Nowadays, weed is more than 300 percent stronger than the stuff from the mid-1990s. “THC content has gone through the roof while the amounts of CBD and other constituents that might mitigate the negative effects of THC have gone down,” says Staci Gruber, Ph.D., associate professor of psychiatry at Harvard Medical School and director of the Marijuana Investigations for Neuroscientific Discovery program at McLean Hospital in Massachusetts.
Why is weed much stronger today?
In the early 1990s, the average amount of THC in marijuana seized by the U.S. Drug Enforcement Administration was less than 4 percent; in 2018, it averaged more than 15 percent. The average THC potency (or strength) in products available in some state marijuana dispensaries can be even higher.
One of the reasons weed is getting stronger is because the cannabis industry has shifted focus on the production of sinsemilla. Sinsemilla is the tops of female plants that have not been fertilized, which contain the highest levels of THC. Harvesters cut off trimmings and remove all large leaves that contain lower levels of THC. Strain selection has also contributed to the increase in potency. Strains with high levels of cannabinoids are favored by growers. Unlike the landrace strains that dominated the ’60s to the ’80s, today’s strongest strains are hybrid crosses of other popular strains. New, more potent combinations are being discovered every day.
What exactly does marijuana do to the brain?
Marijuana works by interacting with the endocannabinoid system, which is composed of chemicals and receptors located all throughout the body. There are endocannabinoid receptors, or “feel-good” antennas in our liver, intestines, fat, vasculature, and every single cell and organ system, and especially our brain.
THC attaches to and activates CB1 receptors, which influence everything from mood to metabolism to memory to movement. Most of marijuana’s acute impacts on the brain and body are driven by the activation of CB1.
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. Cognitive impairments in adult rats exposed to THC during adolescence are associated with structural and functional changes in the hippocampus. 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.
Imaging studies of marijuana’s impact on brain structure in humans have shown conflicting results. Some studies suggest regular marijuana use in adolescence is associated with altered connectivity and reduced volume of specific brain regions involved in a broad range of executive functions such as memory, learning, and impulse control compared to people who do not use. Other studies have not found significant structural differences between the brains of people who do and do not use the drug.
An increased risk of psychosis, changes in the brain’s reward system and the scrambled neuron signals that may underlie “the munchies” are just some of the many potential effects of marijuana use on the brain.
“The biggest risk related to the use of marijuana is the increased risk of psychosis,” said Dr. Scott Krakower, assistant unit chief of psychiatry at Zucker Hillside Hospital in Glen Oaks, New York. Another significant risk, for those who use marijuana during their teenage years, is an increased likelihood of an IQ drop.
Several studies, including two large longitudinal studies, suggest that marijuana use can cause functional impairment in cognitive abilities but that the degree and/or duration of the impairment depends on the age when a person began using and how much and how long he or she used.
Among nearly 4,000 young adults in the Coronary Artery Risk Development in Young Adults study tracked over a 25-year period until mid-adulthood, cumulative lifetime exposure to marijuana was associated with lower scores on a test of verbal memory but did not affect other cognitive abilities such as processing speed or executive function. The effect was sizable and significant even after eliminating those involved with current use and after adjusting for confounding factors such as demographic factors, other drug and alcohol use, and other psychiatric conditions such as depression.
More research will be needed to answer definitively whether marijuana use causes long-term IQ losses and whether factors that weren’t measured in the prior research, such as the increasing amounts of THC in cannabis and the emergence of new cannabis products, are relevant.