seems pretty obvious, really. Whatever goes in is what the baby gets. Codeine depresses the central nervous system. You can feel it happening, if you ever feel like you have to take the stuff.
These discoveries by researchers and doctors are always rather humorous. If you think about it, they only pander to perpetuating an "I just don't know" attitude, which serves the system very well. Meaning -- they exhibit a rather slow evolutionary development vis a vis what's plain and simple common sense.