The evidence regarding the co-morbidity of chronic hepatitis C, psychiatric illness and intravenous drug abuse is reviewed from the literature. Also the occurrence and the treatment of psychiatric side effects during treatment with interferon in patients with a history of drug abuse are reviewed. There is insufficient evidence for a specific hepatitis C induced depression or fatigue, but a direct link between hepatitis C and cerebral dysfunction is not excluded. Immune system activation rather than drug use may explain cerebral symptoms. In HCV positive substance users anxiety and depression are more prevalent than in HCV negative substance users. During treatment with regular or pegylated (PEG) interferon depression is a frequent side effect (ca 30%) and occurs independently from pre-existing psychiatric disorders or drug abuse. A history of drug abuse per se does not increase the risk of depression as a side effect of interferon treatment. It is extremely important to monitor symptoms of depression in the early weeks of treatment and to start antidepressant treatment as early as possible. Antidepressants should be continued throughout the interferon treatment period. There are insufficient data to assess these situations in which preventive antidepressant treatment should be started before interferon treatment. Clinical judgement can, however, lead to preventive antidepressant treatment, even at subclinical levels of depression. A cut off score of > 10 on the Beck Depression Inventory before interferon treatment is associated with a higher risk of depression during treatment. Both selective serotonin reuptake inhibitors and other classes of antidepressants can be used.