A computerized medication monitoring system for alerting and warning of potential adverse drug reactions is described. The system integrates computerized data on each hospital patient (medications, clinical laboratory, blood gas, ECG, ALLERGIES, DIAGNOSIs, etc.) and returns to the pharmacist warning messages and suggestions regarding patient drug therapy. The broad data base allows for nearly complete drug therapy monitoring. When a warning message is received, the pharmacist contacts the physician or nursing staff and explains the potential problem. The system also generates prescription labels and patient drug profiles which are used in a unit dose dispensing system. Five percent of 13,727 patients monitored have had drug alerts. Of these alterts, 44.9% were drug-laboratory contraindications and only 28.9% were drug-drug interactions. Of the 690 alerts received, 77% resulted in the physician changing the patient's therapy.