Long-term marijuana use beginning in the teen years can lead to a lower IQ, according to a New Zealand health study. The study showed that frequent and continued use of pot starting before the subject was 18 was linked to an eight-point decline in IQ for those involved in the study.
The decline was not documented in users who began smoking pot either in early adulthood or later in life, reports WebMD. The study’s findings suggest that long-term marijuana use can result in long-term harm to thinking abilities, like intelligence, memory, and attention span.
Temple University professor of psychology Laurence Steinberg, PhD stated of the study that:
“Many people today, especially young people, believe that marijuana is risk free, but this research tells us that this is not the case.”
The study included almost 1,000 adult participants who were followed from birth. IQ tests were performed at ages 7, 9, 11, 13, and 38. The latest analysis comes from Duke university postdoctoral researcher Madeline H. Meier, PhD, along with her colleagues, who compared IQ scores at age 13 (before most of the participants were introduced to marijuana) with those at age 38, after many had experienced the drug.
NBC News notes that about 5 percent of the participants began smoking marijuana as a teenager. Those who smoked pot four or more times a week throughout their life saw their IQs drop by an average of eight points. Even the people who eventually quit smoking pot still had lower IQs than they did at the beginning of the study.
The study’s findings are the first that associate a decline in intelligence with the use of marijuana. While past work has linked a low IQ with marijuana, they were not able to rule out the possibility that those who smoke pot are inherently less smart than abstainers. The
While it does not give a reason why pot can be bad for the teenage brain, one possibility is that it affects brain development in the teen years, meaning that it is relatively safe for adults, who saw no change in IQ after they took up smoking marijuana.