How does Pain Relievers work?

just a thought, how does my advil know i have a headache? and should i use something like head on?

If you take aspirin for a headache, the aspirin moves through the blood, and then somehow alleviates the pain there, while not affecting other areas.
How does this pain relieving process work once it gets there?
It doesn't. Any drug like aspirin does what it does wherever it goes, although some of its effects may be specific because its real target molecules aren't everywhere.
But the basic answer as to why it only acts on your headache, is that you only have a headache. That's where you have pain, so that's where it relieves pain. Its effects elsewhere aren't felt by you because you don't have pain there. If you did, the aspirin
would work there, too.

See the site below:

Pain relievers work by blocking certain chemical or neurotransmitters that are known to cause pain in your head, brain. Hope that helps. I am sure there are millions of sites on the net that will help you.

Depends on the pain killer. Most are in some way anti-inflammatory. There are OTC's like aspirin, ibuprofen, naproxen etc. these are true pain killers. There are also steroids which also reduce inflammation, but you have to see a doctor and these have some serious side effects. There is tylenol (in a class by itself). And, finally, there are opiates/opioids. These just get a person too high to care about their pain. True pain relief comes in the form of reducing inflammation. This may take time, and may not be effective when pain is absolutely devastating. Opiates can't actually kill pain, but they can shut a person down long enough to get effective treatment. The problem is that they're addictive with ugly withdrawal, and they're used way too often.