There is a specific reason that some people are unaffected by Codeine.
Codeine (3-Methylmorphine) produces insignificant effects on its own, if any. It must first be demethylated in the body. This process renders the Codeine into Morphine, the latter of which is responsible for essentially all effects produced by Codeine.
There is an enzyme CYP2D6 that is responsible for metabolizing Codeine into Morphine. Like all enzymatic activity, heredity/genetics plays a major role in how effective this enzyme is at converting. Individuals with multiple copies of said enzyme can metabolize significantly more Morphine from the same given dose of Codeine. An individual with a weak or absent CYP2D6 function will thus not receive much effect from the use of Codeine.
For the vast majority of people on this planet, Codeine is pegged at 10% the strength of Morphine. So, 100mg Codeine = 10mg Morphine and so on. Individuals with more copies of CYP2D6 will thus have a higher ratio of conversion.
I am one of those people for whom Codeine is ineffective. I´ve never gotten tested or anything, but I´ve taken 10g of Codeine down the hatch in a single sitting following half a day of CWE hundreds of little pills. I was in Iraq at the time. It was my first day there and I was unable to find any Opioids stronger than Codeine. I knew I had had little reaction in the past and had already suspected enzymes to be the culprit. I took that huge dose as I was already dependent on Heroin and I wanted to know if I should ever fuck with Codeine again. I believe I felt a small twinge of relief from withdrawal following the ~10g dose, but I´d be lying if I said I was certain.
I´m not positive, but I believe it´s 10% of Caucasians will have an issue with their CYP2D6. As already implied, race/ethnicity is a huge predictor of potential enzymatic mutations.