Pegasys (peginterferon alfa-2a) is an antiviral medicine used to treat chronic hepatitis C and chronic hepatitis B. It is a pegylated form of interferon, a protein that naturally helps the immune system fight viral infections. Pegylation (attaching polyethylene glycol to the molecule) slows its breakdown in the body, allowing it to be given once weekly instead of multiple times a week.

For hepatitis C, Pegasys is used in combination with other antiviral medicines. In some situations it is still used; however, newer direct-acting antiviral regimens (such as Epclusa and Harvoni) have largely replaced interferon-based therapy due to better tolerability and higher cure rates.

Pegasys is given as a subcutaneous (under the skin) injection once weekly, usually on the same day each week. Your doctor or nurse will show you how to inject.

Common injection sites include the thigh or abdomen. Rotate injection sites weekly.

The treatment duration varies: 24 to 48 weeks for hepatitis C, and up to 48 weeks for hepatitis B. Adherence is important throughout the course.

If you miss a weekly dose, inject as soon as you remember. Then resume your weekly injection schedule from that new day. If almost a week has passed, contact your specialist for advice. Never inject two doses in one week.

Contact your doctor or emergency services immediately.

• Store in a refrigerator between 2°C and 8°C
• Do not freeze
• Keep in the original carton to protect from light
• Take out of the fridge 30 minutes before injecting
• Keep out of reach of children

Side effects are very common with interferon-based therapy. Many patients find these difficult to manage.

Very common side effects:

• Flu-like symptoms: fever, chills, aching muscles, and headache, usually starting within hours of each injection and lasting a day or two
• Fatigue: often severe throughout treatment
• Depression and mood changes: interferon can cause significant psychological effects including depression and irritability
• Low blood counts: anaemia, low white blood cells, and low platelets
• Injection site reactions: redness, bruising, or pain

Serious side effects:

• Severe depression or suicidal thoughts: immediate medical attention required
• Serious infections due to low white blood cells
• Thyroid problems (hyperthyroidism or hypothyroidism)
• Eye problems: rare cases of retinopathy have been reported. Baseline and monitoring eye exams are recommended
  

Blood tests including full blood count, liver function, and thyroid function are done regularly during treatment.

• Telbivudine: combination with Pegasys significantly increases risk of peripheral neuropathy. Avoid
• Didanosine (DDI): increased risk of liver failure and mitochondrial toxicity. Avoid
• Ribavirin: commonly combined with Pegasys for hepatitis C treatment. Can cause anaemia and is harmful to a developing baby
  

Tell your doctor about all medicines, vitamins, and supplements before starting Pegasys.

Why is Pegasys still used when newer hepatitis C medicines exist?

For most patients with hepatitis C, newer all-oral direct-acting antiviral regimens (such as Epclusa, Harvoni, or Maviret) are now preferred. They have higher cure rates, fewer side effects, and shorter treatment courses. However, Pegasys-based regimens may still be considered in certain situations, such as specific hepatitis B cases where interferon is appropriate, or where access or cost considerations apply.

How do I manage the flu-like symptoms after my injection?

Many patients find it helpful to inject on Friday evenings so the worst symptoms occur over the weekend. Paracetamol taken before and after the injection can help with fever, aches, and headache. Staying well hydrated also helps. These symptoms typically lessen after the first few weeks as your body adapts to the treatment.

Ace provides accurate and independent information medically reviewed on prescription medications. This material is provided for educational purposes only and is not intended for medical advice, diagnosis or treatment.