Anecdotally, I can offer my experience.
I have coadministered kratom and Harmala/caapi many times, especially lately as my daily level of pain requires me to use an opioid at least once a day (and with that I am still in quite a bit of pain- usually it is 2-3 times daily). I don't LIKE to do it, not for any fear of health problems, but rather I feel that it blunts the experience and leaves me with less of an Impact. This could be psychological.
I have also taken Harmala with 10mg hydrocodone (prescribed of course), with a similar experience as with kratom. I feel it takes away from the experience. But, then again, so does being wracked with pain. So I feel a little like my hands are tied. It's lame, I don't even like taking these things daily AT ALL and I pray one day I won't need them.
And finally I once took 5mg oxycodone at the end of a Jurema/Harmala experience, for pain, and had no negative consequences.
However of course YMMV and its better to not mix them, just for the sake of the experience
بسم الله الرحمن الرحيم
Fairly responsible Kratom user.
"whenever he drank ayahuasca, he had such beautiful visions that he used to put his hands over his eyes for fear somebody might steal them."
in between the grinding-brakes of a train crash while aluminum-foil robots make obnoxious sex noises on a static-filled walkie-talkie radio.