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Generic Name: peginterferon alfa-2a (peg in ter FEER on AL fa 2 a)
Brand Names: Pegasys, Pegasys ProClick Autoinjector

Medically reviewed by Last updated on Feb 18, 2019.

What is Pegasys?

Pegasys (peginterferon alfa-2a) is made from human proteins that help the body fight viral infections.

Pegasys is used to treat chronic hepatitis B or hepatitis C in adults, and to treat chronic hepatitis C in children who are at least 5 years old.

Pegasys may also be used for purposes not listed in this medication guide.

Important information

You should not use Pegasys if you have liver failure or autoimmune hepatitis.

Do not use Pegasys together with ribavirin if you are pregnant, or if you are a man and your sexual partner is pregnant. Prevent pregnancy while using this medicine, and for at least 6 months after you stop using it.

Pegasys can cause life-threatening infections, autoimmune disorders, serious mood or behavior problems, or a stroke.

Call your doctor at once if you have: unusual changes in mood or behavior, chest pain, trouble breathing, sudden numbness or weakness, or signs of infection (fever, chills, cough with mucus, or burning when you urinate).

Before taking this medicine

Pegasys is often used together with other medicines. To make sure these medicines are safe for you, tell your doctor about all your medical conditions, and if you are pregnant or breast-feeding.

You should not use Pegasys if you are allergic to peginterferon alfa-2a, or if you have:

  • liver failure, or autoimmune hepatitis.

Pegasys can cause life-threatening infections, autoimmune disorders, serious mood or behavior problems, or a stroke. Tell your doctor if you have:

  • a weak immune system (caused by disease such as HIV or AIDS);

  • heart disease, high blood pressure, or a history of heart attack, blood clot, or stroke;

  • a hemoglobin blood cell disorder such as sickle-cell anemia or thalassemia;

  • an autoimmune disorder such as rheumatoid arthritis, systemic lupus erythematosus (SLE), or psoriasis; or

  • history of depression, mental illness, or addiction.

To make sure Pegasys is safe for you, tell your doctor about all your other medical conditions, especially:

  • liver problems other than hepatitis;

  • a history of breathing problems;

  • diabetes, or a thyroid disorder;

  • colitis (an intestinal disorder);

  • kidney disease;

  • high triglycerides (a type of fat in the blood);

  • cancer;

  • heart disease, high blood pressure, or a history of heart attack, blood clot, or stroke;

  • a hemoglobin blood cell disorder such as sickle-cell anemia or thalassemia;

  • history of liver or other organ transplant; or

  • if you have both hepatitis B and hepatitis C or HIV.

Pegasys may be harmful to an unborn baby. Tell your doctor if you are pregnant. Pegasys is often used together with another medicine called ribavirin. Ribavirin is known to cause birth defects or death in an unborn baby. You may need to have a negative pregnancy test before taking these two medicines together and every month during your treatment.

  • If you are a woman, do not use Pegasys and ribavirin if you are pregnant. If you are a man, do not use Pegasys and ribavirin if your sexual partner is pregnant. An unborn baby could also be harmed if a man fathers the child while he is taking ribavirin.

  • Use at least 2 effective forms of birth control while either sexual partner is using Pegasys with ribavirin. Keep using 2 forms of birth control for at least 6 months after treatment ends.

  • Tell your doctor right away if a pregnancy occurs while either the mother or the father is using Pegasys and ribavirin.

It is not known whether peginterferon alfa-2a passes into breast milk or if it could harm a nursing baby. You should not breast-feed while using this medicine.

Pegasys with ribavirin can affect growth in children. Talk with your doctor if you think your child is not growing at a normal rate.

How should I use Pegasys?

Pegasys is usually given once per week. Follow all directions on your prescription label. Do not use this medicine in larger or smaller amounts or for longer than recommended. If your doctor changes your brand, strength, or type of interferon, your dosage needs may change. Do not change your doses of Pegasys or your medication schedule without your doctor's advice.

