Prescribers are often unaware of possibly dangerous previous medical histories (PMHs) of their patients. Data from a study of nonsteroidal anti-inflammatory drug (NSAID) users served to identify factors associated with this lack of awareness. In this study, we analyzed the factors that may have led prescribers to report the absence of some PMHs that the patients reported as being present. Of 26,618 patients prescribed an NSAID, 469 (1.7%) reported a PMH of unstable angina, 648 (2.4%) reported heart failure, 2,244 (8.4%) reported gastric or duodenal ulcer, 489 (1.8%) reported upper gastrointestinal tract bleeding (UGIB), 5,343 (20.0%) reported gastroesophageal reflux disease (GERD), and 7,832 (29.4%) reported dyspepsia. Between 64 (GERD) and 92% (UGIB) of these patient-reported PMHs were absent in the corresponding prescribers' reports. This discordance was associated with the following factors: patients of younger age, female patients, less frequent patient-prescriber contact, prescription of NSAID by a specialist, no recent specialist consultation, hospitalization or surgery related to the PMH, and no dispensation of proton-pump inhibitors (PPIs) for digestive disorder-related PMHs. The study showed that a substantial proportion of prescribers seemed unaware of the presence of risk-related PMHs that the patient reported when asked.