There are now a wide variety of drugs available that are able profoundly to reduce the production of gastric acid. These drugs are currently widely prescribed for the treatment of peptic ulceration and gastro-oesophageal reflux disease. One of the main functions of gastric acid is to kill ingested bacteria. Colonization of the gastric lumen occurs in patients on anti-secretory medication, the degree of bacterial overgrowth depending upon the degree of elevation of the pH. There have been concerns that these bacteria may produce carcinogenic nitrosamines and increase the risk of gastric cancer, but there is at present no definitive evidence in support of this. A profound suppression of gastric acid may also facilitate the colonization of the upper small intestine, leading to deconjugation of the bile salts and malabsorption. There is some evidence that profound gastric acid suppression may decrease the number of ingested pathogens required to produce enteric disease. This chapter discusses these potential bacterial complications of therapeutic acid suppression and the evidence for them.