How fast does Acetaminophen start working?

I'm wondering because when I get headaches, I'll take two 500mg tablets and within seconds my head will stop hurting. I can't think that it would work that fast, so I thought maybe it's the placebo effect? But it's strange because even when I tell myself that there's no way it could be... show more I'm wondering because when I get headaches, I'll take two 500mg tablets and within seconds my head will stop hurting. I can't think that it would work that fast, so I thought maybe it's the placebo effect? But it's strange because even when I tell myself that there's no way it could be kicking in that fast, the pain doesn't return. So I'm wondering does it really work that fast? Or is it actually the placebo effect?
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