It has to do with the way the molecules bind. The hydro molecules are hydrogen and the oxy is oxygen. Typically, people will tolerate both well or neither because they are both metabolized in the same location. However, in my circumstance, I can tolerate all opiates well including morphine. . .but, I have moderate allergic reactions to codeine.

So, when it comes down to it, it really depends on your body type and what you can tolerate.

As always, please be careful when taking any opiod analgesic as it can be hazardous to your health including addiction and/or fatal overdose.