I believe I read somewhere that Big Pharma lobbied against weed back in the day because they saw it as a threat to pharmaceuticals. They essentially used scare tactics and made it seem like a drug that made people "evil" and do terrible things. People started believing it and here we are.

I'll edit this comment if I find some sources....

Update:

It pretty much says that weed helps a lot of people and that Big Pharma would rather have those people take pills than weed.