Even though the advantages of CPOE (computerized physician order entry) have been extensively extolled in the informatics literature in considerable detail, there is still some conflict whether CPOE actually reduces medication errors. I recently re-read Ross Koppel et al's paper in JAMA on the role of CPOE in facilitating, as opposed to mitigating medical errors . Ross and his colleagues found "22 previously unexplored medication-error sources that users report to be facilitated by CPOE", predominantly due to human-computer interface flaws and errors of information.
This conclusion was controversial, and fueled much discussion at the time. Certainly the paper did have some shortcomings (most notably, as David Bates pointed out , a major limitation was that Koppel measured perceptions of errors rather than the errors themselves). But it did raise an important line of reasoning: that CPOE probably isn't as fantastic for improving safety as its initial sales pitch proclaimed. And more importantly, that these systems are quite complex and operate in dynamic environments and we need to be cautious and deliberate before jumping to conclusions.
Of course, there have always been medication errors, even before EHRs and CPOE. The challenge is to show how CPOE has influenced these errors, especially since we only started measuring these errors in an extensive way after EHRs came along. To complicate matters further, workflows vary from one institution to another, which can make it difficult to discern if the error is because of the CPOE or the workflow in that particular organization. It has been six years since the Koppel paper was published, and we haven’t formulated a definitive answer yet.
One of the selling points of CPOE is that it adds a level of safety, and allows errors to be discovered and fixed before the patient is harmed. But this also often adds extra documentation for the clinician, and in today's point-and-click world, often makes using the EHR more tedious. For example, I know of one institution that requires RNs to document any CPOE discrepancies. One of the RNs who was somewhat frustrated by the additional load of documenting said " I bet you Florence Nightingale wouldn't have liked to spend most of her time clicking away at a computer screen instead of taking care of people."
From what I have read about Florence Nightingale, the observation is probably accurate.
 Koppel R, Metlay J, Cohen A, Abaluck B, Localio AR, Kimmel SE, et al. Role of Computerized Physician Order Entry Systems in Facilitating Medication Errors. J Am Med Assoc 2005;293(10):1197–203.
 Bates DW. Computerized physician order entry and medication errors: finding a balance. J Biomed Inform. 2005 Aug;38(4):259-61.