It's fundamentally the responsibility of the FDA to regulate "definitive claims that a compound is safe", whereas the standard for approval for use in a clinical trial is a bit different.
And this is a general issue with prophylactic or anti-aging style 'drugs'. Demonstrating a compound is safe enough to not cause undue harm while helping to rid the body of an accute ailment is a very different kind of 'safe' than something one might be expected to take every day for years.