Heroin addiction is a chronic relapsing disease that is difficult to cure, but stabilisation and harm reduction can importantly increase the life time expectancy and the quality of life of the patient, his immediate vicinity and society in general. Currently, no proven effective pharmacological interventions are available for cocaine addiction, and treatment has to rely on existing cognitive behaviour therapies combined with contingency management strategies. Substitution therapy, however, is effective in caring for heroin addicts. Methadone is a synthetic opioid that counteracts withdrawal symptoms of heroin. Buprenorphine is a derivative of the morphine alkaloid, thebaine, and is a partial opioid agonist at the micro opioid receptor in the nervous system. A substitution treatment program effectively reduces and often eliminates heroin injection behaviour, rendering patients more socially stabilised. Reduction in the number of viral co-infections can be observed. Methadone undergoes oxidative biotransformation in the liver, but is also stored in the liver and released into the blood in unchanged form. The usual dose can be continued in patients with stable chronic liver disease, including advanced cirrhosis. In acute liver disease or acute decompensation of chronic liver disease, close clinical observation for signs of narcotic overdose or withdrawal is necessary. A modest alteration in methadone dose may be appropriate for some patients. Buprenorphine can cause liver dysfunction after sublingual and even more after intravenous administration. It is advised to follow the liver function during buprenorphine treatment and to warn the clients for intravenous use of buprenorphine. Neither methadone nor buprenorphine do influence the effect of interferon and ribavirin during the treatment of chronic hepatitis C patients. It may be necessary to increase the dosage of methadone during interferon treatment.