I don't use much caffeine. Maybe a coke now and then, nothing regular, rarely have an energy drink, and especially on vacation when I am catching up on sleep had almost no caffeine at all.
I am not surprised. I have been told by a couple of different pain doctors that I have a high natural resistance to certain groups of drugs, like narcotics. When I was undergoing treatment for cancer, I had a major surgery where they removed the tumor from my spine and fused together 6 vertebrae in my neck and upper back. It was a 16 hour surgery and the pain was unbelievable afterward. My doctor had me on a morphine drip, but he said he could tell from my vitals and my reactions even while nearly unconscious that it wasn't working well. When I came to he asked me my pain level (the standard 1-10 scale, 10 being worst pain imaginable) and he said he expected me to say 5 or 6 with the morphine in place. When I told him 9 he was really worried. They checked everything out with the surgery, found it was all what it should be, and that I just wasn't responding to the morphine. He ended up upping my dose on the machine to the max he could safely give me and it took my pain down to about a 7. And it never really gave me that euphoria or "high". I mean I felt it for sure, but not like others describe it. I have also been on strong regimens of percocet and methadone after other surgeries, and always end up with much higher doses to take care of the pain. Oxycontin was the only thing I have found that really hit my pain very well, and that was at the highest dose they could give me and still didn't do what it should have. As a result, I have a very high tolerance for pain.
I have kind of wondered if my reaction with kava was tied to that. Not sure how kava works actually, but valium does nothing for me, literally nothing. Doesn't even make me tired, same with codein.