u/Intelligent_Pass205

Promethazine moderately inhibits CYP2D6 which means less morphine through demethylation and it is also an weak to moderate inhibitor of D2 dopamine receptors which means less dopamine from the morphine.
If the point is helping with the histamine release. why not use something like cetirizine? It essentially inhibits no major enzymatic pathways for morphine production in the liver and still blocks histamine as efficiently.

Promethazine moderately inhibits CYP2D6 which means less morphine through demethylation and it is also an weak to moderate inhibitor of D2 dopamine receptors which means less dopamine from the morphine.
If the point is helping with the histamine release. why not use something like cetirizine? It essentially inhibits no major enzymatic pathways for morphine production in the liver and still blocks histamine as efficiently.