Peginterferon (Pegasys, PEG-Intron)

Reviewed by: HU Medical Review Board | Last reviewed: May 2023

Peginterferon is a prescription drug used to treat HCV. It is given by injection and may be used with other drugs like ribavirin. Common brand names include Pegasys (peginterferon alfa-2a) and PEG-Intron (peginterferon alfa-2b).1-4

Peginterferon is an alfa interferon drug. Interferons are naturally produced in our bodies to fight viruses and other unwanted microorganisms. Manufactured interferon boosts the immune system. This helps the body fight the hepatitis C virus (HCV).1,2

What are the ingredients in peginterferon?

Pegasys contains the active ingredient peginterferon alfa-2a. PEG-Intron contains the active ingredient peginterferon alfa-2b.2,3

How does peginterferon work?

There are 3 types of interferons. Interferon alfa (sometimes spelled “alpha”) is used to treat HCV infection.1

Interferons are proteins that are naturally made by the body. They play an important role in the immune system. They are made by cells in response to viruses, bacteria, and other harmful invaders. Interferons trigger an immune response and rally the cells to fight back. Interferons “interfere” with the virus’s ability to reproduce.1

Interferon does not remain active in the body for very long. This is why scientists modified it to increase the amount of time it stays in the body and remains active. Polyethylene glycol (PEG) was added to interferon alfa to increase its activity. This is known as pegylated interferon alfa (or peg interferon, for short).1

What are the possible side effects of peginterferon?

Side effects can vary depending on the specific drug you are taking.

The most common side effects of PEG-Intron include:2,4

  • Fatigue
  • Headache
  • Fever
  • Nausea
  • Muscle aches or joint pain
  • Mood changes, such as depression, anxiety, and irritability
  • Injection site reactions

The most common side effects of Pegasys include:1,4

Peginterferon has a boxed warning, the strictest warning from the U.S. Food and Drug Administration (FDA). This is because it may cause severe side effects, including:1,4

  • Mental health problems – Peginterferon may cause mood or behavioral problems, including depression, irritability, aggressive behavior, relapse into drug addiction or overdose (if you are a former drug addict), and thoughts of self-harm.
  • Restricted or reduced blood flow – Peginterferon may cause heart problems, high blood pressure, or stroke.
  • Autoimmune disorders – Peginterferon may cause autoimmune issues like psoriasis, rheumatoid arthritis, low platelet levels, and lupus.
  • Infections – Peginterferon may cause bacterial, viral, or fungal infections.

These are not all the possible side effects of peginterferon. Talk to your doctor about what to expect or if you experience any changes that concern you during treatment with peginterferon.

Things to know about peginterferon

If you are allergic to peginterferon or any of its ingredients, do not take peginterferon. Signs of an allergic reaction include rash, swelling under the skin, coughing, wheezing, and shortness of breath.1,4

If you have certain liver problems or a kind of hepatitis called autoimmune hepatitis, you should not take peginterferon.1,4

Peginterferon taken in combination with ribavirin can harm an unborn baby. If you or your partner can become pregnant, you should use birth control during treatment and for some time after the last dose of peginterferon/ribavirin. Talk to your doctor about your options for birth control while taking peginterferon/ribavirin.1,4

There is not enough data to know if peginterferon is safe to take while breastfeeding. Talk to your doctor about your options for breastfeeding while taking peginterferon.1,4

Before starting treatment with peginterferon, tell your doctor if you have:1-4

  • Liver problems other than HCV
  • Lung problems
  • Thyroid problems
  • Diabetes
  • Colitis (intestinal inflammation)
  • Cancer
  • HIV infection
  • Kidney problems
  • Cardiovascular problems
  • Elevated blood triglycerides
  • An organ transplant

Before beginning treatment for HCV, tell your doctor about all your health conditions and any other drugs, vitamins, or supplements you are taking. This includes over-the-counter drugs.

For more information, read the prescribing information for Pegasys and the prescribing information for PEG-Intron.

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