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In addition, researchers now have a good understanding of how marijuana in particular affects the brain. The chemical in marijuana responsible for producing mood elevation and relaxation, THC, interferes with the exchange of information between neurons. Regular exposure to THC in adolescents can permanently change neuropathways that are related to cognition, including learning, attention and emotional responses. In some adolescents, it can also lead to long-term dependence.

This is why the American Academy of Pediatrics has warned against both medical and recreational use of marijuana by adolescents. (In adults, some medicinal forms of THC are approved for specific indications, such as nausea caused by cancer chemotherapy . The major nonpsychoactive component of marijuana, CBD, has been medically approved for specific types of epilepsy and other uses.)

The risk that marijuana use poses to adolescents today is far greater than it was 20 or 30 years ago, because the marijuana grown now is much more potent. In the early 1990s, the average THC content of confiscated marijuana was roughly 3.7 percent. By contrast, a recent analysis of marijuana for sale in Colorado’s authorized dispensaries showed an average THC content of 18.7 percent.

The proposals for legalizing marijuana under consideration in New York and New Jersey allow for use starting at age 21. While society may consider a 21-year-old to be an adult, the brain is still developing at that age. States that legalize marijuana should set a minimum age of no younger than 25. They should also impose stricter limits on THC levels and strictly monitor them. Educational campaigns are also necessary to help the public understand that marijuana is not harmless.