- Background The incidence and severity of Clostridium difficile infections are increasing. Acid-suppressive therapy has been suggested as a risk factor for C difficile, but this remains controversial. Methods We conducted a pharmacoepidemiologic cohort study, performing a secondary analysis of data collected prospectively on 101 796 discharges from a tertiary care medical center during a 5-year period. The primary exposure of interest was acid suppression therapy, classified by the most intense acid suppression therapy received (no acid suppression, histamine 2 -receptor antagonist [H 2 RA] therapy, daily proton pump inhibitor [PPI], and PPI more frequently than daily). Results As the level of acid suppression increased, the risk of nosocomial C difficile infection increased, from 0.3% (95% confidence interval [CI], 0.21%-0.31%) in patients not receiving acid suppressive therapy to 0.6% (95% CI, 0.49%-0.79%) in those receiving H 2 RA therapy, to 0.9% (95% CI, 0.80%-0.98%) in those receiving daily PPI treatment, and to 1.4% (1.15%-1.71%) in those receiving more frequent PPI therapy. After adjustment for comorbid conditions, age, antibiotics, and propensity score–based likelihood of receipt of acid-suppression therapy, the association persisted, increasing from an odds ratio of 1 (no acid suppression [reference]) to 1.53 (95% CI, 1.12-2.10) (H 2 RA), to 1.74 (95% CI, 1.39-2.18) (daily PPI), and to 2.36 (95% CI, 1.79-3.11) (more frequent PPI). Similar estimates were found with a matched cohort analysis and with nested case-control techniques. Conclusions Increasing levels of pharmacologic acid suppression are associated with increased risks of nosocomial C difficile infection. This evidence of a dose-response effect provides further support for the potentially causal nature of iatrogenic acid suppression in the development of nosocomial C difficile infection.