Peginterferon (Brands: Pegasys, PEG-Intron)

Peginterferon (brand names: Pegasys® (peginterferon alfa-2a) and PEG-Intron (peginterferon alfa-2b) is a prescription medication called an alfa interferon. Interferons are naturally produced in our bodies to fight invasion by viruses and other pathogens (unwanted microorganisms). Manufactured interferon boosts the immune system and helps the body to fight hepatitis C virus (HCV). To increase the amount of time it stays in the body, interferon was modified, making it pegylated interferon alfa (or peginterferon, for short). Peginterferon is given by injection and may be used with other medications (like ribavirin) to treat HCV.1,2

What are the ingredients in peginterferon?

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

How does peginterferon work?

Interferons are proteins that are naturally produced by the body and play an important role in the immune system. They are made by cells in response to the entry of 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. There are three types of interferons. Interferon alfa (sometimes also spelled “alpha”) is the one used to treat HCV infection. Interferon does not remain active in the body for very long, so scientists modified it to increase the amount of time it stays in the body and remains active. Polyethylene glycol or PEG, for short, was added to interferon alfa to increase its activity, making it pegylated interferon alfa (or peg interferon, for short).1

What are the possible side effects of peginterferon?

Peginterferon can cause a number of side effects, including some serious ones. Peginterferon may cause serious side effects. This is not a complete list of possible adverse effects, but some side effects can include:

  • Depression and suicidal thoughts, which can be fatal
  • Relapses of drug abuse, which may lead to overdose and can be fatal
  • Thyroid problems
  • Blood sugar problems (some people who take Pegasys may develop diabetes and experience symptoms including increased thirst, fatigue, frequent urination, weight loss)
  • Serious eye problems
  • Liver problems, which can result in liver failure and death; Symptoms of liver problems to watch out for include nausea, loss of appetite, fatigue, yellowing of your skin, swelling in stomach area, confusion, sleepiness
  • Lung problems (trouble breathing, pneumonia, pulmonary hypertension)
  • Inflammation of pancreas, with symptoms including stomach pain, back pain, nausea, vomiting, and fever
  • Inflammation of intestines, with symptoms including severe stomach pain, bloody diarrhea or bowel movements
  • Allergic (hypersensitivity) reactions, including rash, a hive-like swelling under the skin, coughing, wheezing, and shortness of breath
  • Nerve problems, including numbness, or a tingling or burning sensation in the extremities
  • Cardiovascular and cerebrovascular problems (including an abnormal heart rate, stroke, or heart attack)
  • Infections (bacterial, viral, or fungal)
  • Bone marrow toxicity, which can suppress the function of the bone marrow and result in severe reductions in blood cells (low red blood, white blood, and platelet counts)
  • Autoimmune disorders, including psoriasis, rheumatoid arthritis, thrombocytopenia, and systemic lupus erythematosus
  • Growth effects in children, including delayed weight gain or height decrease.1-4

The most common less serious side effects with Pegasys include:

Things to know about peginterferon

There are several warnings and precautions you should know about before you start treatment with peginterferon. If you experience any of the problems listed below, you should contact your doctor immediately.1,4


If you are allergic to peginterferon or any of its ingredients, you should not take peginterferon. Signs of a hypersensitivity (allergic) reaction to peginterferon include rash, a hive-like swelling under the skin, coughing, wheezing, and shortness of breath.

Liver problems

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


Peginterferon taken in combination with ribavirin can cause birth defects or death to your fetus. You should not take peginterferon with ribavirin if you are a woman who is pregnant or planning to become pregnant or if you are a man with a sexual partner who is planning to become pregnant. If you are woman taking peginterferon in combination with ribavirin, you should wait for 6 months after treatment is completed before becoming pregnant. If you are man taking peginterferon in combination with ribavirin, you should wait for 6 months after treatment is completed before you and your partner attempt to become pregnant. For both men and women who take peginterferon with ribavirin, during treatment and for the period of 6 months after treatment, you should use proper birth control measures to prevent pregnancy. If you or your sexual partner becomes pregnant during treatment or within 6 months after completing treatment, you should contact your doctor immediately and notify the Pregnancy Registry at 1-800-593-2214.

Additionally, it is not known if peginterferon passes into breast milk and poses a risk to your infant. Women who are breastfeeding or planning on breastfeeding should talk to their doctor about the risks to their child.

Mental health problems

Taking peginterferon may result in the development of 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 suicide or hurting others.

Cardiovascular and cerebrovascular problems

Taking peginterferon may cause some people to develop heart problems (abnormal heart rate, chest pain, heart attack), high blood pressure, or stroke (or related symptoms, including weakness, loss of coordination, or numbness).


Taking peginterferon may cause you to develop an infection with symptoms, including fever, chills, burning and painful urination (also frequent urination), and coughing up yellow or pink mucus.

Before starting treatment with peginterferon, you should make sure to tell your doctor if you have:

  • 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 transplant1

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

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Written by: Emily Downward | Last reviewed: August 2020