An antacid is a substance that neutralizes stomach acidity and it is used to relieve heartburn, indigestion, or an upset stomach. Some Antacids have been used in treating constipation and diarrhea. Marketed Antacids contain salts of aluminum, calcium, magnesium, or sodium. They come as liquid or chewable tablets and can be bought from pharmacies and shops without a prescription. They give quick relief of symptoms for a few hours but do not treat the underlying cause and long-term use of it is not recommended.