There is a common misconception among opioid users in this forum, other forums, & on the street, and that is: a good potentiator for opiates or opioids is cimitidine (more commonly known as tagamet).
The theory is that because cimitidine inhibits the CYP3A4 liver enzyme which is responsible for breaking down many opiates/oids into inactive metabolites (noroxycodone, norhydrocodone, etc..) that the opiate/opioid in question will have an increased effect & longer duration. While this is true to a certain extent, there is a vital peice of information regarding the pharmokinetics of cimitidine & it's enzyme inhibition that is widely overlooked by the opiaficionados out there.
See not only does cimitidine inihibit the CYP3A4 enzyme it is also an inhibitor of a wide range of isozymes of the P450 enzyme system. In addition to the CYP3A4 enzyme, cimitidine (tagamet) also inhibits CYP1A2, CYP2C9, CYP2C19, CYP2E1, & most importantly the CYP2D6 liver enzyme.
The opiates/opioids codeine, hydrocodone, & oxycodone are all CYP2D6 substrates meaning they all involve metabolism by the enzyme CYP2D6 on some level. Specifically codeine as many, if not all, of us well know metabolizes into morphine via CYP2D6, hydrocodone is metabolized into hydromorphone via CYP2D6, & oxycodone is metabolized into oxymorphone via CYP2D6.
These metabolites I have just mentioned are all much stronger than codeine, oxycodone, & hydrocodone alone thus they are NOT good candidates for cimitidine (tagamet) potentiation unlike many benzodiazapines.
Hope that helps...Remember kids, eat your grapefruit.