Helicobacter pylori and non-steroidal anti-inflammatory drugs (NSAIDs) each cause peptic ulcers but by different mechanisms. As a result, the effect of both of these risk factors together is not a synergistic enhancement of injury, ulceration or rates of complications. Indeed, there are circumstances under which patients infected with H. pylori are less prone to NSAID-induced ulcers than those who are not infected or who have undergone eradication treatment. This may be because of opposite effects on gastric mucosal prostaglandin synthesis or for other reasons. Reluctance to accept that there may be specific circumstances where H. pylori is beneficial may arise because of the psychological process of cognitive dissonance.