Growing up, many adolescents are told not to take drugs or drink alcohol by their parents and teachers. While some kids stay away from these substances, others ignore the advice of their elders and go on to drink alcohol and smoke marijuana. These individuals may want to take an IQ test when they are older, as new research shows that engaging in this illegal activity for an extended period of time could have a negative effect on users' brains that could last throughout their lives.

To arrive at this conclusion, the researchers behind this study, which is set to appear in an online issue of "Alcoholism: Clinical & Experimental Research," followed 92 adolescents between the ages of 16 and 20 for 18 months. These individuals were divided into two groups - those who used alcohol and marijuana extensively before adolescence, and those who engaged in little, if any, substance abuse.

Following extensive testing, study subjects who used more alcohol and marijuana were found to have white matter tissue in their brain of a lower quality, compared to individuals who avoided the substances. Furthermore, one of the major connection roadways in the brain - the superior longitudinal fasciculus - was compromised among those who used alcohol and marijuana during their youth.

"When the connections between brain areas are severely damaged, those areas of the brain cannot properly function," said Duncan Clark, associate professor of psychiatry at the University of Pittsburgh Medical Center.