Coming back into the country from Canada with Brassard's prescriptions did require showing the American customs officer the bottles and the specific prescription info, and some back and forth about the doctor and his location (checking to be sure we really had been to his hospital?). And that was for antibiotics and anti-inflammatories as well, not just the controlled painkillers. The impression I got was that since they worry about drug smuggling from other countries, it was OK if and only if the *prescription* was from a Canadian doctor.
But who knows, that's one story about one border guard. I didn't research the legal policies - because it was all right if they chose to confiscate the meds, my wife's local doctor would happily replace it all - so it's just an anecdote.
