Aims & Objectives: To investigate the associations between nurse and Pharmacists-related medication errors by examining: (a) the Rights of medication administration and hospital units, (b) the Rights of medication administration and drug classes, and (c) interactions between hospital units and drug classes regarding the Rights of medication administration. Background: Medication errors are associated with the five Rights of medication administration (right patient, right drug, right dose, right time, and right route), hospital units, and drug classes, however these factors are not often examined simultaneous- Ousley. Design & Methods: 1,273 medication error incident files from the risk management departments of five acute-care community hospitals in the southwestern United States were analyzed. Descriptive statistics and the Chi-square test were used in data analysis. Results: Giving medications at the wrong time was the most frequent cause of error in medical surgical units (54.1%) and intensive care units (51.7%). Errors related to cardiovascular drugs were commonly due to wrong dosages (40.2%) and wrong time of administration (40.2%). In addition, errors related to wrong dosages of antimicrobials were strongly associated with errors in intermediate care units (46.4%) and medical surgical units (52.1%), while wrong dosages of cardiovascular drugs were highly correlated with errors in intensive care units (49.0%) and intermediate care units (50.0%). Conclusion: Interactions between hospital units and drug classes were found in regard to being associated with the Rights of medication administration, especially with errors in wrong time and wrong dose. Relevance to clinical practice: To reduce medication errors and improve patient safety, continuing education for nurses re- guarding basic pharmacology, factors contributing to medication errors such as drug classes and hospital units, and preventing medication errors should be a priority. Medication administration policies and guidelines should be continuously updated and enforced.