Alka-Seltzer is an over-the-counter medication used to relieve heartburn, acid indigestion, and upset stomach. It works by combining an antacid (sodium bicarbonate) with aspirin to provide fast relief from common digestive issues. Alka-Seltzer is available in effervescent tablets that are dissolved in water before being consumed. It is a popular choice for those seeking quick and effective relief from stomach discomfort.