Alka-Seltzer is an over-the-counter effervescent antacid and pain relief medication. It typically contains aspirin, sodium bicarbonate, and citric acid, which combine to relieve heartburn, indigestion, and headaches. When dissolved in water, it produces a fizzy solution that helps neutralize stomach acid and provides relief from discomfort. It's important to follow dosage instructions and consult a healthcare professional if symptoms persist.