Tracking over 1,000 people from New Zealand, the new study
, titled Persistent cannabis users show neuropsychological decline from childhood to midlife
found that teens who used marijuana persistently - more than once per week - suffered a loss of intelligence that stayed with them into adult life. They lost upon average 8 points on IQ tests and even those who'd quit marijuana later in life, the study found
, did not regain their IQ points.
Teens who used the drug from a young age but not often, and those who did not begin marijuana use until past the age of 18, regardless of their frequency of use, were not prone to suffering loss of IQ. Participants were tested at age 13 and again at 38;
at five intervals in between they were interviewed about their marijuana usage.
Lead researchers: "Marijuana is not harmless"
Lead researcher Madeline Meier of Duke University said that their study is a strong indicator that the developing mind is endangered by smoking pot and that parents must be quick to point out harmful effects of the drug to their teens. “Marijuana is not harmless, particularly for adolescents," Meier said in a written statement.
Some health professionals are responding
to the study by stating the safest route for teens is to avoid marijuana entirely. They suggest that given the study did not determine precisely how much usage will begin to damage the developing brain that avoidance is the best choice.
Multiple studies on dangers of smoking pot
It's not the first study that has found marijuana usage by teenagers is harmful. A study
from a team of researchers from Germany, the Netherlands, Switzerland and the U.K published in the British Medical Journal in March of 2011 found that teens and young adults who used cannabis are almost two times as likely to exhibit psychotic behaviors at some point in their lives than those who did not use cannabis.
from 2011, lead by Dr. Matthew Large of the Univ. of New South Wales, concluded marijuana hastens the onset of psychotic illnesses and is especially harmful to young people. A 2007 British study
involving brain scans found marijuana can trigger temporary psychotic episodes in the brain and using it is particularly dangerous for young people.
The study on marijuana usage and IQ loss in teens was published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).