Patients who are critically ill in the in the Intensive Care Unit (ICU), especially those who need a breathing machine, can develop ulcers in the stomach that bleed. To prevent bleeding, many such patients around the world receive a drug called pantoprazole that decreases acid production. However, today, compared to decades ago, critically ill patients rarely develop gastrointestinal bleeding. This decrease is likely due to modern medicine, better resuscitation and earlier feeding. There may also be harms associated with pantoprazole and other drugs that reduce acid levels in the stomach including lung infections (pneumonia) and bowel infections (clostridium difficile). Studies in this area are old and of modest quality. Therefore, it is difficult to know whether pantoprazole does decrease stomach bleeding these days, or whether the possible harms of lung and bowel infections are actually more common and more serious problems. The goal of this international study is to determine if, in critically ill patients using breathing machines, the use of pantoprazole is effective in preventing bleeding from stomach ulcers or whether it causes more problems such as lung infection (pneumonia) and bowel infection (Clostridium difficile), or whether pantoprazole has no effect at all. Whether the harms are worth the benefits, and whether the benefits are worth the costs, will be determined by an economic analysis to inform patients, families, clinicians, and healthcare systems globally.