The Brain and Marijuana

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Although the United States federal government still deems marijuana illegal, several states in recent years have passed laws legalizing it, substantially increasing its documented recreational use in the States.

Marijuana comes from the dried leaves, flowers, stems, and seeds of the Cannabis plant. The plant contains the mind-altering chemical tetra­hydrocannabinol, or THC, which distorts perception and alters a person’s sense of time, space, and self. With­in minutes of smoking marijuana, THC travels from the lungs to the blood and then into the brain. Eating foods containing THC can also create a high, usually within an hour of ingestion.

Marijuana is not harmless. Neuro­scientists have discovered that regular marijuana use is linked to abnormal neurobiology in brain regions related to reward, cravings, and thought con­trol — all key players in addiction. Marijuana use during the teen years can have long-lasting effects on think­ing, memory, and learning. Although cannabis-use disorders have been less studied than other addictions, some known harms include higher stress lev­els due to craving and withdrawal, in­ability to think clearly, missing school or work, and risky behaviors while intoxicated. As with other addictions, heavy marijuana use seems to increase vulnerability to further drug use in susceptible people, through physical changes in the brain circuits of reward systems. In some users, long-term marijuana use has been linked to schizophrenia.

Our brains make a natural form of THC called anandamide, which acts through cannabinoid receptors in the body that help coordinate movement. This may explain why people’s driving is impaired after smoking marijuana. The hippocampus, involved in mem­ory and learning, also contains many THC receptors, possibly explaining the effects of marijuana on short-term memory. While relatively little research has been conducted on the role and usefulness of marijuana in treating medical conditions, some studies sug­gest that another active compound in marijuana called cannabidiol, or CBD, which does not produce a high, can control epileptic seizures, relieve pain and inflammation, and possibly even treat mental illness and addictions.

Many people with post-traumatic stress disorder (PTSD) self-medicate with marijuana to cope with anxi­ety, stress, and insomnia, and a few research studies appear to support this strategy. These studies show that marijuana might reduce anxiety, im­prove sleep, and erase trauma-related memories in people with PTSD, but it is unknown whether this is due to CBD, THC, or another ingredient. More research is needed to explore these findings. Similarly, marijuana has been widely regarded as a treat­ment for reducing nausea associated with chemotherapy. However, 2017 information from the National Cancer Institute (NCI) says there is currently not enough evidence to recommend using cannabis to treat this side effect of cancer therapy.

Adapted from the 8th edition of Brain Facts by Alison Davis.

About the Author

Alison Davis

Alison Davis is a senior science writer and science-policy strategist. Alison earned a B.S. in biochemistry from Virginia Tech, a Ph.D. in pharmacology from Georgetown, and did a post doc in Developmental Biology at Stanford, after which she completed the Science Communication program at the University of California, Santa Cruz.