning of individuals along that spectrum was compared with that of subjects who never used cannabis or reported they had used it, but never regularly.

Those who had used marijuana earliest and those who had used it most persistently through life showed the greatest loss of cognitive function. That collective loss of brainpower among pot smokers held even after the researchers adjusted for differences in the education levels achieved by those who did and those who didn’t smoke pot.

While those who had never used tended to pick up a little less than a single IQ point between 13 and 38 years old, those who had smoked pot most persistently -- or who had been declared marijuana-dependent at three or more time points of the study -- shaved an average of five to six points from their IQ, an “effect size” that the researchers characterized as midway between small and medium.

In speed of processing and laboratory tests that gauge “executive function,” persistent pot smokers and those who had started early also turned in significantly lower scores than those who had not smoked pot at age 38. Consistently, individual participants who were considered to have shown signs of cannabis dependence before the age of 18 had lower scores and lost IQ points with the transition to adulthood. But those stark effects were not apparent for participants who started smoking pot regularly after the age of 18.


The impairment of those who smoked pot early and often was also impairment to those around them, the researchers found. When people who participants said “knew them well” were asked whether the subject had shown memory or attention problems in the past year, their responses lined up neatly with extent of pot use.

New Zealand has a slightly higher rate of marijuana dependence than has been seen in the U.S. But the deleterious effects of pot smoking can’t be put down to more potent weed, the study authors said: marijuana busts in both countries have turned up marijuana of roughly equal potency.

This is bad news for American kids (not to mention those who grew up in the 1960s and 70s). Between 2007 and 2011, rates of marijuana use among American middle and high school students has increased steadily and significantly: in 2011, 25% of eighth-, 10th- and 12th-graders say they have smoked marijuana at some point in the past year, according to the National Institute on Drug Abuse. Among 10-graders, 3.6% report they smoke pot almost daily, and 6.6% of 12th-graders do.