Marijuana and nicotine hinder memory and learning ability when used separately, but together they can improve memory, say scientists Getty

Scientists from the University of Texas at Dallas have discovered that nicotine changes the way in which marijuana affects the brain. The hippocampus is a part of the brain which affects learning ability and memory. It has been widely accepted that the smaller the hippocampus, the less strong these traits are.

It is known that the use of marijuana or tobacco products – used separately from each other – often leads to this specific part of the brain shrinking, and thus weaker learning and memory ability. However, the scientists' research, which was published in Behavioural Brain Research, says that this is opposite when marijuana and tobacco is combined. It says that as the hippocampus gets smaller, the memory gets stronger. Furthermore, it adds that the more cigarettes smoked a day, the smaller the hippocampal volume and the greater the memory performance.

The team, led by Francesca Filbey, director of cognitive neuroscience of addictive behaviours at the Center for BrainHealth, say that the affects of marijuana on the brain are not usually analysed with tobacco taken into consideration, despite the two often going hand in hand.

Filbey said: "Approximately 70% of individuals who use marijuana also use tobacco. 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. 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."

"We have always known that each substance is associated with effects on the brain and hypothesized that their interaction may not simply be a linear relationship. Our findings confirm that the interaction between marijuana and nicotine is indeed much more complicated due to the different mechanisms at play. Future studies need to address these compounding effects of substances.

"The combined use of marijuana and tobacco is highly prevalent. 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 behaviour."