Pegylated Interferon: Definition, Types, Uses

A hepatitis medication with added PEG improves dosage requirements

If you take a weekly injection of pegylated interferon, you owe thanks to a chemist. This is because without the chemistry of pegylation you would need to inject interferon up to three times a week. In short, pegylation makes taking your medicine easier and actually makes it work better.

A pegylated interferon drug
F. Hoffmann / La Roche AG

Pegylated Interferon Definition

Pegylated interferon, usually called peginterferon, is a chemically modified form of the standard interferon that treats hepatitis C and rarely hepatitis B. The difference between interferon and peginterferon is the PEG, which stands for a molecule called polyethylene glycol. The PEG prolongs the half-life of the drug. By attaching to the interferon, it will stay in the body (specifically the blood) much longer.

This process benefits hepatitis patients taking interferon because, rather than needing to inject interferon up to three times a week, only one weekly injection is needed. In addition, it yields a higher rate of sustained virologic response.

There are two versions of pegylated interferon, which are marketed individually as Pegasys (also known as peginterferon alfa-2a) and PegIntron (also known as peginterferon alfa-2b). Both have similar pharmacokinetic actions, with only minor differences between the two.


Prior to the introduction of direct-acting antivirals (DAAs) in 2013—which include the drugs Sovaldi, Harvoni, Daklinza, and Viekira Pak, among others—the most effective way to treat hepatitis C was with a combination of peginterferon and ribavirin. The dual therapy is known to have a high level of treatment side effects, some of which can be profound. Still, the combination therapy was able to achieve a sustained virologic response (the clinical definition of a cure) in about 50 percent of cases.

Today, with the newer class DAAs, the use of pegylated interferon has fallen away significantly, although it is sometimes still prescribed in cases of advanced liver disease and/or in patients who have had previously failed therapy.


Pegylated interferon is injected once weekly. The exact dose will vary based on a number of factors, including the genotype of your virus, the type of peginterferon you’ll be using, and the accompanying drug(s) it will be co-administered with. Your nurse will teach you how to correctly administer the injection, usually subcutaneously (beneath the skin). Peginterferon must always be refrigerated, and needles must never be reused or shared.

Side Effects

The side effects of peginterferon will vary from person to person, with some individuals managing will minimal ill events while others experience symptoms they consider intolerable. The most common side effects associated with pegylated interferon use are:

  • Flu-like symptoms (a headache, fever, chills, muscle aches)
  • Insomnia
  • Depression and mood changes
  • Irritability
  • Nausea and vomiting
  • Skin rashes
  • Extreme tiredness
  • Loss of appetite
  • Weight loss

Patients on treatment will often speak of Riba Rage, a condition typified by general irritability and sudden, sometimes explosive emotional outbursts. While the term suggests the cause is ribavirin, it is more commonly associated with peginterferon. Antidepressants are sometimes prescribed to treat the condition, particularly in patients with pre-existing psychiatric issues.


A full course of peginterferon treatment is expensive but luckily, with newer class DAAs, the duration of treatment is far shorter than before.

Medicaid, Medicare and private insurance will generally cover the cost of interferon when hepatitis C treatment is approved. Co-pay assistance can be sought through the drug manufacturer for qualified patients, as well as patient assistance programs (PAPs) for uninsured or low-income patients.


Some people with hepatitis C should not take pegylated interferon. These include persons with the following conditions:

  • Advanced liver, heart, kidney or lung disease
  • Some autoimmune disorders
  • Severe depression or other psychiatric conditions
  • Pregnancy
  • Breastfeeding

Always be sure that your healthcare provider has your complete medical history, including information about any other chronic infections you may have (e.g., HIV, diabetes) or any plans you may have to conceive.

Frequently Asked Questions

  • What is the difference between pegylated interferon and interferon?

    Interferon is a substance that helps the body fight infection. Your body makes interferon naturally, but man-made versions are available by prescription. Pegylated interferon is a type of interferon to which a molecule called polyethylene glycol (PEG) has been added. This allows the drug to stay active in your body for a longer period of time, which means you need to take it less frequently.

  • Is pegylated interferon an antiviral?

    No. Interferon is a substance called a cytokine, which plays a role in controlling our body's immune system and inflammation, among other things. The addition of polyethylene glycol (PEG) to interferon helps the drug stay in your body longer. Peginterferon, when given with antiviral medications, can make those drugs last longer in the body.

  • What are pegylated interferon and ribvirin?

    Pegylated interferon and ribvirin are powerful drugs used together in the treatment of conditions like hepatitis C. This combination treatment is widely recognized as a standard of care for patients with hepatitis.

  • Can interferon cause dementia?

    Research is limited regarding the impact that log-term interferon use can have on mental health, including conditions like dementia.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Baker DE. Pegylated interferons. Rev Gastroenterol Disord. 2001;1(2):87-99. PMID:12120178

  2. Tsubota A. Peginterferon and ribavirin treatment for hepatitis C virus infectionWJG. 2011;17(4):419. doi:10.3748%2Fwjg.v17.i4.419

  3. Schmidt F, Janssen G, Martin G, et al. Factors influencing long-term changes in mental health after interferon-alpha treatment of chronic hepatitis CAlimentary Pharmacology & Therapeutics. 2009;30(10):1049-1059. doi:10.1111/j.1365-2036.2009.04123.x

Additional Reading
  • Brenner GM, Stevens, CW. Pharmacology, 2e. Philadelphia, Saunders Elsevier, 2006.
  • Genentech. "Pegasys: Highlights of Prescribing Information." 
  • Katzung, BG. Basic and Clinical Pharmacology, 10e. New York, McGraw-Hill, 2007.
  • Merck. "PegIntron: Highlights of Prescribing Information."

By Charles Daniel
 Charles Daniel, MPH, CHES is an infectious disease epidemiologist, specializing in hepatitis.