Doctors won't prescribe enhancers just for enhancements, but all stimulants (except modafinil I guess) were OTC for at least a time, so we'd expect an enhancer (if safe) to be regulated as a supplement. Caffeine is "the one good drug" and I don't see that changing, and that's even though there are fatal overdoses on it and the side effects are pretty bad (but not psychosis bad), so it's not a terribly high bar to clear for a new compound.

It'll be interesting to see if new compounds have tolerance or withdrawal effects -- something that made you smarter for 4 hours but unable to tie your shoes for 2 days would be a non-starter, as would a smart drug that then permanently lowered your non-drug IQ and raised your chance of Alzheimer's. If anything if I remember my course on the subject correctly pretty much all psychiatric drugs have withdrawal effects that are worse or longer lasting than stimulants do, and while that's not something that would cause them to be made illegal, it would make doctors very reluctant to prescribe them "just to try" the way some have with ADD. (I also don't think a compound that has fatal withdrawals would be allowed as a food or supplement today, alcohol is pretty much grandfathered in.)