OBJECTIVE: This study aimed to investigate which categories of medication are most commonly implicated in overdose, to compare this information with prescription data and to explore how the medications used in overdoses are typically acquired. METHODS: A 12-month audit (11/2003-10/2004) of all medication overdose presentations to an inner-Melbourne ED was conducted and the medications compared to published population-based prescription data. Interviews were conducted with 31 patients who attended the ED following a medication overdose and typical stories regarding the acquisition of medications reported. RESULTS: The same broad categories of medications identified in earlier studies were found to contribute to the majority of overdoses in this study, namely benzodiazepines, antidepressants, analgesics and antipsychotics. Two benzodiazepine medications, diazepam and alprazolam, appeared to be over-represented in the overdose data relative to their population rates of prescription. Patient interviews revealed three main reasons for the original acquisition of the medications used in overdose: treatment purposes (77%); recreational use (16%); and overdose (7%). The most common source of medications (68%) used in overdose was prescription by the patient's usual doctor. CONCLUSION: The high representation of benzodiazepines among medications used in overdose is of ongoing concern. IMPLICATIONS: The time of medication prescription and dispensing may be an ideal opportunity for overdose prevention, through judicious prescribing, consideration of treatment alternatives, patient education and encouraging the safe disposal of unused medications.