If someone is allergic to codeine...

[SpeedyBaboon]

... would they definitely be allergic to hydrocodone too? I know they're closely related chemicals, but would it be possible for someone to be allergic to codeine and NOT to hydro?

 

[Bliss26]

Yes it's possible either way. I think codeine and morphine are more closely related. I had a pharmacist argue with me, saying it's impossible for me to be allergic to morphine but not codeine. My body disagrees.
Still, if you're allergic to one and they're chemically similar I would proceed with extreme caution.