Famotidine (Pepcid^®) is used for the treatment of conditions affecting the digestive tract, including heartburn, GERD, and ulcers. Available in both prescription and over-the-counter forms, the drug can be taken once or twice a day, or as needed. It works by blocking the action of a certain histamine receptor in the stomach, which reduces the amount of acid produced. Some potential side effects of famotidine include constipation, dizziness, and headaches. Among the conditions you should let your healthcare provider know about before you take famotidine are kidney problems and any allergies you may have.