Non-scheduled drugs? I don't think that poses a problem. I fill scripts for birth control from out-of-staters quite regularly. Problems arise with scheduled drugs. I know that specifically, about 8 states such as California and New York require schedule II scripts to be written on official triplicate forms. A schedule II written on a normal prescription blank in any of the other 42 states brought into one of those "triplicate states" like California, can't be filled.
There are more knowledgeable, practicing pharmacists on this board who can probably shed better light on this 😉