Alpha interferon is a natural glycoprotein produced by the body to fight infections; recombinant interferon alfa is an artificially produced copy of the natural protein. Recombinant interferon alfa, which was first approved for use in chronic hepatitis C in 1991, interferes with virus replication and stimulates the immune system to fight infections.

Pegylated interferon (plus ribavirin) is emerging as the treatment of choice for patients with hepatitis C.

Pegylation, a process in which polyethylene glycol (PEG) molecules are attached to a protein, increases the duration of action of interferon in the body because the PEG molecules are ignored by the body and the rate of elimination of interferon from the body is accordingly much slower. Because pegylation converts interferon from a relatively short-acting to a long-acting medication, the time between injections is extended. Instead of three injections of standard interferon a week, the patient injects pegylated interferon only once a week.