Peg-interferon (Pegasys)


Pegasys is a medication used to treat hepatitis C. It is used in combination with other medications to cure people of the Hep C virus. It is taken by injection once a week. Common side effects include fever, headache, tiredness, irritability and sometimes depression. This medication must be stored in the fridge. Hepatitis C treatment can cure a person from hepatitis C. However, a person could become infected again if they are exposed to the hepatitis C virus in the future.

What is Pegasys?

Pegasys is a medication used to treat hepatitis C (Hep C). It is used in combination with other medications to cure people of the hepatitis C virus.

Pegasys is commonly referred to as peg-interferon. It is a specific type called peg-interferon alpha-2a. It is sold together with ribavirin, another Hep C medication.

How does Pegasys work?

Hep C is a disease of the liver that is caused by a virus. A virus is a very tiny germ that makes copies of itself to survive by a process called replication. The Hep C virus has at least six different strains, which are also known as genotypes.

Pegasys is the name of a long-lasting form of alpha interferon. Alpha interferon is a chemical messenger that is made by the body. It protects your cells from infection by Hep C and helps your immune system fight Hep C.

Hep C treatment can cure a person from Hep C. However, a person could get infected again if they are exposed to the Hep C virus. Being cured from Hep C does not make you immune from getting the Hep C virus again.

How do people use Pegasys?

Pegasys is taken as an injection once per week. It must be used in combination with ribavirin. It can also be taken with some direct-acting antiviral medications (DAAs). DAAs are a group of medications that directly attack the ability of a virus, such as hepatitis C, to make copies of itself. In Canada, Pegasys is approved for use with the following drug combinations:

For more information on the medications that are taken with Pegasys, see the drug factsheets by clicking on the drug name.

The combination a person takes depends on which genotype they have, whether they have been treated before, the amount of liver damage and other issues.

Pegasys is approved in Canada for people with genotype 1, 2, 3, 4, 5 or 6 hepatitis C virus.

How long is treatment?

As access to newer Hep C treatments improves in Canada it will be less likely that a person would take Pegasys and ribavirin without an additional DAA. To find out the length of treatment for a DAA taken with Pegasys and ribavirin see the drug factsheets for the DAAs listed above.

When Pegasys and ribavirin are prescribed together without an additional DAA they are taken for either 24 or 48 weeks depending on what genotype a person has and other factors.

How is Pegasys taken?

Pegasys is injected under the skin once per week. This drug is usually taken at bedtime.

Your nurse will teach you how to inject Pegasys under the skin. The best places on the body to inject Pegasys include the following:

  • upper leg or thigh
  • outer part of the upper arm
  • abdomen (but not the belly button or waist)

Do not inject Pegasys into the same place all the time; you should change injection sites in a regular pattern. If you prefer, some clinics or health centres may let you come in once a week so a nurse can inject you. Speak to your hepatitis nurse or doctor to find out if this option is available.

Sticking to treatment

All medications work best when they are taken exactly as prescribed and directed. This means taking the medications in the right amount and at the right time for the entire time that your treatment lasts. Skipping doses or stopping treatment altogether means that the treatment may not work as well and the chance of being cured is lower.

What can you do if you forget to take your medication?

If you miss taking a dose of Pegasys and remember within two days of when you were supposed to take it, take it as soon as possible. If it is more than two days after you were supposed to take your dose, ask your doctor what you should do.

If you find that you are not able to take your medication as prescribed and directed, talk to your nurse or doctor. You can also check out the section “Tips for staying on track with treatment” in CATIE’s Hepatitis C: An in-depth guide.

How likely is a cure from treatment that includes Pegasys?

Hep C treatment can cure a person from Hep C.

In late-stage clinical trials of Pegasys and ribavirin, cure rates ranged from 45% to 80% depending on the genotype of the participants. In clinical trials of Pegasys and ribavirin in people who are co-infected with Hep C and HIV, cure rates were lower compared to those in people who are mono-infected with Hep C.

In clinical trials when peg-interferon and ribavirin are tested with other DAAs, the cure rates are higher than those in people taking only peg-interferon and ribavirin. For more information on the cure rates for these combinations, see the drug factsheets.

Sometimes in real life the cure rates can be lower than in clinical trials.

