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.
The cut-off is the concentration at which a target drug is determined to be present and detected (true positive) or absent, not detected (true negative).