The
metabolism of codeine results in other opioid metabolites, including morphine,
and a minor amount of hydrocodone. In fact, this is how codeine works -- by itself, codeine is ineffective. But because it is metabolized into a small amount of morphine, it is effective against pain.
Depending on the laboratory method used by your doctor's testing service, false positives can be reported if their "cut-off level" is set too low.