Non-steroidal anti-inflammatory drugs have been clearly shown to increase the chances of peptic ulcer and its complications. Risk factors associated with this propensity are as yet poorly and inconsistently defined. Clinicians have long accepted that non-steroidal anti-inflammatory drug (NSAID) treatment may cause peptic ulcer and its complications. In assessing the chances it is necessary to define what we mean by risk, to examine the features which provide convincing evidence of causality and to determine whether the studies available are of a sufficient standard to be compelling.