Wiki gives a good chunk of metabolism ratio:
Quote:
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Roughly 5-10% of codeine will be converted to morphine, with the remainder either free, conjugated to form codeine-6-glucuronide (~70% ), or converted to norcodeine (~10% ) and hydromorphone (~1% ).
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Seeing as it's a good 70% or so, there must be atleast some effect. Compared to Morphine, I (used to

) get so much more from an equivalent dose of Codeine.