Earlier quoted context omitted.
As someone who has used opiates for well over a decade (for medical reasons), heroin and stronger synthetic opioids can absolutely cause an opioid naive individual to have withdrawal symptoms after even a week. This is why codeine OTC recommends no more than three consecutive days of use.
Wait, you've got over-the-counter opiates in the US (I assume it's the US)? What?
Codeine is available with a pharmacist's approval alone in my state. So while it may not technically OTC by that definition (or is, I don't know how OTC is defined), it can be had without a prescription by visiting a participating pharmacy and simply requesting codeine+guaifenesin. Done it a few times myself.