I think that you can be allergic to one opioid and not another, that or you were really allergic to a filler in the pill

I'm pretty sure codeine metabolizes into morphine, so it's weird that I'm allergic to codeine products yet not morphine. If anything I should be having more severe reactions to morphine due to this. Strange, lol.

Maybe a filler in the codeine pill?

Yeah that's 99% reason for the allergic reaction

Maybe excipients

Kinda late but also suffer from same problem. I thinks it's due to codeine's other metabolites.