Codeine
CYP2D6 in the liverMorphine - Ultra-rapid metabolisers make more morphine — risk of morphine toxicity
- Poor metabolisers make little morphine — weaker pain relief
Codeine relieves pain only after CYP2D6 in the liver converts it to morphine — and people differ in how much they convert.
vs
Codeine phosphate
A prodrug: it works once the liver turns it into morphine
Dihydrocodeine tartrate
Active in its own right: the BNF gives no CYP2D6 warning
Key takeaway
Codeine has to be turned into morphine by CYP2D6 — and that one step explains most of its extra warnings.