Simplicity said:
It depends on the person. I drink maybe 3-4 sodas a day (38 mg*4 < 2 Excedrin), and I can definitely feel the effects (headaches) when I go a day without. Then again, I have been drinking sodas for a prolonged period.
There is significant variation of sensitivity, sure. But as you note, you've got a long-term pattern of intake. You should still not be seeing withdrawal symptoms after a single dose of Excedrin.
Also, we must beware confounding bias. For example - 4 sodas is about three-quarters of your recommended daily intake of
water. If you go without much of that water, you'll be dehydrated, and that'll also cause headaches.
Now, let us attach the two. Say you've got a day where, for some reason, you're prone to a headache. You take caffeine-laden Excedrin. Caffeine, among other things, is a mild diuretic. So, if you don't also increase your water intake, you can end up slightly dehydrated afterwards. Then the Excedrin wears off, and you now have yet another thing making you prone to the return of a headache.
Better living through chemistry does require we be careful about the fiddly details.
