Perhaps in response to the legalization of marijuana in Colorado and Washington last month, more teens are lighting up than ever before. However, one study suggests that parents have less to fear from marijuana than from alcohol. The study found that while marijuana had no effect on the health of teenagers' brain tissue, alcohol did.

The researchers, from the University of California, San Diego and the University of Pittsburgh Medical Center, performed the study on 92 16- to 20-year-olds. The scientists scanned their brains both before and after an 18-month period. Over the course of the 18 months, half of the teens, who already had an extensive track record with alcohol and marijuana, continued their vices as they had before. The other half continued to abstain or drink a minimal amount, like they too had done before the study.

In addition to the brain scans, the study also required a detailed toxicology report and substance use assessment. The teens also were interviewed every six months. Researchers did not check the teens' cognitive ability, but simply took brain scans.

The researchers found that, after the year and a half was over, kids who had drank five or more alcoholic beverages twice a week had lost white brain matter. That means that they could have impaired memory, attention, and decision-making into adulthood. The teens that smoked marijuana on a regular basis had no such reduction.

This damage occurs because brains continue to develop white matter well into adolescence and early adulthood. That brain tissue is integral to decision-making and self-control, which means that drinking alcohol could lead to further substance abuse problems in the future.

Indeed, consistent studies have shown that alcohol is bad for teens' health. However, the effect that marijuana can have on growing brains has been far more inconsistent. In fact, two studies earlier this year linked marijuana with lower IQ scores and reduced white matter in the brain.

However, Duncan Clark, a doctor who was unaffiliated with the study, said to the Huffington Post that previous studies simply administered brain scans one time. This study, on the other hand, which administered brain scans both before and after substance use, can more accurately depict the effect that these substances have on the brain.

In addition, researchers were better able to monitor differences in the adolescents' lives. Even still, other factors, like genetics or even the consumption of other drugs, could not be completely eliminated.

The study will be published in the April issue of Alcoholism: Clinical and Experimental Research.