Background: Some hospital Computerized Physician Order Entry (CPOE) systems support interprofessional communication. The aim of this study was to investigate the effectiveness of pharmacist-physician messages sent via a CPOE system. Method: Data from the year 2012 were captured from a large university teaching hospital CPOE database on: 1) review messages assigned by pharmacists; 2) details of the prescription on which the messages were assigned; and 3) details of any changes made to the prescription following a review message being assigned. Data were coded for temporal, message and prescription factors. Messages were analysed to investigate: 1) whether they were signed-off; and 2) the time taken. Messages that requested a measurable action were further analysed to investigate: 1) whether they were actioned as requested; and 2) the time taken. We conducted a multivariable analysis using Generalised Estimating Equations (GEE) to account for the effects of multiple factors simultaneously, and to adjust for any potential correlation between outcomes for repeated review messages on the same prescription. All analyses were performed using SPSS 22 (IBM SPSS Inc., Chicago, IL, USA), with p<0.05 considered significant. Results: Pharmacists assigned 36,245 review messages to prescriptions over the 12 months, 34,506 of which were coded for analysis after exclusions. Nearly half of messages (46.6%) were signed-off and 65.5% of these were signed-off in ≤ 48 hours. Of the 9,991 further analysed for action, 35.8% led to an action as requested by the pharmacist and just over half of these (57.0%) were actioned in ≤ 24 hours. Factors predictive of an action were the time since the prescription was generated (p<0.001), pharmacist grade (p<0.001), presence of a high-risk medicine (p<0.001), messages relating to reconciliation (p = 0.004), theme of communication (p<0.001), speciality, (p<0.001), category of medicine (p<0.001), and regularity of the prescription (p<0.001). Conclusion: In this study we observed a lower rate of sign-off and action than we might have expected, suggesting uni-directional communication via the CPOE system may not be optimal. An established pharmacist-physician collaborative working relationship is likely to influence the prioritisation and response to messages, since a more desirable outcome was observed in settings and with grades of pharmacists where this was more likely. Designing systems that can facilitate collaborative communication may be more effective in practice.