A new study from the University of Texas has identified differences in the brains of people who smoke pot and those who smoke both pot and tobacco.

In recent years, research on the effects of marijuana use on the brain has produced some fascinating results, however, one area of research that has remained relatively unexplored concerns the approximately 70 percent of cannabis users who also use tobacco, the majority of whom have been excluded from joining the participant pools of marijuana studies. This study broke down that barrier in marijuana research and could impact future studies. 

What experts found

Researchers at the Center for BrainHealth at the University of Texas found that the combined use of these substances is associated with significant changes in brain functioning that are unique compared to what happens to the minds of singular pot smokers. Specifically, they looked at the size of the hippocampus—a region of the brain responsible for memory and learning—and found that, though both groups tend to have smaller hippocampal brain volumes than non-users, the memory of people who indulged in both substances showed improved functioning.

"We would anticipate that an individual with small hippocampal volume would perform more poorly on a memory function task, but instead individuals in our study who use marijuana as well as tobacco demonstrated the reverse," Dr. Francesca Filbey, the study's lead investigator, told ATTN:. "The smaller the hippocampus size, the greater the memory performance."

"Because existing studies have noted that nicotine's cognitive enhancing effects are observable only in those with the reduced memory performance, we speculate that we may be observing short-term cognitive enhancement as a result of nicotine use, that is especially greater in those with smaller hippocampal volumes," she added.

In other words, it is not that combined users have better memory functioning than non-users; rather, because marijuana use is associated with smaller hippocampal volume in the first place, the scientists believe that the seemingly counterintuitive result is an observable phenomenon that becomes even more observable in people who smoke more pot (i.e. have smaller hippocampal volume). The study discovered that the more cigarettes that combined users smoked per day, the greater the hippocampal shrinkage—and the greater the memory performance.

How the study works

The research team used magnetic resonance imaging (MRI) to examine the brains of participants, who were given substance use history assessments and neuropsychological tests and separated them into four groups: nonusers, chronic marijuana users, frequent nicotine users, and chronic marijuana plus frequent nicotine users.

"Research has primarily been dedicated towards determining unique effects of substances, which results in either excluding combined users or controlling for their effects through statistical means," Filbey explained. "But I think that it is equally important to determine combined effects of substances, particularly when it is so prevalent."

"This study is one of the first to provide evidence that the confounding effect of tobacco on marijuana use is significant. As the prevalence of marijuana use and its potency is rapidly increasing, so is the knowledge in the scientific community that individuals are very rarely using only one substance. We hope that our findings will really influence the way future studies on the effects of marijuana on the brain are conducted."

Related: What Marijuana Really Does to Your Brain