Interesting experience: usually I dose my promethazine about half an hour before taking my codeine, and I never really figured it to have a real impact on the high other than reducing nausea.
Tonight, I decided to wait until after dosing the codeine to take the prometh. Not only did I get a significantly stronger buzz than expected (which leads me to believe that the promethazine actually was weakening the codeine somewhat), but it seems to have been extended significantly. I'm still feeling a slight afterglow 6 hours later, the high itself has faded for the most part but the analgesia is still going strong, usually my back starts hurting again about 3 - 4 hours after dosing, but it's been 6 hours and I feel, well, quite good.
I'm going to play around with this some more, but maybe the promethazine was affecting the metabolism of codeine more than I thought.