Since 1996, California's (and other states later laws) had legal medical cannabis, and since then, many tens of millions of Americans (and Canadians) have been using medical cannabis under medical supervision for more than 25 years, and no real epidemics or raises in the rates of schizophrenia or cancer has been occurring due to cannabis use.
Add the many other millions regularly using cannabis through state legalization, and still, no increases in disease or death resulting from cannabis use.

But, further studies are required to determine the EFFICACY (as it's now known to be GRAS) of cannabis as medicine should be done. 'Cuz 100 million people using cannabis will little ill effect just isn't convincing.