My mother always told me that taking an aspirin before bedtime was the best birth control pill in the world.
It was only after I got married that she told me to place it on the inside of one knee and hold it tightly with the other...
That's a really old joke, but I guess this is the place for it.
In reality, aspirin is acidic, and could cause acid burns from too long contact. Some folks have discovered this in treating toothaches with contact aspirin. Their temporary treatment caused permanent damage.
Since the stomach is already acidic, aspirin does less harm there. Still, I feel that aspirin is more of a known quantity than the newer pain relievers, and aspirin reduces irritation of tissues, which is a component of muscle aches.