As far as methadone goes, i know for a fact that Grapefruit juice greatly increases both peak values and half-life of the drug. I was even told this by my methadone clinic doctor. Cimetidine does the same for methadone.
Another note is using two kinds of opiates at the same time: bad idea. Eventhough they both are mu opioid receptor agonists, the small difference in chemical structure only allows one of them to activate the receptor, they cant both fit onto it, so you create a "competition" between the two and the one with a higher affinity or a greater amount will win and basically block and "waste" the other opiate you took. This is especially true when mixing tramadol with other opiates or methadone with other opiates.
Codeine, which is worthless in my book anyway, is a strange drug mecause it is metabolised to morphine, heroin also is quickly converted to morphine and so for these drugs that are ultimatley just changed to morphine in the body, might not have the same reaction to grapefruit juice and cimetidine. They both work great for benzos too. But remember, be very carefull, they can increace plasma levels by as much as 50% so you could eisily overdose playing with these two things. Also they inhibit the metabolism of common other high blood pressure and antidepressant drugs ans countless others. So if you're on other meds you could be manipulating thier levels too. Grapfruit juice is dangerous to toy with when it comes to drugs, you'd be surprised at how many drugs are metabolised by the enzyme that it inhibits. Many prescription bottles say "do not drink grapefruit juice while taking this medication".
On another note, In my opinion and experience, things like diphenhydramine (benadryl) and promethazine (phenergan), just ruin an opiates high. Instead of the crystal clear euphoric fuzz, its transformed into this shitty, groggy, detached, foggy headed feeling. For me, an opiate lover, I just want to increase the pure opiate feeling, not add other impure effects to it with things like the above mentioned or benzos or weed or alcohol. that's why I stick to GF juice and cimetidine (Tagamet). I hate it when a doctor in the E.R. comes in and says I'll get you a shot off demerol and phenergan to control its neusea, I make up the excuse that I'm allergic to phenergan just to get the pure demerol, it's so much better.