New research confirms what many people already know about marijuana. In spite of its increasing popularity, weed is not the benign, all-purpose nutrient that the industry promotes.
Damage to the brain from cannabis use includes a person’s response time, executive functioning and residual verbal memory. In laymen’s terms this means that the cannabis user’s decision-making, problem-solving ability and recall of something spoken or written which was already learned can be compromised by the effects of the drug on the brain. A review of new research by Drs. Johnston, Tsuang and Shrivastava on the effects of cannabis on specific cognitive functions shows these among other important neuropsychological activities are impaired by its use.
These verified cognitive deficits present a widespread problem, because according to international statistics, cannabis is one of the most commonly abused illicit drugs, with the World Health Organization documenting that nearly 3% of the world’s adult population abuses cannabis (and adolescents using at higher levels on average).
The implications of this review of research, therefore, are valuable, particularly because the documented negative effects of cannabis use extend beyond a person’s health, both acute and long-term, to include, as well, one’s psychological functioning and cognitive development.
The report contains studies showing that THC (tetrahydrocannabinol, the active metabolite responsible for cannabis’s effects) induces both psychical and physical dependencies, but the perception of withdrawal (craving and varying symptoms of discomfort) is weak (in comparison to other addictive drugs), due its slow elimination from the body. According to the authors, “There is much debate about the nature of cannabis dependence, as it is considered non-addicting, due to the absence of a withdrawal state. This presumption has also been proved wrong.”
Some of the studies reviewed point to a marked cognitive decline in cannabis abusers (especially long-term users), compared to non-abusers and healthy controls, and included the damaging effects of cannabis on prospective memory ability in young adults.
These and other studies cited demonstrate “the use of cannabis during critical developmental periods in the still maturing brain may induce persistent alterations in brain structure and brain function.”
The reviewing authors summarized their findings regarding cannabis’s influence on neurological development by saying, “The effects of frequent cannabis use during adolescence could be different from and more serious than those during adulthood, an issue increasingly recognized in the field of cannabis research.”
One of the results of a meta-analysis cited in this review was the identification of residual verbal memory impairments as the most prevalent deficit associated with chronic cannabis use in otherwise healthy users.
Interestingly, the review’s authors found evidence that the cognitive decline is related to dose and the amount of consumption. That is, what was categorized as “very heavy use” of marijuana is associated with continuous decline in neurocognitive performance, slower response times and decreased motor control with increasing THC doses.
In conclusion, this review of research showed a range of cognitive deficits associated with cannabis use, as well as negative interactions with neurodevelopment stages. They acknowledged that further research in the area of cannabis’s effects on cognitive functioning is warranted and “will enhance our understanding of underlying pathophysiology and improving treatments for substance abuse and mental illness. Cognitive functions may provide a guide to treating marijuana addiction.”
According to treatment expert John Harden LCSW, CAP, MPH in Florida, “more research is not needed to determine whether or not cannabis use causes brain damage. This new research and decades of prior research shows that, or you can just ask any heavy user after they detox and get into recovery. They will tell you their regrets. It is a known-known, like heavy alcohol use damages the liver, cigarettes cause lung cancer and water is wet. The only people that are arguing heavily for the myth that weed is benign are the sellers, the enablers and those that are hooked.”
 Solowij N. Cannabis and cognitive functioning. Cambridge: Cambridge University Press; 1988; The residual cognitive effects of heavy marijuana use in college students – PubMed (nih.gov)
 Dose-related neurocognitive effects of marijuana use – PubMed (nih.gov); Cognitive and psychomotor effects in males after smoking a combination of tobacco and cannabis containing up to 69 mg delta-9-tetrahydrocannabinol (THC) – PubMed (nih.gov)