With one Prilosec OTC pill in the morning, block your heartburn all day and all night. Heartburn happens when stomach acid refluxes into the esophagus, often causing a burning sensation in the chest and throat. Prilosec OTC’s omeprazole works differently by preventing excess acid production. Prilosec is not intended for immediate relief of heartburn; this drug may take 1-4 days for full effect. Active ingredient: Omeprazole Delayed-Release Tablet 20 Mg