* Photo: Thomas Hannich * Aluminum hydroxide

In the acidic confines of your stomach, Al(OH) 3 acts as a relatively strong base, with three hydroxide (OH-) groups eager to hook up with any excess hydrogen ions (H+) floating around. When combined, all those Hs and Os form harmless water. But too much aluminum can cause constipation, so Mylanta has to deploy a counterattack.

Magnesium hydroxide

You know Milk of Magnesia? The chalky white liquid has been sold since the 1880s as a remedy (albeit a harsh one) for "irregularity." In your gut, this ingredient neutralizes acid just like aluminum hydroxide. But downstream, it causes you to retain water, which accumulates in the colon, building pressure until ... well, you get the idea. With equal amounts of Al(OH) 3 and Mg(OH) 3 in Mylanta, their constipating/laxative effects should cancel each other out. In some people, however, they simply alternate.

Simethicone

An antigas agent made from polydimethylsiloxane, the polymer goop that makes Silly Putty, bathtub caulk, and breast implants possible. When diluted with water to 50 parts per million, it reduces the surface tension of all those little gas bubbles in your belly so they can merge into one big bubble that's more easily burped up. Excuse you.

Butylparaben

Sometimes it's a preservative (the probable use in Mylanta). Sometimes it kills the bacterium responsible for dental plaque. And sometimes butylparaben is absorbed into the bloodstream of lab mice, where it mimics estrogen and retards sperm production. Perhaps Mylanta has a future as a rodent contraceptive.

Hypromellose

This stuff gained fame as "artificial tears," an eye lubricant used in ophthalmic surgery. In Mylanta, its plant origins are put to work: The cellulosic lattice structure holds some of the active ingredients hostage, releasing them slowly over time.

Carboxymethylcellulose Sodium

Another cellulose product. In libraries and museums, it's used as a "bandage" to cover small rips in antique papers. Here it likely serves as both a backup to hypromellose and a thickener. Watery antacid just wouldn't seem as effective.

Microcrystalline Cellulose

A nearly perfect oral drug carrier, MCC is stable, inert, and tasteless, with a creamy mouthfeel. And like simethicone, it destabilizes gas bubbles. Just don't tell anyone it's made from wood pulp — some people are squeamish about eating lumber.

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