This is how research works. Years ago, scientists first noted an association between cigarette smoking and lung cancer. Only later were they able to figure out exactly how cigarette smoke damaged the lungs and other parts of the body, causing cancer and other diseases.
The research on marijuana and the brain is at a much earlier stage. We do know that THC, one of the active compounds in marijuana, stimulates the brain and triggers other chemical reactions that contribute to the drug’s psychological and physical effects.
But it’s not clear how marijuana use might lead to psychosis. One theory is that marijuana may interfere with normal brain development during the teenage years and young adulthood.