Funny...I belong to several Kratom forums, and nearly everyone uses 100% white grapefruit juice as a potentiator. I've got candida, and can't drink juice without the candida flaring up, so for me, I use turmeric extract with black pepper essential oil as potentiator. It definitely works. Doesn't make the pain killing effect any greater for me--but it makes it last longer--4 hours instead of 2.
Has to be white, not pink/red grapefruit juice. White has some special enzyme that makes it work. And make sure it's 100% juice--no sugar or other juices added. (Read the ingredients on the bottle).
I didn't know it might potentiate other pain meds. Just learned that here. I know that antihistamines (like benadryl) potentiate pain meds. And I also just read recently that magnesium glycinate, taken on a regular basis, not only mildly potentiates the pain meds, but also helps prevent addiction.