OverviewSome substance users actively seek out both marijuana and nicotine, but many marijuana users unknowingly consume tobacco, often a key ingredient in smoked marijuana joints. The individual effects of marijuana and nicotine on the brain have consistently shown reduced hippocampal volume and poor memory performance. Prior studies have yet to characterize the independent versus combined effects of marijuana and nicotine use on the brain.This study found participants who used marijuana both individually and combined with tobacco had smaller hippocampal volumes compared to both tobacco users and nonusers, with combined users having the smallest hippocampal volume. Reduced hippocampal volume is thought to be due to a potentially toxic effect of marijuana on cells of the hippocampus. In nonusers, reduced hippocampal volume was associated with poorer memory performance, while in combined users, smaller hippocampal volumes were associated with relatively higher memory scores. As the effects of combined substance use continue to be investigated, other factors such as intensity of use, age of onset, method of use and frequency of use could potentially highlight more differences in the effects of single and combined substance use.
Figure 1 shows that the combined effect of both marijuana and nicotine was associated with the smallest hippocampal volume among participants.