New evidence published in Proceedings of the National Academy of Sciences shows that persistent cannabis (marijuana) use is linked to a mental decline and loss of up to 8 IQ points.

The study was conducted over the course of 20 years in New Zealand and followed a group of 1,037 individuals from the age 18 to 38.

“IQ was tested at age 13, before cannabis use, and again at age 38, after some study members had used cannabis for many years,” said Madeline Meier, a postdoctoral researcher at Duke University and lead researcher for this study.

The results showed that those who began smoking marijuana before age 18, termed adolescent-onset users, “showed marked IQ decline of 8 points from childhood to adulthood,” Meier said.

Quitting or reducing cannabis use did not appear to restore lost mental function.

“The findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” Meier said.

To put the 8 point reduction in IQ points into perspective, an average person has an IQ of 100, putting them in the 50th percentile of intelligence. Losing 8 points drops them into the 29th percentile.

“Individuals who lose 8 points in their teens and twenties may be disadvantaged, relative to their same-age peers, in most of the important aspects of life and for years to come,” Meier said.

Third-party informants, people who knew the study members well, reported noticing attention and memory problems that kept them from performing daily tasks to the best of their ability.

While only approximately 5 percent of the 1,037 study members were affected by the fall in IQ points, “the findings are still concerning given that fewer adolescents believe that cannabis use presents a serious health risk,” Meier said.

Given the negative impacts of marijuana as presented in this study, “I hope our research can be used to inform a person’s decision to use marijuana. Studies like ours can also inform policy,” Meier said.

Meier wants to be clear, however, that the findings are not conclusive.

“We do not know how much cannabis needs to be consumed and across what ages before impairment occurs. Given that the brain undergoes dynamic changes from the onset of puberty through early adulthood, this developmental period should be the focus of future research on the age at which harm occurs,” Meier said.

This study also has not convinced some students.

“It’s not a credible study. It does not prove that cannabis lowers IQ,” said Miranda Webb, president of Students for Sensible Drug Policy. “If anything, it just proves that people who have higher IQs just wait until they are older to smoke, or there is the fact that they could lie on the test. It is only correlation, not causation.”

“No actual scientist would take a study like that seriously,” Webb said.

Contact Christina Bucciere at [email protected].