The number of medication errors reported to The Danish National Board of Health in Denmark exceeds 5000 per year. It is well known that computerized physician order entry (CPOE) with addition of decision support tools may reduce the frequency of medication errors. The primary scope of the work in Denmark has been to help health care professionals avoiding harmful errors. Using data primarily from The Danish National Board of Health, based on the reports of errors from Danish hospitals, and with our previous foundation in the international literature, we analyzed the errors which led to harmful conditions or death. In the process we developed a methodical consensus for identifying which medicines should have a warning attached, and we systematized the different kind of warnings. The following validation of the data resulted in a final list of 14 classes of drugs or drug substances, which all have been involved in serious medication errors. At present time there is a total of 136 different medicines with warnings found in the drug database for health professionals from Infomatum A/S (www.medicin.dk). In a parallel matter other decision support tools from Infomatum A/S Infomatum A/S is a company jointly owned by The Danish Drug Information and The Danish Medical Association. Infomatum A/S publishes drug information for health professionals and the public. are available or in progress e.g. ensuring use of correct dosage based on normal range, information about drugs used during pregnancy, etc. A major challenge when implementing decision support for medication processes has been to ensure useful coding of the medicines, A/S there does not exist one unique identification number for each drug substance.