Transitions of care may result in medication errors, when information about a patient's medications is not communicated sufficiently. In this clinical record review study, we aimed to evaluate the frequency of undocumented medication discrepancies at discharge from hospital and evaluate which patient characteristics could be associated with undocumented medication discrepancies. Preadmission medication lists were compared against the medication list in the discharge letters, taking into account medication changes documented in the patient record throughout the inpatient stay and in the discharge summary. Out of 200 patients, 174 (87%) were affected by at least one undocumented medication discrepancy, mostly for regular medication. Of the 1972 medications used, 744 (38%) medications were changed without documentation in the patient record, the majority being over-the-counter supplements and herbal medications. Polypharmacy at admission and discharge was associated with increased undocumented medication discrepancies. This study indicates a lack of medication reconciliation during inpatient stay. Correct and complete medication lists at admission and discharge may resolve many of these discrepancies, supporting patient safety at transitions of care.