Famotidine, including both 20 and 40 milligram doses, treats gastroesophageal reflux disease, or GERD, and other conditions that cause acid to back up into the esophagus from the stomach, according to Drugs.com. Doctors also prescribe famotidine to treat and prevent stomach and intestinal ulcers and Zollinger-Ellison syndrome.
Famotidine is a histamine-2 blocker that works by reducing the amount of stomach acid the body produces, notes Drugs.com. Common side effects of famotidine usage include headache, constipation, diarrhea and dizziness. Some common famotidine products include PEPCID, PEPCID AC and Mylanta AR. Doctors typically prescribe 20 milligrams of famotidine taken by mouth twice daily for up to six weeks as a treatment for GERD.