Pot users show drop in IQ from adolescence to adulthood

Marijuana plants lay in a pile before being burned by the army in the Sierra de Juarez in Ensenada, Mexico, Thursday Sept. 30, 2010. According to Army Gen. Alfonso Duarte Mujica, soldiers have destroyed 73 hectares of marijuana in the northern region of Mexico's Baja California peninsula in 2010. (AP Photo/Guillermo Arias) (AP)

While smoking marijuana is sometimes touted for its ability to dull pain, research has emerged showing that it also significantly dulls the mind – especially in teenagers.

A new study of more than 1,000 children in New Zealand, following them from birth to the age of 38, found that those who began using cannabis during their adolescence showed a decrease in their general intellectual ability, as measured by IQ tests, from childhood to adulthood.

A person's IQ - or intelligence quotient - does not measure a person's amount of knowledge, but rather represents a person's ability to comprehend concepts, as well their capacity to process information.  Typically, IQ does not change significantly over the course of a person's life, unless as a result of severe brain damage from trauma or disease.

Although previous research has found adolescent cannabis use to have an effect on IQ, this is the first study to rule out pre-cannabis performance explaining the results.  Researchers initially tested the participants at the age of 13 – before they began using cannabis – ultimately comparing this IQ to later tests throughout their adulthood.

While drug tests were not used in this study, detailed questionnaires given to the participants at five different ages throughout their lives helped to establish their marijuana use.

“We assessed cannabis use in two ways,” the study's lead author Madeline Meier, of the Department of Psychology and Neuroscience and Duke Transdisciplinary Prevention Research Center at Duke University, Durham, told FoxNews.com.  “At [various] ages, they were asked to go back and report for that past year how frequently they used cannabis.  They were also given diagnostic interviews, where we diagnosed cannabis dependence.  Dependence generally reflects someone who has been using cannabis and experiencing health, social and/or legal problems – but are still continuing to use.”

Each individual was categorized into one of various groups depending on their cannabis use and dependence.  The two main groups the researchers focused on were adult-onset cannabis users and adolescent-onset cannabis users. Adolescent-onset was defined in two ways – dependence on cannabis before the age of 18 or weekly cannabis use before 18.  Adult-onset was defined the same way for after the age of 18.

Upon determining the age of cannabis-use onset, the researchers then analyzed the participants’ various IQ tests to establish trends in intelligence over time.  For adult-onset cannabis users, there was ultimately no change in their IQ over time.  But individuals who started using cannabis between the ages of 13 and 18 experienced an average IQ drop of eight points.

Not only did intelligence decrease, but the changes seemed to be permanent.  Quitting or toning down cannabis use did not help adolescent-onset cannabis users to recover their lost IQ points.

According to Meier, an eight-point drop may sound small, but it can actually be a significant decrease with lasting effects.

“Take an average person – an IQ of 100 puts them in the 50th percentile of intelligence,” Meier said. “If this person loses eight IQ points, it drops them down to the 29th percentile.  IQ is a strong determinate of a person’s access to college education, getting a job, performance on the job, tendency to develop heart disease.  So those individuals who lose IQ points may be disadvantaged toward the most important aspects of life.”

While the results may be staggering, Meier said there is still much work to be done.  The researchers did not determine how much cannabis a person needs to use in order to impair intelligence, as well as the age range that is most vulnerable.  However, Meier feels that future research should focus on cannabis use during the onset of puberty – a crucial time for the brain’s development.

Even though more information is needed to establish the exact effects of marijuana on the developing mind, Meier noted it’s important for adults and teenagers alike to have a better understanding of just what cannabis does to their minds and bodies.

“[People] really need to be aware that cannabis isn’t harmless for adolescents,” Meier said.  “2011 was the first year in which adolescents smoked more marijuana than cigarettes.  They’re getting the message that cigarettes are dangerous and that marijuana is not.  They need to understand that it is not harmless.”