How do you ensure they don’t sell it? How do you ensure vulnerable people aren’t taking it? How do you ensure undesirable strains do infiltrate the market? How do you ensure people know what they’re taking? Etc. Like most drugs, used in the wrong way by the wrong people, there’s a lot of scope for harm. Prohibition basically doesn’t work, but it could be regulated in a way which protects people and stills allows most people to smoke to their heart’s content.