['A computerized physician order entry (CPOE) system with embedded clinical decision support can reduce medication errors in hospitals, but might increase the time taken to generate orders.', 'We aimed to quantify the effects of temporal (month, day of week, hour of shift) and other factors (grade of doctor, prior experience with the system, alert characteristics, and shift type) on the time taken to generate a prescription order.', 'A large university teaching hospital using a locally developed CPOE system with an extensive audit database.', 'We retrospectively analyzed prescription orders from the audit database between August 2011 and July 2012.', 'The geometric mean time taken to generate a prescription order within the CPOE system was 11.75\u2005s (95% CI 11.72 to 11.78). Time to prescribe was most affected by the display of high-level (24.59\u2005s (24.43 to 24.76); p<0.001) or previously unseen (18.87\u2005s (18.78 to 18.96); p<0.001) alerts. Prescribers took significantly less time at weekends (11.29\u2005s (11.23 to 11.35)) than on weekdays (11.88\u2005s (11.84 to 11.91); p<0.001), in the first (11.25\u2005s (11.16 to 11.34); p<0.001) and final (11.56\u2005s (11.47 to 11.66); p<0.001) hour of their shifts, and after the first month of using the system.'] Do temporal and other factors that influence the time doctors take to prescribe using an electronic prescribing system?