Benzodiazepine use by patients in methadone and buprenorphine substitution treatment is common, despite safety concerns regarding these drug interactions. The relative safety of diazepam use by methadone- or buprenorphine-treated patients has not been systematically examined. This study aimed to examine the effect of single diazepam doses, within normal therapeutic range (doses: 0, 10, and 20 mg), upon physiological, subjective, and performance measures in stable methadone and buprenorphine-treated patients. In a double-blind, randomized crossover design, methadone- or buprenorphine-treated patients were administered their normal opioid dose and either placebo, 10-, or 20-mg diazepam, in balanced order over 3 sessions. Eight methadone- and 8 buprenorphine-prescribed patients with no concurrent benzodiazepine dependence or significant comorbidity were recruited from an outpatient addiction clinic in London. Measures were taken at baseline and for 6 hours after dosing, and included physiological responses (pulse rate, blood pressure, pupil size, respiratory rate, and peripheral pO2), subjective drug effects (Addiction Research Center Inventory subscales, visual analog scales of strength of drug effect, drug-liking, and sedation), and performance measures (simple reaction time, cancellation task, digit symbol substitution task, and balance). The 10- and 20-mg diazepam doses resulted in comparable subjective experiences of greater sedation and strength of drug effects in both patient groups, and had minimal impact on physiological parameters. However, diazepam had greater peak effects on performance measures (simple reaction time, digit symbol substitution task, and cancellation time) in methadone-treated than in buprenorphine-treated patients. Diazepam may significantly alter the response to opioid substitution treatment with methadone or buprenorphine.