22 papers on clinical decision support (CDS) for computer physician order entry (CPOE) and the ability to reduce medication errors were reviewed. Among the 22 original clinical trials, 21 demonstrated a reduced number of medication errors after the implementation of CDS. The effect was strongest for 2nd and 3rd generation of the CDS-systems. CPOE with CDS is time consuming and may generate new medication errors in itself. All the trials had poor designs. A Danish data source for CDS has not yet been established.