Famotidine is a histamine-2 blocker used to decrease acid in the stomach, according to Drugs.com. It is prescribed to treat ulcers and gastroesophageal reflux disease. It can be purchased over-the-counter in tablet or liquid form for the treatment and prevention of heartburn or sour stomach.
Famotidine may cause mild to severe side effects, states Medline Plus. These effects include dizziness, constipation or diarrhea. More severe side effects include hives and difficulty breathing. Some of the brands offered over-the-counter are Pepcid, Pepcid AC and Fluxid. Famotidine is often used as part of a treatment program along with diet or lifestyle changes.