Stress ulcers are acute superficial inflammatory lesions of the gastric mucosa induced when an individual is subjected to unusually high physiologic demands. In recent years, use of acid suppression therapy (AST) for stress ulcer prophylaxis (SUP) in inpatient settings other than intensive care has become increasingly common, leading to increased drug cost and an avoidable increased risk of adverse events such as hospitalacquired pneumonia. To assess the effects of a clinical pharmacist intervention including AST prescribing and adherence to a SUP guideline that was developed by clinical pharmacists for use in the infectious disease ward of a teaching hospital based on the 1999 American Society of Health-System Pharmacists (ASHP) guidelines for use of SUP. This was an exploratory, prospective pre- and post-intervention study of all patients admitted to the infectious disease ward of Imam Khomeini Hospital, the major referral hospital for infectious disease patients in Iran, which is affiliated with Tehran University of Medical Sciences. The study intervention consisted of the use of an internal guideline for SUP that was prepared by clinical pharmacists in accordance with ASHP guidelines, followed by education provided to the physicians who monitored and visited the hospitalized patients in the infectious disease ward. For the 4-month pre-intervention (August 1, 2008, to December 1, 2008) and post-intervention (February 1, 2009, to June 1, 2009) periods, the following data were collected: admitting diagnoses, number and type of SUP risk factors for AST, and type of AST medication used (omeprazole or ranitidine). Exclusions included (a) patients using AST for appropriate gastrointestinal diagnoses at admission (n = 4 in each period), and (b) patients who died during the hospital stay because of a cause other than a gastrointestinal disorder (n = 3 pre-intervention and n = 1 post-intervention). Rates of AST use were measured for the sample overall, and for patients with and without an indication for SUP. Appropriate use was defined as 1 primary (absolute) risk factor (i.e., coagulopathy, mechanical ventilation, or history of gastrointestinal bleed in the last 12 months) or 2 or more secondary (relative) risk factors (e.g., use of heparin). Pre- and post-intervention results were compared using the Pearson chi-square test. AST use declined from 80.9% (212 of 262) infectious disease ward patients in the pre-intervention period to 47.1% (113 of 240) patients in the post-intervention period (P < 0.001). Of 23 patients in the preintervention period with an indication for SUP according to our ASHP-based guideline, 78.3% (n = 18) received AST versus 85.7% (n = 12 of 14) in the post-intervention period (P = 0.575). Of the patients without an indication for SUP, 194 of 239 (81.2%) received AST in the pre-intervention period versus 101 of 226 (44.7%) in the post-intervention period (P < 0.001). Of the patients who received AST, 194 of 212 (91.5%) did not have an indication for SUP in the pre-intervention period versus 101 of 113 (89.4%) in the post-intervention period (P = 0.528). In this pre- and post-intervention study without a comparison group, the introduction by pharmacists of a treatment guideline for SUP in the infectious disease ward of Imam Khomeini Hospital was associated with reduction in use of AST overall and in patients without an absolute indication for SUP. However, there was no significant change in either the proportion of patients with an indication for SUP who received AST or in the proportion who received AST without an indication for SUP.