The bioavailability of AlkaSeltzer with aspirin... (there are many other types now for other uses) is unique.

We studied it years ago... on volunteers the blood levels of aspirin were almost equivalent to an IV administration in onset.

Adding a buffering agent is not new. There used to be a product called Bufferin on the market. I don't know if they still make it.

I don't take it every day... or all day long. Just once a day for 2 or 3 days when I have a flare of burning. The blast of bicarb is also useful for gout, and food allergies. Environmental allergists use TriSalts (the bicarb portion) which does not have aspirin added to abort allergic responses from the environment.
http://www.drhoffman.com/page.cfm/537
notice the magnesium content...similar to Ionic Fizz in fact.

I personally am guarded about aspirin everyday.

The mechanism that aspirin and NSAIDs use that promotes bleeding is not only local. It is metabolic throughout the whole body affecting cytokine production everywhere as an anti-inflammatory. This whole body effect also reduces platelets, and that is how aspirin can prevent blood clots. In people with damaged intestines, bleeding will result, because of this platelet action. So the buffering is only good as a temp measure.

I think the bicarb blast temporarily neutralizes the acids in the blood which are mediators of pain. There have been some studies recently on "acid" being the culprit of pain generation.
I'll see if I can find those. One was on a chronic pain journal site about 3 yrs ago, if some of you want to look for it too. There is also evidence that people with mild renal tubular acidosis, do better when using some bicarb daily, than those that don't. High doses are not necessary.

I've had elevated uric acid for many years with only 2 gout attacks. I've seen several sites for gout recommending the bicarb solution too. So while AlkaSeltzer is a common remedy targeting "hangovers" it really is a complex drug delivery product! Don't abuse it, therefore, but don't ignore it either, as it may be helpful when used wisely.