The hippocampus is a region of the brain that is partly responsible for our memory. Marijuana and nicotine are known to reduce the size of the hippocampus. So, it would make sense that among users of both substances, a smaller hippocampus would lead to a reduced memory function, right? Surprising new research has found that’s not necessarily the case.

A remarkable study published in the journal Behavioural Brain Research, by scientists from the Center for BrainHealth at the University of Texas at Dallas, looked at the effects of combined marijuana and nicotine (tobacco) use, something that has been poorly studied before.

The findings were that, among participants who used both marijuana and nicotine, the smaller their hippocampus, the better their memory compared to others with a bigger hippocampus who also used both. And those that smoked the most tobacco had the smallest hippocampus but best memory compared to the others. The same was observed to a lesser degree among those who solely used marijuana or tobacco. 

"That’s the surprising effect we found," lead researcher Francesca Filbey told IFLScience. "It’s not what we expected."

The research, which just looked at short-term memory, involved participants in four groups: 16 nonusers (no marijuana or tobacco in three months), 36 chronic marijuana users (four or more times per week), 19 frequent nicotine users (10 or more times daily), and 19 chronic marijuana plus frequent nicotine users. All performed memory tests and had their brain scanned using a magnetic resonance imager (MRI).

It should be noted that the marijuana and tobacco users had the worst memory of the four, followed by marijuana users, then the control group, then the nicotine users. Filbey points out, though, that the results of the memory tests between the groups were not significantly different.

^{The research focused on the hippocampus, illustrated. Public Domain/Wikimedia Commons.}

The reason for the effect is not entirely clear. Filbey speculates that nicotine, which is known to have some "protective cognitive effects," could be beneficial in people who have memory impairment already, possibly the result of marijuana. "Marijuana users would have an impaired cognitive condition and a worse performance," she said.

In addition, the hippocampus is not the only part of the brain responsible for memory function. While this study focused on this area, it might be that using nicotine and marijuana affect other parts, such as the prefrontal cortex (PFC). "That is another important region [to be studied]," said Filbey.

The major implication of the research is that there is still much we don’t know about the effects of marijuana and tobacco, both together and separately, on the brain. "The combined use of marijuana and tobacco is highly prevalent," said Filbey in a statement. "For instance, a 'blunt' is wrapped in tobacco leaf. A 'spliff' is a joint rolled with tobacco. We really need to understand how the combined use changes the brain to really understand its effects on memory function and behavior."

She notes, though, that these findings are somewhat limited by the number of participants. Further research will certainly be needed to investigate the surprising conclusions further. "Our findings confirm that the interaction between marijuana and nicotine is indeed much more complicated due to the different mechanisms at play," added Filbey in the statement. "Future studies need to address these compounding effects of substances."