The University of San Diego conducted a study. And in the spirit of what generally seems to be the rage these days, the ganj came out on top:
Neuroscientists at the University of San Diego studied the brains of 92 teenagers by scanning their brains over time, according to the Huffington Post. Half of the group, which already had some experience with marijuana and alcohol, was ordered to abstain or go easy on the substances for about 18 months. The other half continued smoking and drinking in varying amounts. The teens were subjected to toxicology tests before and after the study and were interviewed every 6 months about their substance use during the study. Researchers compared brain scans before and after substance use (or abstention).
Researchers found that booze had an obvious effect on the brain that weed did not. Teens consuming five or more drinks at least twice a week showed reduced white matter brain tissue health. The white matter brain tissue of teens only smoking weed, in some cases up to nine times a week, didn't suffer at all.
To be real though, ordering teens “to abstain or go easy on the substances for 18 months” sounds like it'd produce the exact opposite effect of what was desired. These are teenagers we're talking about–a species who is programmed to yell at their parents multiple times a day because it's not fair, mom. Insert terrible role-model neighbor down the street gets to toke up all the time. With his parents.