Provider order entry systems (POE) often incorporate active decision-support component for drug dosing. The efficacy of automated alerts that suggest dose amounts to the clinician in real time depends in part on how well they are timed to fit into the decision process and on their representational structure. We have conducted a cognitive evaluation of an interaction with a POE system that offered active decision support for heparin dosing with the goal of characterizing its effectiveness and opportunities for error. Two researchers completed a cognitive walk-through of an ordering task based on a clinical scenario. In addition, seven clinicians were asked to enter a set of orders in an experiment using the same scenario. The analysis revealed that users without a solid conceptual knowledge of the ordering system followed patterns of inefficient interactive behavior resulting in delays and some errors. Physicians often did not take full advantage of automatic dose computation provided by a decision support component and used it largely as reference. The calculated dose was not perceptually salient in the generated alert and required users to engage in meaning interpretation of the displayed information. Better visual presentation of the alert message would likely result in faster and less cognitively demanding interaction.