A cure for Hep C is also known as a sustained virological response (SVR). This is when the Hep C virus is no longer detected in the blood 12 or 24 weeks after treatment ends.


Below is a list of warnings about conditions that may or may not occur while you are taking Pegasys. This list is not a complete list of warnings. Talk to your doctor, nurse and pharmacist about possible side effects and other issues that you may experience while you are taking Pegasys.

1. Combination therapy

Pegasys must be taken in combination with ribavirin, so all of the warnings that apply to ribavirin also apply to people considering Pegasys. For more information on peg-interferon and ribavirin, see the ribavirin factsheet.

2. Mental health

Some people who take Pegasys find it affects their mental health. For example, some people may experience irritability, brain fog, anxiety or hallucinations.

A common side effect of treatment with Pegasys is depression. People who have a history of depression can talk to their doctor about the possibility of starting antidepressants prior to starting treatment with Pegasys. Let your doctor know if you have a history of depression or anxiety. Talk to your doctor about treatment options for depression.

Once you have started treatment, if you notice any of these problems, talk to your doctor right away:

  • you become easily upset or angry
  • you have unexpected feelings of sadness
  • you feel hopeless
  • you have thoughts about harming yourself or others
  • you have thoughts about suicide

3. Pregnancy

If you are pregnant or think that you might be pregnant and you are taking Pegasys and ribavirin, talk to your doctor right away. These drugs should not be used by the following people:

  • pregnant people
  • people who could become pregnant and their sex partners

This is because the combination of these drugs can severely damage the fetus. Pregnancy should not be planned while you or your partner are on this therapy. If you are planning to have a baby, you should wait six months after therapy has been stopped. Ribavirin that is taken with Pegasys can also be toxic to sperm.

4. Breastfeeding/nursing

People with infants and who are taking Pegasys should not breastfeed or nurse their children.

5. Heart, stroke and blood problems

Although this is rare, some people who have used Pegasys have developed heart problems, including low blood pressure, rapid heartbeat and chest pain.

Pegasys can also lower levels of white blood cells, increasing the risk of developing infections. As well, this drug can reduce levels of platelets in your blood. Because platelets are needed to help your blood clot, having lower-than-normal levels of platelets increases the risk of bleeding. Tell your doctor right away if you have any problems with blood clotting or are getting regular infections.

Some people who have used Pegasys have developed bleeding in the brain or a disruption of blood flow to the brain (a stroke). Signs and symptoms of a stroke include:

  • sudden numbness or weakness in the face, arm or leg, especially on one side of the body
  • sudden confusion or trouble speaking or understanding speech
  • sudden trouble seeing in one or both eyes
  • sudden trouble walking, dizziness or loss of balance or coordination
  • sudden severe headache with no known cause

If you have any of these symptoms, go to your nearest emergency department immediately.

6. Lung problems

In rare cases, people taking Pegasys have developed lung problems, such as lung infection (pneumonia) and inflammation of the lung tissue (pneumonitis). If you experience any of the following problems, seek medical attention immediately:

  • chest pain
  • shortness of breath
  • coughing up blood
  • having your skin, fingernails or toenails turn blue or dark
  • dizziness
  • fainting
  • swelling in the legs and ankles
  • fatigue
  • racing heart rate

7. Infections

A temporary fever is a normal reaction to Pegasys treatment. However, a high fever or a fever that does not go away needs to be investigated to ensure that there is not some other cause, such as an infection. If you have a high or persistent fever while using Pegasys, tell your nurse and doctor.

8. Skin rash

Rash can occur in people using Pegasys. In rare cases the rash may be serious. If you have a severe rash with blisters, fever, or sores in your mouth, nose or eyes (or your eyes become red), contact your doctor right away.

9. Inflammation of the pancreas (pancreatitis)

Some people who use Pegasys may develop inflammation of the pancreas. People who have pancreatitis may develop abdominal pain, nausea, vomiting, unexpected sweating, fever and anxiety. If this happens, tell your doctor right away.

10. Inflammation of the colon (colitis)

Some people who use Pegasys may develop inflammation of the colon. People who develop colitis typically experience abdominal pain, bloody diarrhea and fever. If this happens, tell your doctor right away. Colitis usually goes away one to three weeks after Pegasys is discontinued.

