i use quinine to potentiate codeine, didn't know people used it with much else though. i found this;
Quinine and quinidine accentuate the buzz of opiates but only directly help with codeine metabolism by boosting the amount changed into morphine. Tagamet and grapefruit juice will weaken codeine substantially. The prescription antihistamine promethazine (Phenergan) helps out codeine even more, and is found with codeine in the Schedule V cough syrup Phenergan VC With Codeine. Quinine will tend to have its effect without impairing hydrocodone but does seem to burn off the buzz more quickly and may flatten the dose-to-response curve.