Limestone is not directly used in antacids. The active ingredient in many antacids is calcium carbonate, which is a chemical compound found in limestone.

Here's why calcium carbonate is used in antacids:

* Neutralizing Stomach Acid: Calcium carbonate reacts with stomach acid (hydrochloric acid) to form calcium chloride, water, and carbon dioxide. This reaction neutralizes the acid, reducing heartburn and indigestion.

* Antacid Properties: Calcium carbonate is a weak base, meaning it can absorb excess acid in the stomach.

* Availability: Calcium carbonate is readily available and relatively inexpensive, making it a cost-effective ingredient for antacids.

It's important to note that while limestone contains calcium carbonate, it is not directly consumed as an antacid. Instead, purified forms of calcium carbonate are used in pharmaceutical preparations.