An allergic reaction to Codeine would mean that you start having rashes for 1-2 days after taking the pills, but the question is if this means you’re allergic to Vicodin, Morphine or only to opiates?

The answer is that Vicodin being a semi-synthetic opiod that contains hydrocodone does not have cross reactivity with the true opiates, so this means that you’re probably not going to have an allergic reaction while taking Vicodin.

As for the morphine, it would be possible for your body’s antibodies to not recognize morphine as an antigen. This might be because Codeine and morphine have somehow different chemical configurations, as even if Codeine has the basic structure morphine does, it also contains methyl groups might cause your immunoglobulins to react allergic to it, but not to morphine which doesn’t have this.