The long term risks of tobacco use were not suspected until after the invention of the cigarette rolling machine. Lung cancer was once a rare disease.
Marijuana users have typically smoked far less than tobacco smokers, and because of its illicit status, accurate and unbiased observational research has been difficult. There may be long term consequences that have not been detected. With legalization, we will see increased individual consumption. We should be recording data like cancers, heart and lung disease, birth defects, etc., so that in ten years we are not as ignorant as we are now.