The Teenage Mind

The internal experience of the young adult

Does Marijuana Impair Cognition? Implications for Prop 19

Does Marijuana impair thinking?
California voters have an important decision to make this fall. On Nov. 2, Californians will decide whether or not to legalize marijuana. Proposition 19 will allow people 21 years old or older to possess, cultivate or transport marijuana for personal use. Marijuana will be regulated and taxed by local governments. And, it is expected to generate hundreds of millions of dollars annually. Driving under the influence is still strictly prohibited and so is possessing marijuana on school grounds or giving it to someone under 21. Prop 19 is supported by judges and police chiefs because it will frees police to fight violent crime instead of clogging the system with pot smokers.

Is voting YES on Prop 19 a good idea? There are many ways to analyze and think through a change in the law. Attorneys think through the legal consequences. Police think through the criminal consequences. Some people think through the financial consequences. I think through the health implications and even more specifically, the mental health implications. In terms of health, medical marijuana has helped thousands of cancer patients sleep better, eat better, and cope with pain. But, what about its effect on thinking? Does marijuana impair cognition?

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A study conducted by Hart, et al. (2001) and published in Nueropsychopharmacology examined the effects of marijuana on complex, cognitive tasks. Experienced marijuana users were given a single marijuana cigarette (0%, 1.8% or 3.9% Delta (9) –THC w/w) which was provided by the National Institute of Drug Abuse. After smoking, the experimental subjects took a valid and reliable computerized nueropsych test called The Micro-Cog. This test is a standardized measure of reaction time, attention, memory, visuospatial processing, mental calculation, digital recall, and other similar abilities.

Data analyses provide comparisons between high THC levels, low THC levels, and zero THC levels on the cognitive tasks. Interestingly, math answers under the 3.9% THC condition were accurate but slower. In other words, subjects still got the math problems right but it took them longer. While this study has some serious limitations like small sample, subjects who were experienced users, and lack of replication, it provides some useful findings. And even though there continues to be a need for more scientific research, slowed reaction time is consistent with anecdotal observations.

So, what is the take home message? There are clearly some situations that high smokers should avoid. DON’T DRIVE. Driving, especially in crowded metropolitan areas, requires quick reflexes. And driving under the influence is illegal under Prop 19. On the other hand, this one small study implies that there are no serious cognitive effects to getting high. OK, I am still unable to find any solid scientific evidence indicating that using marijuana for recreational purposes like listening to music at home is harmful. Then, YES or No on Prop 19? I guess in the end, each individual has to make that decision for themselves but based on the very limited scientific evidence that exists, I can find no health reason justifying criminalizing marijuana. And, in fact, Prop 19 is supported by Joycelyn Elders, United State Surgeon General (Ret.).

Hart, C.L., van Gorp, W., Haney, M., Foltin, R. W., and Fischman, M. W. (2001). Effects of Acute Smoked Marijuana on Complex Cognitive Performance. Neuropsychopharmacology. 25, 757-65.10. 1038/s0893-133x(01)0 00273-1.

Jann Gumbiner, Ph.D., is a licensed psychologist and clinical professor at the University of California, Irvine College of Medicine.

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