Substantial evidence from animal research and a growing number of studies in humans indicate that marijuana exposure during development can cause long-term or possibly permanent adverse changes in the brain. Rats exposed to THC before birth, soon after birth, or during adolescence show notable problems with specific learning and memory tasks later in life. 32–34 Cognitive impairments in adult rats exposed to THC during adolescence are associated with structural and functional changes in the hippocampus. 35–37 Studies in rats also show that adolescent exposure to THC is associated with an altered reward system, increasing the likelihood that an animal will self-administer other drugs (e.g., heroin) when given an opportunity (see “Is marijuana a gateway drug?”).
Marijuana, Memory, and the Hippocampus
Also, the ability to draw definitive conclusions about marijuana’s long-term impact on the human brain from past studies is often limited by the fact that study participants use multiple substances, and there is often limited data about the participants’ health or mental functioning prior to the study. Over the next decade, the National Institutes of Health is funding the Adolescent Brain Cognitive Development (ABCD) study—a major longitudinal study that will track a large sample of young Americans from late childhood (before first use of drugs) to early adulthood. The study will use neuroimaging and other advanced tools to clarify precisely how and to what extent marijuana and other substances, alone and in combination, affect adolescent brain development.
A large longitudinal study in New Zealand found that persistent marijuana use disorder with frequent use starting in adolescence was associated with a loss of an average of 6 or up to 8 IQ points measured in mid-adulthood. 43 Those who used marijuana heavily as teenagers and quit using as adults did not recover the lost IQ points. People who only began using marijuana heavily in adulthood did not lose IQ points. Two shorter-duration prospective longitudinal twin studies found that youth who used marijuana showed significant declines in verbal ability (equivalent to 4 IQ points) and general knowledge between the preteen years (ages 9 to 12, before use) and late adolescence/early adulthood (ages 17 to 20); however those who went on to use marijuana at older ages already had lower scores on these measures at the start of the study, before they started using the drug. Also, no predictable difference was found between twins when one used marijuana and one did not. 44
Any type of prolonged use of a psychoactive substance has the potential to cause neurological imbalances or impairment—and marijuana is no exception. The exact nature and extent of brain damage or other lasting effects can depend on a number of factors including:
Though marijuana may not be as overtly damaging as alcohol, meth, or cocaine (substances that drastically alter brain chemistry and uncoincidentally, are highly addictive), there are still plenty of undesirable cognitive side effects to be had.
How Marijuana Works
That same study found that the most significant factor was when those participants began using marijuana. Those who started smoking as adults rather than teenagers faced no decrease in IQ. Adolescents with their still-developing brains have the most to lose cognitively. Rather than killing off brain cells, however, marijuana impedes brain development, putting young adults at a number of neurocognitive deficits (the human brain isn’t fully developed until age 25).
This system that marijuana is able to exert influence on is known as the endocannabinoid (EC) system. It affects virtually every part of the brain, including the amygdala (emotion regulation), brain stem (pain sensitivity), and hypothalamus (hunger and sexual impulses), all of which are responsible for those hallmark signs of a marijuana high.
4 Ways That Marijuana Affects Cognition
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