["General practitioners have a key role in updating their patients' medication. Poor communication regarding patients' drug use may easily occur when patients cross health care levels. We wanted to explore whether such inadequate communication leads to errors in patients' medication on admission, during hospital stay, and after discharge, and whether these errors were potentially harmful.", 'Exploratory case study of 30 patients.', 'General practices in central Norway and medical ward of Innlandet Hospital Trust Gjøvik, Norway.', '30 patients urgently admitted to the medical ward, and using three or more drugs on admission.', "Discrepancies between the patients' actual drugs taken and what was recorded on admission to hospital, during hospitalization, at discharge, and five weeks after hospital stay. The discrepancies were grouped according to the NCC Merp Index for Categorizing Medication Errors to assess their potential harm.", 'The 30 patients used a total of 250 drugs, and 50 medication errors were found, affecting 18 of the patients; 27 errors were potentially harmful, according to NCC Merp Index: 23 in category E, four in category F. Half of the errors originated from an incomplete medication list in the referral letter.'] Does poor communication on patients ' medication across health care levels lead to potentially harmful medication errors?