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Dr Munir Pirmohamed, Professor of Molecular and Clinical Pharmacology at the University of Liverpool, in reply to an enquiry:-
“Codeine is a pro-drug and it is converted in the body to morphine which is what leads to the pain relief. This conversion is undertaken by an enzyme called CYP2D6, which is absent in about 7-8% of the UK population. Thus, if you lack the enzyme, you cannot convert codeine to morphine, you do not get pain relief.
In relation to dextropropoxyphene: (a) it is not a pro-drug and is supposed to act directly; (b) it is not broken down by the same enzyme as codeine (CYP2D6) and thus will not be liable to the same issues as codeine; and (c) it is broken down by another enzyme called CYP3A4, which varies in the body, but all people have the enzyme.”
in other words, if you are genetically unable to produce the enzyme to metabolise it, codeine will do nothing for you. We have to make clear co-proxamol, being easily metabolised and direct acting, is a valuable treatment or we will lose it!
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