It drove me crazy that in Japan OTC painkillers had a minimum of 4 active ingredients and up to 6 in one case. I could not find any that did not contain caffeine, for example.
The pharmacist explained that caffeine reduces headaches. Well that's great, but I didn't have a headache, I had muscle aches, and I was trying to sleep. He looked at me like I was crazy.
My understanding is that this is also to prevent abuse / recreational use. For example OTC cough pills containing dihydrocodeine (an opiate), but with enough caffeine and others to make recreational doses of the opiate unpleasant even for those with high caffeine tolerance. Or DXM, which is only found with enough guanfecine (IIRC) to make you feel like crap if you take too much. Kind of like how industrial ethanol is mixed with things that would make you vomit.
This is something that really irks me. The reason you always get paracetamol packaged with codeine is to prevent abuse of the codeine. They'd literally rather you destroy your liver and die than risk getting addicted to codeine.
One meta-theory I had is that the caffeine has been determined to help with headaches only because studies done on it are for people who are ill and need pain management, and ill people tend to change their coffee habits.
When I'm healthy and working, I regularly drink about three espresso shots a day. If I'm sick, that plummets to zero while I'm lying in bed and feeling miserable. Invariably, I'll get a killer headache, which the paracetamol alone doesn't fix. I know this, and either I just ignore the headache or make myself some instant coffee at home.
But a badly run scientific study would not control for this quantifiable effect: Sick people get headaches, and dosing them with caffeine reduces the headaches! Ergo... caffeine reduces headaches.
However, I very much doubt that caffeine has a headache reducing effect in sick people that aren't also having caffeine withdrawal.
Imagine a world where most office workers would regularly partake in, say, Opiates to "take the edge off" their stressful jobs. In that world, researchers would confidently conclude the OTC pain relief medication should include Naloxone!
This isn't even that outlandish a scenario. There are countries where people living at high altitude habitually chew Coca leaves to counteract altitude sickness. Should OTC pain killers in those countries include Propranolol? Is it right to force tourists wanting ordinary Paracetamol to also take drugs for acute Cocaine withdrawal just because most locals feel better if they get it?
It's absurd to mix drugs like this, if you think about it a little...
I had a chat with my chemist about why a small bit of caffeine was in the more expensive brands of acetaminophen / paracetamol. I asked if it was used as a kind of pick me up to feel more awake.
He said that the dose of caffeine was very very small and not for that, but it actually helps the paracetamol work better or faster.
The pharmacist explained that caffeine reduces headaches. Well that's great, but I didn't have a headache, I had muscle aches, and I was trying to sleep. He looked at me like I was crazy.