AIMS: To examine concurrent buprenorphine and benzodiazepine consumption and to compare opioid toxicity symptoms induced by methadone and buprenorphine, examining factors associated with the reporting of these symptoms. DESIGN: Self-report cross-sectional survey. SETTING: Five needle syringe programmes and five opioid substitution treatment services in Melbourne, Australia. PARTICIPANTS: A total of 250 people who had experience with methadone or buprenorphine. Eligibility criteria were current or previous methadone or buprenorphine use. MEASUREMENTS: Structured questionnaire covering: demographic characteristics; current treatment and drug use; concurrent use of buprenorphine and benzodiazepines, including route of administration and source of medications; and opioid toxicity symptoms reported in association with methadone and buprenorphine consumption. FINDINGS: Of those reporting buprenorphine use, two-thirds reported concurrent benzodiazepine use, with a median dose reported of 30 mg diazepam equivalents. A greater number of opioid toxicity symptoms were reported in relation to methadone consumption compared with buprenorphine. Those reporting opioid toxicity with buprenorphine were more likely to report intravenous use compared with those reporting opioid toxicity with methadone. CONCLUSIONS: The risk of opioid toxicity appeared greater with methadone compared with buprenorphine, despite high levels of benzodiazepine consumption and injection being reported in relation to buprenorphine use. The prevalence of buprenorphine injection and the normalization of methadone-induced sedation are two findings that merit further investigation. Establishing recommendations as to the safest and most effective way to manage benzodiazepine-using people in opioid substitution treatment is necessary for the optimization of treatment for opioid dependence in polydrug-using individuals.