Few will be surprised that teens can be struck dumb by heavy pot smoking. What's interesting is the question that new research raises: Can kids under 16 get baked every now and then without limiting their long-term brain power?

Sixteen-year-olds who regularly enjoy marijuana show somewhat diminished grades, according to a new analysis of data on 2,235 British children born in 1991 and 1992. What the study considers moderate users -- teens who have smoked cannabis fewer than 50 times -- showed no similar losses on exams.

The children are members of an ongoing study at the University of Bristol that began with their mothers' pregnancies. They were given IQ tests when they were eight years old and again at 15, when they were also asked to complete surveys on cannabis use.

The new analysis, led by a researcher at College University London, linked that information to a U.K. national student database. That allowed the team to compare academic performance before and after the the onset of the disclosed drug use.

The new work, which is supported by the U.K. Medical Research Council, suggests that pot smoking may be a symptom of a larger problem. “Cigarette use, alcohol use and other recreational drug use, as well as cannabis use,” the authors write, “are all associated with IQ and educational decline.” Lifestyles that encourage pot smoking may be a hidden force behind the drop, “rather than a specific effect of cannabis consumption,” they said.

Claire Mokrysz, the lead researcher and a University College London PhD student, presented the work at an academic conference in Berlin Tuesday. The research, which she is conducting with five other scientists, is still being prepared for publication in a peer-reviewed journal.

Analyses like this “can never make definitive causal conclusions," Mokrysz said in an email. What the scientists will say, interpret it as you like, is that they found nothing to suggest a link between academic or cognitive slumps and getting high fewer than 50 times. And where they found one, among heavier users, the link was weakened once they factored in the similar impacts of tobacco, alcohol and other drugs on education.

The new work also addresses a 2012 study that drew widespread attention for finding a connection between cannabis use and cognitive decline. The researchers suggest that the previous paper studied too few people and didn't take into account other explanations for the cannabis-cognition relationship.

Madeline Meier, an Arizona State University assistant professor and the lead author of the 2012 study, hasn't seen the new work, only the press release, and said it "looks interesting." She also clarified a difference she said she saw between the two projects. Her paper found that adults who used cannabis more than three times a week during the 20 years after adolescence had lost IQ points by age 38, a much longer period of study. "There is no reason to expect," she said, "that teens who have used cannabis only 50 times would already show a loss of IQ points by age 15

http://www.bloomberg.com/news/2014-10-23/teenage-pot-smoking-and-iq-researchers-smoke-out-the-links.html