Depends entirely on the metabolic fate of your chosen poison and its enzymatic pathway.
For the typical pro-drugs (Codeine/Hydro) induction of 2D6 will potentiate the effects but shorten the duration.
Playing around with your friendly neighbourhood cytochrome biocatalysts is damn efficient if done correctly but requires some give and take.
Codeine is metabolized mostly by UGT2B7, only 5% of it is metabolized by CYP2D6. GFJ is completely useless here. Promethazine and Cimetidine are minor players as well.
GFJ can be used for Hydrocodone, since it acts on the CYP3A4, which is the major isoform responsible for Hydro's conversion into Norhydrocodone. CYP2D6(conversion to Hydromorphone) is a minor actor in Hydrocodone's metabolism. Cimetidine inhibits both CYP3A4 and CYP2D6, which makes it superior to GFJ.
Cimetidine inhibits P450 - CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4. While GFJ only inhibits P450 CYP3A4.
Oxycodone, 47% is metabolized by CYP3A4 into Noroxycodone, this is the major player. Then from CYP3A4 it is further metabolized by CYP2D6, 11%, into Oxymorphone. So, once again, Cimetidine is superior to GFJ.
Methadone is metabolized by CYP3A4, CYP2B6 and CYP2D6. GFJ will only inhibit one enzyme, while Cimetidine will inhibit two.
We can go into Morphine, which is metabolized by UGT2B7, both GFJ and Cimetidine are useless.
If you're not using Cimetidine, you're doing it wrong.