Imitation Alka Seltzer for a Fraction of the Cost

I first tried to make tablets using the bath bomb principle: mix in a small amount to liquid (in this case I replaced the bath oils with agave syrup) and smush the mixture together. Perhaps with more patience the right balance could be achieved, but when the cute little balls started swelling, and ended up the next day as a bubbly, sticky mess, I opted for this dry, powdered version instead. Much less labor intensive to make, just as pleasant to consume.

Put the following ingredients into a bowl and mix thoroughly:

1/4 cup citric acid (aka sour salt or lemon salt)
1/2 cup minus 1 teaspoon baking soda
1/2 cup fructose (**see the notes on ingredients in the next step!)
3 tablespoons cornstarch

Store in an airtight container. When needed, pour 2 teaspoons into a small glass of water, stir, and drink. Make sure your measuring spoon is dry and you don't get any water in your jar...

This recipe will yield about 66 doses. If you'd like to try it out with a single dose, mix these quantities into a small cup, then add water:

1/4 heaping teaspoon citric acid (aka sour salt or lemon salt)
1/2 teaspoon baking soda
1/2 teaspoon fructose

This makes a slightly smaller dose than the regular amount, but when it comes to medicine it's better to err on the conservative side.

Filler
Cornstarch and sweeteners are there just for taste and to prevent clumping. Basically to have a dose of 2 teaspoons, you'll want to put in 1/2 cup and 3 tablespoons of filler.

If you want to you can cut out the filler entirely. One dose of your mix will be 1 teaspoon instead of 2. It won't taste as good, but it will still work...

Fructose

I used fructose as a filler/sweetener because it dissolves faster in water than granulated sugar. You can replace it with powdered sugar (aka confectioners sugar, icing sugar or caster sugar), but powdered sugar clumps a little when you combine your mixture with water. The end result is fine, it's just not quite as satisfying to watch. The real Alka Seltzer uses aspartame, which is sold in supermarkets as Nutrasweet. If you want to avoid sugar you can use that instead. You could also cut down on the sugar by replacing some of it with cornstarch.

Instructables member WayneR1 wrote the following comment, which I wanted to include here:

In a lot of people , fructose will cause strong heartburn.

I use glucose if anything, which does not cause any heartburn. The body metabolizes the two in quite different ways. Sucrose too can cause heartburn as it is made of one fructose unit bonded with a glucose unit, and the first step in digesting it, is the decoupling of those two units.

Citric acid
This can be found in the kosher section of supermarkets under the name "sour salt." In Brooklyn, New York, Sahadi's on Atlantic Avenue carries it but they call it "lemon salt." Apparently it can also be found at brewers supplies stores. I've also seen it in regular supermarkets (if you call Garden of Eden regular) with their other spices and even at a cake supply store (but it cost more than $4 per pound). If all else fails, buy a large stash online and make borsh, bath bombs and dishwashing detergent as well as alka selzer.

UPDATE:
Several people have commented that plain baking soda can work too -- although it is a common home remedy, adding citric acid has two advantages:
1. when mixed with water and an acid, baking soda has a chemical reaction which produces carbon dioxide. This is the fizz which is produced when this powder is mixed with water. When this reaction happens in your stomach (rather than in the glass, where the gas dissipates into the air) it can cause swelling and tenderness, and it might force more of the acidic stomach juices up the esophagus -- which worsens the heartburn rather than help it.
2. the citric acid acts as a chemical buffer. This means it will not make the Ph of your insides fluctuate wildly as the base (baking soda) hits the acid (stomach juice).
A commenter who tried both methods said the plain baking soda seems to work faster but that soda combined with citric acid prolonged the effect.
Then of course it's simply much more fun (and tasty) to mix a fizzy, sweet drink rather than downing a glass of lukewarm salty water...