You can prove it yourself: marijuana is harmful. You can prove it both logically and historically.
Logical: Marijuana makes you feel good at the same time that it is distorting your perception of reality. Therefore, your mind is being conditioned chemically to have a distorted perception of reality.
Historical: The Native Americans controlled the usage of marijuana. They allowed it very seldom, at special religious gatherings. This way, they kept it by regulation out of ordinary use.
You don’t need the scientific studies.
FWIW the Americans Indians didn’t have cannabis and it’s use wasn’t widespread in the USA until the 1920’s when they Mexicans fleeing the Mex Revolution showed up with it and then the blacks picked it up from there.