Yes, caffeine does benefit many pain relievers. (I usually take coffee with Advil … it works better.) Caffeine also has opposite effects on some people: coffee actually calms me down! (I think I could be a bit “hyper-active” and that’s what I’ve been told is a typical reaction in folks like me).
I think if you get jittery from caffeine, then perhaps pain is increased. Again, this could only be psychological.
Interesting about voltage and pain. I’m not sure I “buy” the concept that voltage alone causes pain (I'm checking it out). In the older machines (DC only) there was no "auto-voltage regulator." For example, if the meter “shot up” we knew the client was sweating (better conductivity), or squeezing the indifferent electrode (because of pain). The shooting-up DC meter meant she was experiencing more pain. When the meter dropped, we knew she was either feeling less pain, or the electrode was drying out.
The “constant current” feature on new machines BOOSTS (or lowers) the voltage to keep the current constant. Problem is, we lost this “bio-feedback” feature. Also, people got WAY too lazy and stopped using a really wet electrode. (We used to use heavy salt water in the sponge electrode to create an excellent contact.)
For years, I have wanted a couple manufacturers to put a +/- meter-indicator on the machine to show when the voltage is being boosted (or reduced). This would be a nice “bio-feedback” feature.
In the old days, I would say: “Oh, I think you are experiencing a bit more pain than normal?” The client thought I was psychic. I was just watching the DC meter!