About 70% of pot smokers also use tobacco, and that sizeable population has typically been excluded from marijuana studies because researchers are, well, studying marijuana—not tobacco, and not marijuana and tobacco.

But a new study from the University of Texas looks at pot smokers who also smoke cigarettes and it finds significant differences between those who smoke both and those who only smoke pot.

Researchers at the university's Center for Brain Health were looking specifically at the size of the hippocampus—a brain region responsible for memory and learning—and found that while both cigarette smokers and pot smokers had smaller hippocampal brain volumes than non-smokers, people who smoked both showed signs of improved functioning.

In people who use only one of the drugs, there was "no significant associations between hippocampal size and memory performance." In non-marijuana and -tobacco users, the size of the hippocampus has a direct relationship to memory function—the smaller the hippocampus, the poorer the memory function. But in people who use both substances, that relationship was flipped: The more cigarettes they smoked each day, the greater the hippocampal shrinkage, but also, and paradoxically, the greater the memory performance.

"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. The smaller the hippocampus size, the greater the memory performance," Francesca Filbey, the study's lead investigator, told ATTN.

"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.

"Our findings exemplify why the effects of marijuana on the brain may not generalize to the vast majority of the marijuana-using population, because most studies do not account for tobacco use," Filbey continued. "This study is one of the first to tease apart the unique effects of each substance on the brain as well as their combined effects."

The study divided subjects into four groups: non-users, chronic marijuana users, frequent cigarette smokers, and chronic marijuana users who were also frequent cigarette smokers. They were given history assessments and neuropsychological tests and subjected to magnetic resonance imaging scans of their brains.

It's not just the findings but the range of subjects that makes the study fairly unusual in the marijuana literature. And it could lead the way to more studies that try to assess the impacts of poly-substance use.

"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. But I think that it is equally important to determine combined effects of substances, particularly when it is so prevalent," Filbey explained.

"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," she said.

In the meantime, smoke 'em if you've got 'em. It may help you remember that you've got 'em in the first place.