Free speech issues aside, there were plenty of doctors who recommended Hydroxychloroquine. I know a (good) doctor who thought that Ivermectin probably had in vivo antivral properties (before studies showed that it didn't). Lots of doctors have weird fringe opinions.

Speech may become illegal if your intent is to hide known risk, but most certainly not for being wrong, otherwise you better be pounding your fist to arrest a huge number more of people honest wrong.

It's worth considering why we don't do that.

When will cigars, cigarettes, and alcohol be made illegal? - they're certainly not good for your health, and the evidence is everywhere.