Linked from
The 15 pages that link to Codeine, each with the reason it gives.
HeroinCompared with: It is a less potent opioid alkaloid that can be metabolized into morphine.
PapaverineCompared with: Codeine is an opioid, whereas papaverine primarily relaxes smooth muscle.
HydrocodoneCompared with: Both drugs rely partly on CYP2D6 metabolism, but codeine is less potent.
DesomorphineCompared with: Codeine is a less potent opioid with different clinical uses and effects.