Pegasys (generic: peginterferon alfa-2a) is a prescription medication used to treat hepatitis B and also to treat chronic (lasting a long time) hepatitis C infection in people who have not taken alpha interferon. Pegasys belongs to a group of drugs called interferons. It works by activating the body's immune system to attack hepatitis viruses.

This medication comes in an injectable form in a prefilled syringe and is given just under the skin. It is usually given once a week, for up to 48 weeks.

Common side effects of Pegasys are flu-like symptoms, tiredness, and stomach problems.