The maximum daily dose of over-the-counter (OTC) famotidine (Pepcid AC, Zantac 360) in adults and children over 12 years of age is 40 mg per day unless your doctor prescribes it otherwise. If your symptoms persist more than 2 weeks, contact your doctor. Higher doses may be used for other conditions such as erosive esophagitis or Zollinger-Ellison Syndrome.