Chemistry - How Do Antacids Work?

(This post assumes a general knowledge of chemistry and organic chemistry).

How does Pepto-Bismol work? Well, Pepto-Bismol is an antacid, meaning that it buffers excess stomach acid. Below is a picture of bismuth subsalicylic acid (pepto-bismol).Now, because bismuth is in the same group as nitrogen, bismuth creates 3 covalent bonds in its most stable Lewis structure. In this case, bismuth covalently bonds to 3 different oxygen atoms. Given that bismuth has an electronegativity of 2.0 and oxygen has an electronegativity of 3.5, the bonds that bismuth has with the oxygen atoms are polar covalent, almost to the point of ionic. In any event, these bonds are exceptionally weak and are prone to break in environments of excess charges (as in the stomach). Also, because bismuth is less electronegative than oxygen (i.e., bismuth is more electropositive than oxygen), bismuth will have a slight delta positive charge, and the oxygens will have a delta negative charge.

When the stomach's parietal cells produce excess gastric acid, the concentration of protons, [H+], increases markedly. Because the oxygen atoms have a delta negative charge, they act as weak nucleophiles; the oxygen atoms are craving some nucleus loving. Well, H+ is nothing but a nucleus. Excess H+ will want to attack the oxygen atom that is directly attached to the benzene ring, producing bismuth subsalicylate!This form of bismuth subsalicylate does 2 things: 1) Acts as a direct buffer by buffering excess H+ via the "aromatic" oxygen; and 2) acts as an indirect buffer by producing a salt of BiO+ and salicylate. As [BiO+] increases, excess positive charge accumulates in the stomach. As far as we know, the human body has no enzymes to convert BiO+ into any electroneutral molecule. So, the body is stuck with BiO+. But, humans have the enzyme carbonic anhydrase (CA) that will convert excess H+ into bicarbonate and eventually CO2 gas (which we eventually exhale out of our bodies). Thus, an increase in [BiO+] will also eventually lead to a decrease in [H+] and therefore less stomach acid.

Also notice that the reaction produces salicylate (i.e., o-hydroxybenzoate), accounting for Pepto-Bismol's slight anti-inflammatory properties. But, salicylate is also able to buffer excess H+ due to its carboxylic and hydroxy groups.