Some authors have proposed that in humans there may be a third set of tooth organs on which this drug could be used. This theory could explain why some people get supernumerary (extra) teeth--in most people these extra organs appear to regress but in some contexts maybe they don't. From what I can tell (again, not quite my expertise) there is not a scientific consensus on the presence or prevalence of this third set of teeth in humans or specifically human adults, which is why this treatment is primarily focused on those people whose teeth never formed--ostensibly their first or second set of tooth buds may still be present and just need to be triggered to develop.

Even if we assume that adult humans have a third set of tooth buds on which this drug could act (and that's questionable), giving the drug in IV form would probably just make all of the buds grow, which would be problematic. But that could be reasonable for a person for whom few or no teeth had grown.