OK. This is sometimes confused with the 2-phase drug metabolism that happens in the liver.
With regards to first pass or not, the 2-phase metabolism of a drug is what it is. The CYP inhibitors (GFJ) are effective on the first phase of all drug metabolism, first pass or not.
Ah, thanks, that makes sense. What I meant was that I had heard it didn't really make a
noticeable difference if you were using an ROA that bypassed first-pass metabolism, because during first pass CYP inhibition makes a much bigger difference, but it makes sense what you said about it potentially making the effects of the drug last longer but not stronger.
I think you meant CYP3A4, that is the one that converts oxy and hydro to an inactive metabolite, that's what GFJ primarily inhibits. The only time you care about CYP2D6 inhibition is if you are taking codeine which is metabolized by CYP2D6 to morphine. If that is inhibited, you won't get any buzz or pain relief
That's a very good point and is mostly correct, I'm sure Effie meant to say CYP3A4. But not everything you said is 100% correct.
Oxycodone is deactivated by CYP3A4 (and CYP3A5) so CYP3A4
is of primary concern when it comes to oxycodone.
As for codeine, it is activated to morphine by CYP2D6, but it is also activated by CYP3A4 into codeine-6-glucuronide which is responsible for a large percentage of the analgesia. So inhibiting CYP2D6 would decrease the "buzz" and the analgesia, but not
eliminate analgesia. One wouldn't want to deliberately take any CYP2D6
or CYP3A4 inhibitors with codeine, although taking one or the other should not make codeine
completely ineffectual.
In addition, codeine is not actually the only opioid where we have to worry about CYP2D6. Tramadol is converted by CYP2D6 to its most active metabolite, O-desmethyltramadol, which is
way stronger and longer-lasting than tramadol. So with tramadol we do not want to inhibit CYP2D6. Hydrocodone is also activated by CYP2D6 to hydromorphone. so again CYP2D6 inhibitors are undesirable with hydrocodone.
But what confuses me about tramadol is that it is available for injection (unlike codeine), which to me does not make sense, why would anyone want to inject tramadol when it appears that would make it
much less effective?
Oxycodone is metabolized by CYP3A4, CYP3A5 and CYP2D6. Oxycodone is deactivated by CYP3A4 and CYP3A5 to noroxycodone and activated by CYP2D6 to oxymorphone. Oxycodone itself is believed to be responsible for much of its effects, however since we know hydromorphine is active, would it be safe to assume that inhibition of CYP2D6 would decrease the metabolization into hydromorphone and therefore be undesirable?
TLDR; version - Don't take CYP2D6 inhibitors with codeine, tramadol, or hydrocodone. Taking CYP3A4 inhibitors (such as grapefruit) with most other opioids can make the effects stronger and/or longer. Question: why is tramadol available for injection?