OverviewSome substance users will actively seek out both marijuana and nicotine, but many others are unknowingly consuming both as tobacco is 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.It was found that those who used marijuana both individually and combined with tobacco had smaller hippocampal volumes compared to tobacco users and nonusers, with combined users having the smallest. Reduced hippocampal volumes are thought to be due to a potentially toxic effect of marijuana on the 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 combined substance use and its effects continue to be investigated, other factors such as intensity of use, age of onset, method of use, and frequency 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.