Mixing moonshine and marijuana is a recipe for academic failure, new research has found.
While heavy drinkers who stay off the weed lag behind in their first semester, they tend to catch up to their sober peers by the end of their university studies, researchers in the US found.
But those who toke as well as drink, despite identical pre-university marks, fall further and further behind as the years go on.
"Doing a lot of both drugs had a significant impact, in terms of lower grades in our study, and in other studies, with number of leaves of absences and those who dropped out of school," says Godfrey Pearlson, senior author of the study.
The effect of heavy marijuana use without alcohol couldn't be measured, as there were "almost no individuals" amongst the 1142 students looked at who used marijuana, but didn't drink.
"This is interesting given the popular belief that marijuana-only users are much more common than individuals who use both substances together," the study, published in journal PLOS ONE, notes.
Students who managed to reduce their consumption of either substance saw improvements in their grades.
The researchers say the findings have "serious implications" for users of both substances, but stop short of saying there's a causal link.
"Given the analysis presented in the paper, all we can infer is that academic performance and substance use was significantly associated."