Patients seeking treatment for opiate withdrawal are commonly also dependent on benzodiazepines, although the interactions between benzodiazepine and opiate dependence and withdrawal syndromes have been subject to little systematic investigation. This is the first study comparing type, severity and course of opiate withdrawal symptoms between opiate dependent patients with, and without, concurrent benzodiazepine dependence. Patients dependent only on opiates (n = 39), and patients dependent on both opiates and benzodiazepines (n = 22), were recruited from consecutive admissions to an in-patient drug treatment unit. Quantity and duration of prior opiate use was similar for both groups. Patients completed daily self-ratings of opiate withdrawal (SOWS) for the duration of a standard in-patient detoxification treatment. Co-dependent patients were detoxified from benzodiazepines and opiates concurrently. Co-dependent patients reported a more severe withdrawal symptoms than patients withdrawing from opiates alone. Co-dependent patients had significantly more severe opiate withdrawal symptoms. Concurrent benzodiazepine withdrawal exacerbates opiate specific withdrawal symptoms. Possible psychological and neurophysiological mechanisms for the observed sensitisation are discussed.