Pegasys Is a Once-Weekly Injectable Interferon Used to Treat Chronic Hepatitis B, Chronic Hepatitis C, and Blood Disorders Like Polycythemia Vera

If your doctor has mentioned Pegasys, you probably have questions. What exactly is it? How does it work? What's it going to cost? This guide covers everything you need to know about Pegasys in 2026 — from what it treats to how it's taken and what to expect.

What Is Pegasys?

Pegasys is the brand name for Peginterferon Alfa-2a, a pegylated interferon. It belongs to a class of medications called antiviral interferons. Interferons are proteins that your body naturally produces to fight viruses and regulate your immune system. Pegasys is a lab-made version that has been modified with a polyethylene glycol (PEG) chain to make it last longer in your body, allowing once-weekly dosing instead of multiple injections per week.

Pegasys was originally manufactured by Roche/Genentech and is now produced by pharma& GmbH. It is not a controlled substance and is not available as a generic or biosimilar in the United States as of 2026.

What Is Pegasys Used For?

Pegasys is FDA-approved for:

• Chronic Hepatitis C (CHC) — In adults and children 5 years and older. It's used in combination with other hepatitis C antiviral drugs, or as monotherapy when other antivirals are contraindicated.
• Chronic Hepatitis B (CHB) — In adults with both HBeAg-positive and HBeAg-negative disease with compensated liver function. Also approved for pediatric patients 3 years and older with HBeAg-positive, non-cirrhotic hepatitis B.

Pegasys is also widely used off-label for:

• Polycythemia vera (PV) — A blood disorder where the body makes too many red blood cells
• Essential thrombocythemia (ET) — A condition where the body makes too many platelets
• Other myeloproliferative neoplasms (MPNs)

In fact, the off-label use for MPNs has become one of the biggest drivers of Pegasys demand and is a key factor in the current shortage.

How Is Pegasys Taken?

Pegasys is given as a subcutaneous injection (a shot under the skin) once per week. It can be injected in the abdomen or thigh, and injection sites should be rotated each week.

The medication comes in several forms:

• 180 mcg/mL single-dose vial
• 180 mcg/0.5 mL prefilled syringe
• 135 mcg/0.5 mL prefilled syringe
• 180 mcg/0.5 mL autoinjector
• 135 mcg/0.5 mL autoinjector

Typical Dosing

• Adults: 180 mcg once weekly (standard dose for hepatitis B and C)
• Severe kidney impairment: Reduced to 135 mcg once weekly
• Children: 180 mcg/1.73 m² × body surface area, once weekly (maximum 180 mcg)

Pegasys must be refrigerated at 2–8°C (36–46°F). It can be self-injected at home after proper training from your healthcare provider, or administered in a doctor's office.

Treatment duration depends on the condition:

• Hepatitis C: Typically 24 to 48 weeks
• Hepatitis B: Typically 48 weeks
• Polycythemia vera / MPNs: Often ongoing, potentially indefinite

Who Should Not Take Pegasys?

Pegasys is contraindicated (should not be used) in patients with:

• Hypersensitivity to Peginterferon Alfa-2a or any alpha interferon
• Autoimmune hepatitis
• Hepatic decompensation (Child-Pugh score above 6)
• Neonates and infants (the formulation contains benzyl alcohol)

When combined with Ribavirin, Pegasys is strictly contraindicated during pregnancy or in patients planning pregnancy, as the combination is Category X (known to cause birth defects).

Pegasys should be used with caution in patients with:

• History of depression or psychiatric illness
• Heart disease
• Autoimmune conditions
• Cirrhosis or advanced liver disease

For details on potential side effects, read our guide on Pegasys side effects. For information about medications that interact with Pegasys, see Pegasys drug interactions.

How Much Does Pegasys Cost?

Pegasys is expensive. Here's what you can expect:

• Per vial or syringe: $1,095 to $1,200
• Monthly cost (4 weekly doses): Approximately $4,290 to $4,800
• Annual cost: Roughly $52,000 to $58,000 or more

There is no generic or biosimilar version of Pegasys available in the US.

Insurance Coverage

Most insurance plans cover Pegasys, but typically with conditions:

• Prior authorization is usually required
• It's covered under the medical benefit (if administered in-office) or specialty pharmacy benefit
• Step therapy may be required for hepatitis C (trying direct-acting antivirals first)
• Medicare Part D covers it on a specialty tier, with the $2,000 annual out-of-pocket cap applying as of 2025

Financial Assistance

If you're struggling with the cost, there are options:

• pharma& Patient Assistance Program — Call 855-443-7028 for uninsured or underinsured patients
• NeedyMeds and RxAssist — May list additional assistance programs
• Read our full guide on how to save money on Pegasys

Final Thoughts

Pegasys is a well-established interferon treatment that plays an important role in managing chronic hepatitis B, chronic hepatitis C, and myeloproliferative blood disorders. It requires commitment — weekly injections, regular blood monitoring, and management of side effects — but for many patients, it's an essential part of their treatment plan.

If you've been prescribed Pegasys and need help finding it, Medfinder can help you locate it in stock near you.