Pegasys is injected under the skin, usually once per week. You may be shown how to use injections at home. Do not self-inject this medicine if you do not fully understand how to give the injection and properly dispose of used needles and syringes.

Do not shake the Pegasys bottle or you may ruin the medicine. Do not use if the medicine has changed colors or has particles in it. Call your pharmacist for new medicine.

Use a disposable needle and syringe only once. Follow any state or local laws about throwing away used needles and syringes. Use a puncture-proof "sharps" disposal container (ask your pharmacist where to get one and how to throw it away). Keep this container out of the reach of children and pets.

Pegasys can lower blood cells that help your body fight infections and help your blood to clot. This can make it easier for you to bleed from an injury or get sick from being around others who are ill. Your blood may need to be tested often. Your blood pressure, vision, and lung function may also need to be checked.

If you have hepatitis B you may develop liver symptoms after you stop taking this medicine. Your doctor may want to check your liver function for several months after you stop using Pegasys.

Store Pegasys in the refrigerator, protected from light. Do not freeze. Each single-use vial (bottle), prefilled syringe, or auto-injector device is for one use only. Throw away after one use, even if there is still some medicine left in it after injecting your dose.

What happens if I miss a dose?

Use your dose of Pegasys as soon as you remember the missed dose, then go back to your regular schedule on the day your next dose is due. If you are more than 2 days late in using your injection, call your doctor for instructions. Do not use extra medicine to make up a missed dose.

What happens if I overdose?

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

What should I avoid while using Pegasys?

Avoid drinking alcohol. It may increase your risk of liver damage.

Using this medicine will not prevent your disease from spreading. Follow your doctor's instructions about how to prevent passing the disease to another person.

Pegasys side effects

Get emergency medical help if you have signs of an allergic reaction to Pegasys: hives, or a rash that spreads and causes blistering and peeling; chest pain, anxiety, difficult breathing; swelling of your face, lips, tongue, or throat.

Call your doctor at once if you have:

  • vision loss;

  • severe stomach pain with bloody diarrhea;

  • pale skin, easy bruising, unusual bleeding (nose, mouth, vagina, or rectum), purple or red pinpoint spots under your skin;

  • sudden chest pain or discomfort, wheezing, dry cough, feeling short of breath;

  • mood or behavior changes - depression, anxiety, irritability, confusion, thoughts about hurting yourself, or falling back into a previous pattern of drug addiction;

  • heart attack symptoms - chest pain or pressure, pain spreading to your jaw or shoulder, nausea, sweating;

  • signs of a stroke - sudden numbness or weakness (especially on one side of the body), sudden severe headache, slurred speech, problems with balance;

  • new or worsened autoimmune disorders - skin problems, joint pain or swelling, cold feeling or pale appearance in your fingers or toes;

  • signs of infection - fever, chills, body aches, cough with yellow or pink mucus, pain or burning when you urinate;

  • worsening liver symptoms - upper stomach pain or swelling, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes);

  • pancreas problems - severe pain in your upper stomach spreading to your back, vomiting, fever;

  • thyroid problems - weight changes, skin changes, feeling hot or cold all the time; or

  • high blood sugar - increased thirst or urination, hunger, fruity breath odor, tiredness, weight loss.

Common Pegasys side effects may include:

  • flu-like symptoms (fever, chills, headache, muscle or joint pain);

  • nausea, vomiting, loss of appetite;

  • feeling weak or tired;

  • sleep problems (insomnia);

  • thinning hair; or

  • itching, redness, dryness, or swelling where the medicine was injected.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

What other drugs will affect Pegasys?

Tell your doctor about all your current medicines and any you start or stop using, especially:

  • azathioprine;

  • methadone;

  • telbivudine;

  • theophylline; or

  • any medicine to treat HIV or AIDS.

This list is not complete. Other drugs may interact with peginterferon alfa-2a, including prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed in this medication guide.

Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use Pegasys only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.