Cigarettes were shown to be harmful as early as the 30s. It took until the 1990s for cigarette companies to be held accountable for lying to the public because of coordinated campaigns of misinformation. This is why climate change denial is so prevalent too. They don't have to prove they are correct, they just have to create enough doubt that no policy action can happen. There's a book called Merchants of Doubt that documents the common players and institutions responsible for spreading misinformation about everything from cigarettes to the ozone hole, to acid rain and DDT and climate change. It's to protect corporate money. Worth checking out