This retrospective study aims to determine whether there is a difference in the additional consumption of alcohol between addicts treated with methadone or dihydrocodeine (DHC) and untreated addicts injecting heroin. 1,685 patients admitted for opioid withdrawal between 1991 and 1997 were reviewed. Cross-reference tables and multiple logistic regression analyses were carried out. 28% of patients take more than 40 g of alcohol daily (on average 176 g). We found that patients who are treated with methadone or DHC drink alcohol significantly more often daily than the heroin-dependent patients (p<0.01). Using multiple regression analyses, the results were confirmed. Additionally, we found that co-abuse of alcohol was predicted by male gender, longer duration of drug use, additional daily consumption of tetrahydrocannabinol and daily consumption of benzodiazepines. Alcohol consumption by opioid-addicted patients treated with methadone or DHC presents a serious medical problem. Co-abuse of alcohol will receive more attention Copyright (C) 2003 S. Karger AG, Basel.