Most cases of viral hepatitis in humans are caused by four viruses. These include hepatitis B virus, hepatitis C virus, hepatitis A virus, and hepatitis D virus, or delta-hepatitis. There are, however, distinct differences in these viral agents in terms of their epidemiologic, immunologic, and clinical characteristics. For drug users, hepatitis viruses B and C are the agents of major concern. This is because the major risk of infection is by bloodborne transmission, especially via shared needles, syringes, and other drug paraphernalia. There is also a risk of developing chronic hepatitis, and even - over time - the risk of developing hepatocellular carcinoma (liver cancer).

Drug treatment programs should routinely screen for hepatitis B. Hepatitis C should be tested for in all persons known to have used injection drugs. Liver enzymes should also be measured. Additional testing needs to be done as follows:

• Serologic testing for hepatitis A should be done only in persons with acute hepatitis.
• Serologic testing for hepatitis D virus should be done only in persons with known hepatitis B virus infection who have chronic hepatitis and are hepatitis B surface antigen (HBsAg) positive, since the presence of serologic evidence of infection with hepatitis D does not alter the clinical followup or therapeutic intervention from that of a patient with chronic HBV infection.