Changes to a single enzyme in the genetic code turn the plant's psychoactive compounds into non-high-inducing molecules.
Teens who smoke marijuana see their IQs drop as adults, and deficits persist even after quitting, according to a new study.
"The findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects," study researcher Madeline Meier of Duke University said in a statement.
The study followed 1,037 New Zealand children for 25 years. Subjects took IQ tests at age 13, before any of them had smoked marijuana, and again at age 38. Throughout the study, participants also answered several surveys about their drug use.
Roughly 5 percent of the participants started using marijuana as teenagers. Those who smoked marijuana at least four times a week and used marijuana throughout their life saw their IQ drop an average of 8 points, the equivalent of going from an A to a B student. The drop was not explained by other drug use, years of education, schizophrenia or using marijuana in the day before the test.
People who eventually quit smoking pot still had lower IQs than they did at the start of the study.
Interestingly, people who picked up the habit as adults had no IQ drop, suggesting that marijuana may not be as harmful to the mature brain.
The findings are the first to associate intelligence declines with marijuana use. Past work linked low IQ and marijuana, but couldn't rule out the possibility that people who choose to smoke pot are inherently less smart than abstainers.
It's not clear why pot is bad for teen brains.
One possibility is that teenagers are more vulnerable to marijuana's effects on brain chemistry, said Susan Tapert, a neuropsychologist at the University of California, San Diego, who was not involved in the study.
During adolescence, neural connections are pruned in the hippocampus and the prefrontal cortex, critical regions for learning, memory and planning, Tapert said.
Those regions may also soak up the active ingredient in marijuana."A lot of the areas that are still developing during adolescent years happen to be the areas with high cannabis receptor density," Tapert told LiveScience.
But those who consistently smoke marijuana may simply make less intellectually stimulating choices at critical points in life.
"What people tend to do when they're under the influence is different than they would otherwise," Tapert said.
For instance, pot users may be less inclined to attend classes or do other activities that give the brain a workout. Getting off track early on can also limit future opportunities and thereby reduce IQ, she said. [10 Ways to Keep Your Mind Sharp]
"Teens need to view cannabis as not an entirely benign compound, but as something that can impair your judgment and might not be great for your brain," Tapert said.
The study is detailed today (Aug. 27) in the journal Proceedings of the National Academy of Sciences.