AlkaSeltzer was originally formulated decades ago for hangovers.

Since then we have learned much about effervescent products, and that they deliver drugs (and some vitamins) quickly into the blood. We've also learned about acid/base swings in the blood, which are really kept by our body's systems to a minimum. But at the receptor sites for pain, there is an acid gradient to activate the pain impulse. This was posted on a pain site.

What the bicarb in the AlkaSeltzer does, is block receptor and also that of histamine. Histamine does many things in the body and brain. In the brain it is the signaling molecule for the release of other neurotransmitters.

So today, AlkaSeltzer is much more than a simple hangover remedy-- plop plop fizz fizz, oh what a relief it is!
http://www.youtube.com/watch?v=bxjb2UJZ-5I

Here is a little story from this summer. Our son came upNorth to visit for a short while and we went out to lunch and he and I had a perch basket. Nice perch...I love perch..and they were very generous with it.
I wondered why...since perch is expensive. Then we went back to the island and he took his camera to explore and I picked up my major book Pillars of the Earth, and when he came back he said his feet were burning...and so were mine! I thought...aha...the fish was going old, and had that histamine thing which causes tingling and burning. So we both did the plop plop fizz fizz and within minutes we were back to normal. He was IMPRESSED ... not many things impress him about us anymore! LOL

http://en.wikipedia.org/wiki/Scombroid_food_poisoning
our case was mild.... but this can be a very uncomfortable thing to have from old fish.