There is a stereotype that marijuana smokers have hazy or weak memories as a result of their use. Tobacco smoke, on the other hand, is usually discussed in terms of the positive effect on cognition by virtue of the very mild stimulant effects delivered by nicotine. Over a decade ago, however, researchers in Nice, France showed in rats that administration of nicotine, without smoke, induced a suppression of new neuronal births in the dentate gyrus, the part of the brain that handles about 90% of a person or animal's memories. In the last decade as well, researchers from the Chinese Military, Canada, and Maryland have shown that marijuana actually encourages the birth of neurogenesis in the hippocampus (which is responsible for most remaining memories) by 40%, when a pure THC copycat chemical, named for Hebrew University where the compound was made, is administered to rats. Researchers at Princeton University and the Department of Nutrition in Brazil also showed that there was no effect from THC on the dentate gyrus of rats, even at levels producing "gross behavioural intoxication".
This provides a general understanding of the effect of THC on the brain, which should be reproducible in human or population studies which have been conducted. More recent brain scans have also shown this to be true as conducted with human populations, with negligible changes, positive or negative, found in most parts of the brain, though functional connectivity has also been shown to increase in a significant manner with marijuana use this last year.
Most recently, a study from Spain claimed that while virtually every mode of behavioural measurement was equal between marijuana smokers and control groups, a significant (though still within the realm of normal fluctuation) decrease in memory function was noted. This speaks in the face of research that has been conducted about THC or the marijuana that contains it. Even with patients using 10 times the responsible adult use in the sample, and the average number of marijuana cigarettes, or spliffs, smoked in a lifetime being around 42,000 (with one participant smoking over a massive 256,000 times) among those included in the study, the results do not match with rats who were administered high levels of THC; the results should have been positive and not negative.
There is a difference between the human and rat studies, though which is not addressed. In the Spanish study, noting a decrease in memory function, the research explicitly emphasizes that tobacco users were not omitted from the study, and about 75% of the participants were actively smoking tobacco when the study was conducted.
A little background, for the American marijuana user, is necessary. In Europe, people smoke spliffs generally made with hashish and tobacco. The unfiltered smoke doubles the nicotine intake (in some countries there are lower limits on nicotine levels, but in Spain this probably means quite similar to American cigarettes, as there has not been a strict concentration of absorbed nicotine enforced throughout the European Union yet), and this dominates the marijuana culture there. While more efficient, this also means that it is very difficult to smoke marijuana regularly without crossing the threshold of 5-10 mg of nicotine daily at which point positive effects are eclipsed by neurogenesis suppression, assuming the user is smoking on work breaks or at certain timepoints and not regularly throughout the day (the research indicates that doses of more than 3-5 mg of nicotine in the blood at a time is the crossover point from positive to negative effects).
The research in Nice, France, which originally shows the negative impact of moderate to heavy nicotine use on the dentate gyrus, emphasizes that further research is needed to confirm this neuronal difference actually translates to a cognitive change. Research from Riba et al. should be used in conjunction with other research done in this field to confirm this fact, as has been shown above. It is still worth investigating the impacts of age, obviously at certain ages nicotine has a positive effect, while it can be assumed that while a brain is developing such stunting of neurogenesis must induce extreme cognitive defects. For those concerned about absorbing too much nicotine, in the last 70 years absorbed nicotine in cigarettes has increased almost 3-fold in the USA (though in some parts of Europe the composition of a cigarette is much the same as in the early days of the tobacco industry) as companies and governments attempt to limit tar or air pollution intake (read previous research on cancer mortality and smoking for more), one viable option is to use pipe tobacco instead of cigarette tobacco. While the curing process for pipe tobacco means there is more nicotine per gram in the cigarette or bowl, the wide cut of the leaves lowers absorbed nicotine by around 13 times (or 2-4 times in comparison to low nicotine European cigarettes). Normal use of pipe tobacco is also associated with levels of cancer and smoking-related disease more similar to the general population than to the cigarette smokers.
Works Cited:
Abrous, Djoher Nora, et al. "Nicotine self-administration impairs hippocampal plasticity." The Journal of neuroscience 22.9 (2002): 3656-3662.
Filbey, Francesca M., et al. "Long-term effects of marijuana use on the brain."Proceedings of the National Academy of Sciences 111.47 (2014): 16913-16918.
Jiang, Wen, et al. "Cannabinoids promote embryonic and adult hippocampus neurogenesis and produce anxiolytic-and antidepressant-like effects." Journal of Clinical Investigation 115.11 (2005): 3104.
Kochman, Linda J., et al. "Despite strong behavioral disruption, Δ 9-tetrahydrocannabinol does not affect cell proliferation in the adult mouse dentate gyrus." Brain research 1113.1 (2006): 86-93.
Ling, H. W., and CB Wynn Parry. "The amount of nicotine absorbed in smoking." British journal of pharmacology and chemotherapy 4.3 (1949): 313-314.
Riba, J., et al. "Telling true from false: cannabis users show increased susceptibility to false memories." Molecular psychiatry (2015).
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