Cannabis is easily the most widely used illegal substance in the world. Although it still illegal at federal level, Washington and Colorado have legalized recreational cannabis use. Studies examining the relationship between marijuana use and neuropsychological function should be taken into consideration when making/reforming laws and health policies. I have received multiple questions regarding the effects of marijuana on memory and health and recently found a longitudinal study on this matter.
Prior evidence suggests that long-term, heavy cannabis use may cause enduring neuropsychological impairment beyond the period of acute intoxication (i.e. being high). Moreover, the magnitude and persistence of impairment depends on several factors including: quantity, frequency, duration and age of onset. Greater quantities, more frequent and earlier onset of use are associated with a poorer neuropsychological outcome. However, studies that compare pre-initiation neuropsychological functioning with longitudinal data on post-initiation functioning are scarce.
Meier et. al investigated the association between persistent cannabis use and neuropsychological functioning (assessed over a 20 year period) in over 1,000 individuals. Subjects received neuropsychological testing prior to onset of use (childhood; 1985-1986) and after some had developed a persistent pattern of use (~38 years old; 2010-2012).
Important findings included:
- Subjects with more persistence cannabis dependence showed greater IQ decline. Those who never experienced cannabis experienced a slight increase in IQ.
- Subjects with more persistent cannabis dependence generally showed greater neuropsychological impairment across different areas of mental function: executive function, memory, processing speed, perceptual reasoning and verbal comprehension. The greatest impairments were in the domains of executive function and processing speed.
- Neuropsychological deficits induced by cannabis use were still significant even when the researchers controlled for: past 24 hour cannabis use, past-week cannabis use, persistent tobacco, alcohol and/or hard drug dependence, and schizophrenia (all of which alternative explanations for poorer neuropsychological function).
- The effect of cannabis dependence on cognitive decline remained significant even after controlling for years of education. Persistent cannabis users with a high school education or less experienced greater IQ decline.
- Subjects who had an adolescent onset of use and were diagnosed with dependence prior to 18 years of age tended to become more persistent users. Importantly, adult-onset cannabis users did not appear to experience IQ decline as a function of cannabis use.
- Within-person IQ decline was apparent regardless of whether cannabis was used frequently or infrequently a year before testing. Thus, cessation of cannabis use did not restore neuropsychological functioning among adolescent-onset former persistent cannabis users.
So it looks like persistent use of cannabis is particularly detrimental with adolescent onset. Some have speculated that this may be due to puberty, a critical period of brain development in which circuits related to decision-making, executive-function, and reward are undergoing reorganization/rewiring. Neurotransmitter systems like dopamine are also vulnerable during this period as they have not fully matured yet. Thus, the authors suggest that cannabis use exerts neurotoxic effects during this developmental period.
However, one must remember that although the authors show compelling data, their data correlational and is not sufficient to establish causation. Furthermore, there is no mechanism underlying the negative impact of cannabis use on neuropsychological function- merely speculations (see above). It is also possible that there is another variable related to cannabis use and neuropsychological decline that the authors did not rule out. Another limitation of the study was the heavy reliance on self-reporting measures like self-reported frequency of use. Finally, it is hard to estimate dosages due to the variety of strains and potency of cannabis.
I would personally suggest taking this information for what it’s worth. Neuroimaging studies in adolescents (humans) reveal structural and functional brain differences associated with cannabis use so we know that cannabis use changes the brain. I personally believe that cannabis use has negative effects on memory and general health, but I do not think that it’s as simple as the “Weed will make you stupid.” notion that some adults try to instill in adolescents. After all, we already KNOW about the dangers and costs of alcohol/tobacco use and people still use them. For me, the key is to delay onset of use (if you must use) and to prevent adolescent use of cannabis. If you are a teenager with cannabis dependence, it is never to late to quit and try to remedy the effects.
Source: (Click on the link for abstract)