Mr Blonde
Bluelighter
I don't understand the cimetidine. You say it inhibits metabolism, so the drug stays in the system longer. Isn't this what happens with promethazine? Or is the idea that promethazine is only a weak inhibitor, whereas cimetidine is a strong inhibitor?
They inhibit two different enzymes. Promethazine inhibits CYP2D6, whilst cimetidine inhibits CYP3A4.
Codeine is a prodrug; it's converted into morphine by the liver via CYP2D6. So promethazine would stop as much codeine being converted into morphine.
CYP3A4 metabolizes opioids into less useful substances. By inhibiting that, less of the drug is initially metabolized and goes into the blood stream, and it takes longer for your liver to metabolize what is in your body.
You've got the idea in your edit: that the morphine that codeine is converted to is increased in half-life and blood concentration somewhat. Except that CYP2D6 only demethylates codeine into morphine, CYP3A4 is what then metabolizes both of those into useless things.
I'm very tired and out of it right now so I will have to come back to this and see if it makes sense in a few hours.
