I suspect people generally agree with this, but will extend the "affecting only themselves" a degree out. Did a Father who orphans a child due to drug use only affect themselves? Did the narco terrorists who were funded by his purchase only affect himself?
Do not criminalize the things that make your society worse at scale, rather ensure the positive incentives draw your population in a better direction. I.e. carrot, not stick.
Edit: I'll walk this back to 'is always worth regulating'
On the other hand, sure, regulating and ensuring tax revenue comes from the vice is not a bad idea (and it works ok for gambling, alcohol, and now pot too).
Obviously, even in cases where it is illegal to both buy and sell we still have black markets, but making it ok to buy but not sell seems like a recipe for a giant black market.
Taking a job, quitting to start a company, walking back on a contractual relationship, marrying a partner. While it's true that the direction of impact has outsized and measurable impact on children and dependents, there are other interactions we can't even begin to fathom.
Barry Marshall was criticized in the 80's for his H. pylori hypothesis. Then he drank some to prove his point, and now we know the cause of peptic ulcers and several stomach cancers.
mRRNA self-experimentation is undoubtably risky and I wouldn't do it myself, but it may yield dividends even if it does induce disease state or kill some of those that attempt it.
I don't think we should police it. Not when you can legally buy a car and drive it off a bridge. There are far worse things a person can do.
Compare this to mass scams: If many people are tricked (by a person or by a mass histeria) to do something, then it makes perfect sense to have a regulatory agency step in (e.g. FTC, FDA, SEC, etc). And compare that to people with a medical disability: if someone is permanently or temporarily mentally impaired, then some of these rights are curtailed.