The active ingredient in Pepcid AC is famotidine, which is a histamine H2-receptor antagonist, according to RxList. Pepcid AC is available over the counter in 20 and 40 milligram doses in both regular and chewable tablets.
Famotidine works by decreasing the amount of acid produced by the stomach, according to Drugs.com. Doctors prescribe famotidine for the treatment or prevention of stomach and intestinal ulcers. Other conditions treated with famotidine include gastroesophageal reflux disease, or GERD, which causes a backup of stomach acid into the esophagus; Zollinger-Ellison syndrome; urticaria and upper gastrointestinal hemorrhage. Generic brands of famotidine are also widely available for purchase.