Cannabis is the most widely used illicit drug worldwide. Today, attitudes and perceptions of risks associated with cannabis use are changing, in part due to the increased use of cannabis for medical purposes and the decriminalization of its use in many states and countries. Memory loss is known to be a problem associated with cannabis use, but a new study reports that cannabis might prevent people from knowing if memories are real or make-believe.

Acute cannabis intoxication is correlated with impaired working memory and declarative memory. Most data suggest that these effects last for a few days, but heavy cannabis users may experience memory deficits that last for up to months after the last use.

To explore this consequence of cannabis use, researchers used functional magnetic resonance imaging (fMRI) to assess whether abstinent cannabis users could distinguish between real and false memories. Heavy users (i.e., once daily for at least 2 years) were compared to cannabis-naïve (i.e., less than 50 lifetime uses) participants. All participants were abstinent from cannabis for 4 weeks prior to the study.

The researchers used word lists to induce false memories and they found that heavy cannabis users were more susceptible to forming false memories than non-cannabis users. The participants’ brain images revealed that heavy cannabis users have reduced activation in areas associated with memory processing and in areas associated with attention and performance monitoring.

Together, these findings suggest that cannabis may have longer-lasting adverse effects than originally believed. According to the authors, compromised memory and cognitive control mechanisms involved in telling reality from fantasy may last for a long period after the drug is used.

Chronic cannabis use is known to have serious health implications, including anxiety, depression, psychosis, and cognitive impairment. These implications, as well as the inability to tell real from imaginary memories, are similar to the deficits seen in psychiatric and neurologic conditions, as well as age-related cognitive decline.

Telling truth from fiction may hold serious medical or legal ramifications, but, on an everyday basis, the ability to create, retain, and identify memories affects a person’s self-image, emotional health, safety, and relationships. While many people downplay the long-term consequences of cannabis use, it might be best to remember that there is a lot about this largely recreational chemical that we don’t know – and that’s the truth.

References

Jager G, Kahn RS, Van Den Brink W, Van Ree JM, & Ramsey NF (2006). Long-term effects of frequent cannabis use on working memory and attention: an fMRI study. Psychopharmacology, 185 (3), 358-68 PMID: 16521034

Karila L, Roux P, Rolland B, Benyamina A, Reynaud M, Aubin HJ, & Lançon C (2014). Acute and long-term effects of cannabis use: a review. Current pharmaceutical design, 20 (25), 4112-8 PMID: 24001294

McClure EA, Lydiard JB, Goddard SD, & Gray KM (2015). Objective and subjective memory ratings in cannabis-dependent adolescents. The American journal on addictions / American Academy of Psychiatrists in Alcoholism and Addictions, 24 (1), 47-52 PMID: 25823635

Metzak PD, Lavigne KM, & Woodward TS (2015). Functional brain Networks involved in reality monitoring. Neuropsychologia PMID: 26004062

Riba J, Valle M, Sampedro F, Rodríguez-Pujadas A, Martínez-Horta S, Kulisevsky J, & Rodríguez-Fornells A (2015). Telling true from false: cannabis users show increased susceptibility to false memories. Molecular psychiatry, 20 (6), 772-7 PMID: 25824306

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