Huh, People Who Mix Weed and Booze Have Lower GPAs

A heavy regimen of alcohol and marijuana together might just sink your grade point average in college, according to what might be the most obvious study ever published in PLOS One. 

The researchers tracked 1,100 college students through their freshman year. The amount of weed and booze that these freshmen imbibed varied broadly, but overall the researchers grouped them into people who both drank and smoked moderate to high amounts, people who drank moderate to heavy amounts but didn't smoke weed, and the rest who drank and smoked little or not at all. Both the light users and alcohol-only groups each represented about 40 percent of the students, while the moderate-to-heavy drinkers and smokers filled the remaining 20 percent.

Those moderate-to-heavy drinkers averaged a little more than seven alcoholic beverages a week. The weed-and-alcohol group was averaging closer to 14 drinks a week, and they smoked weed (or, potentially, ate edibles) 14 times a month.

Only the combination of both marijuana and alcohol was linked with substantially worse grades. Drinking heavily without smoking weed was associated with a slightly lower GPA—3.03 versus 3.10 for the light partiers—but the difference between the two was only statistically significant for the first semester of college, and leveled off after that. The weed-plus-alcohol crowd, meanwhile, had an average GPA of 2.66.

The study authors took pains to point out, as is always the case with these kinds of studies, that it wasn't clear whether smoking weed and drinking were necessarily causing lower GPAs. They could only observe that people who drank and smoked the most also happened to be doing poorly in school. And as far as the fact that heavy drinkers seemed to be performing just fine after the first semester, they theorized that the students might gain a better tolerance for alcohol, or may even switch to easier classes after that first report card comes in.

The authors also believe theirs is the first study to look at the combined effects of the two most popular types of mood-altering substance use on college campuses. They also cite previous studies that have found that either drinking alcohol or smoking weed in adolescence can inhibit the development of a still-young brain. One study even found that young teens who smoked marijuana never reached their predicted IQ levels as adults, even if they stopped smoking. (It's worth noting, though, that still other research has noted that students with high academic ability smoke weed and not cigarettes.)

One bright spot in this study is that a good chunk of those heavy drinkers and weed smokers seemed to fairly quickly change their ways. They either shifted into simply drinking—or quit partying altogether.

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