Morris via text asked:
When I take a painkiller, how does it know where the pain is?
When you take a pill like an aspirin or a paracetamol, what it does is to target the inflammatory cascade. What that means is that if you have an injury to a part of the body, you start to make substances in those parts of the body that signal to nerve cells that that part of the body is hurt and that you shouldn't move it around too much. The way the painkillers do it is they block this cascade of inflammation everywhere in the body at the same time so that anywhere that is hurting doesn't hurt as much. So it's not that it homes in purely on you headache; it has its effects everywhere in your body. But you only notice its effects where you had the pain before because it stops being so bad in that area.