11. Changes in vision

Some people who use Pegasys may develop blurred vision or other problems seeing. If this happens, tell your doctor right away.

12. Special populations

Pegasys has not been tested in the following populations:

  • people over the age of 65
  • people who are organ transplant recipients
  • people with hepatitis B

People in the following groups should not take Pegasys:

  • people who have epilepsy
  • people who are hypersensitive to alpha interferons, E. coli-derived products or polyethylene glycol
  • people with autoimmune hepatitis (AH) or a history of AH
  • people with severe liver damage (decompensated cirrhosis)
  • people with genetic health problems that affect hemoglobin (including thalassemia, sickle-cell anemia)
  • people with chronic obstructive pulmonary disease (COPD)
  • newborns and infants
  • people who have severe mental health issues or a history of severe mental health issues
  • people who have a thyroid problem that is not controlled by medication
  • people who have diabetes, high blood sugar (hyperglycemia) or low blood sugar (hypoglycemia) that is not controlled by medication
  • people who are breastfeeding or nursing
  • people with severe kidney problems
  • people who are co-infected with HIV and have cirrhosis (Child-Pugh score1 equal or greater to 6)

Children and youth (3-17 years old)

Pegasys and ribavirin have been tested in small groups of children. Some children who took Pegasys and ribavirin for 48 weeks showed a delay in height and weight while on treatment. For the majority of participants their height returned to normal after treatment but the weight of some children did not.

Side effects

Common side effects of Pegasys include:

This is not a complete list of side effects of Pegasys.

Drug interactions

Always consult your doctor and pharmacist about taking other prescription and non-prescription drugs, including methadone or opiate substitution therapies, herbs, supplements and street drugs.

Drug interactions occur when one medication affects how another is absorbed, used or flushed out of the body. Some drugs can interact with Pegasys, increasing or decreasing the level of one or both drugs in the body. Increased levels can lead to new or more severe side effects. Decreased levels may mean that the drugs won’t be as effective.

When Pegasys or ribavirin is taken with the following medications it could potentially cause significant drug interactions:

  • methadone, an opiate substitution medication
  • the HIV medications, AZT (zidovudine, Retrovir), lamivudine (3TC), stavudine (d4T, Zerit), ddI (didanosine, Videx EC), Combivir, Trizivir
  • theophylline, a medication to treat lung disease
  • azathioprine, a medication to lower immune response or prevent organ transplant failure
  • telbivudine (Sebivo), a hepatitis B medication
  • the Chinese herbal medication sho-saiko/Xiao-Chai-Hu

This is not a complete list of possible drug interactions with Pegasys.

Pegasys can temporarily weaken the bone marrow. Use of the anti-HIV drug AZT (zidovudine, Retrovir and in Combivir and Trizivir) may make this worse.

Talk to your nurse, doctor and pharmacist if you are taking any of these medicines. One way to manage drug interactions is to make sure that your doctor and pharmacist know about everything you are taking, including prescription drugs, over-the-counter drugs, street drugs, herbal medications, supplements or anything else. If you have more than one doctor or pharmacist, it is possible for drug interactions to get missed. If more than one doctor is writing prescriptions for you, let each one know about everything you are taking. If possible, use the same pharmacy for all your prescriptions.


The dose of Pegasys normally used is 180 micrograms (mcg) once a week. The drug is injected under the skin at bedtime. Keep Pegasys in your refrigerator.

The dose of ribavirin used with Pegasys will be adjusted depending on your weight. Ribavirin is taken twice per day orally with meals.


Pegasys has been approved by Health Canada and is available in Canada. Pegasys is manufactured by Hoffman La-Roche Limited.

Your nurse, doctor or pharmacist can tell you more about the availability and coverage of Pegasys in your region.

CATIE’s online Treatment coverage in your region section of Hepatitis C: An in-depth guide contains information about provincial and territorial drug coverage.


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  • 1. Child-Pugh Class A, B and C are measurements of cirrhosis. A score of 6 falls in Class A. In Class A, the liver is still functioning despite being damaged. Child-Pugh B and C are more severe forms of cirrhosis.

Author(s): Anderson S, Hosein SR

Published: 2015