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Epinephrine increases the strength of painkillers. That's why there's caffeine in Excedrin.

I thought the caffeine was there to restrict/narrow blood vessels which usually helps with migraines. Multi-purpose maybe?

AFAIK, I think it depends on where in the body, vasoconstriction in the head vasodilation for the rest of the body.

What does epinephrine have to do with caffeine?

Caffeine directly kickstarts the adrenal glands