Is it still first-line for hepatitis C?

Largely no. Direct-acting antivirals have replaced interferon-based regimens for most hepatitis C patients, though the pivotal trial evidence remains historically important.

What is the boxed warning?

The FDA interferon boxed warning states it may cause or worsen fatal or life-threatening neuropsychiatric, autoimmune, ischemic and infectious disorders, requiring close monitoring.

What are the common side effects?

Flu-like symptoms, fatigue, injection-site reactions and suppressed blood counts are common; depression and suicidal thoughts are serious risks requiring monitoring.

It is a once-weekly subcutaneous injection prescribed and monitored by a specialist; when used for hepatitis C it was historically combined with ribavirin.

EMA-approvedaka Pegasys

Peginterferon alfa-2a — Complete Research Guide (2026)

Last updated 2026-06-30

TL;DR

Peginterferon alfa-2a (also known as Pegasys) is a peptide catalogued under Immune (PEG-IFN-α). Neutral reference entry; EU status: EU-approved prescription medicine.

What is Peginterferon alfa-2a?

Peginterferon alfa-2a is a pegylated (long-acting) recombinant interferon alfa-2a marketed as Pegasys. It is FDA- and EMA-approved for chronic hepatitis C (historically used with ribavirin) and for chronic hepatitis B, in both HBeAg-positive and HBeAg-negative disease.

Its evidence base is human and strong, built on large randomized registration trials (Fried 2002 for HCV; Lau 2005 and Marcellin 2004 for HBV), though in hepatitis C it has largely been superseded by direct-acting antivirals.

How does Peginterferon alfa-2a work?

Interferon alfa binds type I interferon receptors to trigger antiviral and immunomodulatory gene programs that inhibit viral replication and enhance immune clearance of infected hepatocytes.

Pegylation slows clearance, allowing once-weekly dosing and more sustained drug exposure.

What does the research say about Peginterferon alfa-2a?

In chronic hepatitis C, peginterferon alfa-2a plus ribavirin produced higher sustained virological response rates than standard interferon plus ribavirin. ^[1]
For HBeAg-positive chronic hepatitis B, peginterferon alfa-2a achieved higher rates of HBeAg seroconversion than lamivudine alone. ^[4]
In HBeAg-negative chronic hepatitis B, peginterferon alfa-2a produced sustained off-treatment virological and biochemical responses. ^[3]

Clinical research & studies

The references below are the primary sources cited throughout this guide. Each links directly to PubMed or the regulator. Where evidence is preclinical (animal or in-vitro), that is stated rather than implied.

[1] Peginterferon alfa-2a plus ribavirin for chronic hepatitis C virus infection — Fried MW et al., The New England Journal of Medicine 2002. (Randomized controlled trial (registration))
[2] Peginterferon-alpha2a and ribavirin combination therapy in chronic hepatitis C: a randomized study of treatment duration and ribavirin dose — Hadziyannis SJ et al., Annals of Internal Medicine 2004. (Randomized controlled trial)
[3] Peginterferon alfa-2a alone, lamivudine alone, and the two in combination in patients with HBeAg-negative chronic hepatitis B — Marcellin P et al., The New England Journal of Medicine 2004. (Randomized controlled trial)
[4] Peginterferon Alfa-2a, lamivudine, and the combination for HBeAg-positive chronic hepatitis B — Lau GK et al., The New England Journal of Medicine 2005. (Randomized controlled trial)
[5] Suicide risk in hepatitis C and during interferon-alpha therapy: a review and clinical update — Sockalingam S et al., Journal of Viral Hepatitis 2011. (Narrative review)

Dosing context

This is not medical advice or a usage recommendation. Dosing figures are reported research context only, cited from the published literature.

For context only: Pegasys is given as a once-weekly subcutaneous injection, historically for a defined course (for example roughly 48 weeks for genotype-1 hepatitis C or chronic hepatitis B), sometimes combined with ribavirin in hepatitis C.

Actual duration, combinations and monitoring are set by a treating hepatologist and fall outside this encyclopedia's scope.

Side effects & safety profile

Peginterferon alfa-2a carries the FDA interferon boxed warning: it may cause or aggravate fatal or life-threatening neuropsychiatric (including depression and suicidality), autoimmune, ischemic and infectious disorders, requiring close monitoring and discontinuation for severe events.

Very common side effects include flu-like symptoms (fever, chills, myalgia, headache), fatigue, injection-site reactions, and blood-count suppression (neutropenia, thrombocytopenia); depression and suicidal ideation are important risks.

It is contraindicated in autoimmune hepatitis and decompensated liver disease, and when used with ribavirin adds serious teratogenic and hemolytic risks. It is a specialist-prescribed antiviral biologic requiring structured laboratory and psychiatric monitoring.

Stacking & combinations

In hepatitis C it was historically combined with ribavirin under physician direction; it is otherwise not intended for informal combination with other agents.

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Frequently asked questions

Yes. Pegasys is FDA- and EMA-approved for chronic hepatitis C (historically with ribavirin) and for chronic hepatitis B in both HBeAg-positive and HBeAg-negative disease.

References

[1] Peginterferon alfa-2a plus ribavirin for chronic hepatitis C virus infection — Fried MW et al., The New England Journal of Medicine 2002. PMID: 12324553. View sourceStudy: Randomized controlled trial (registration)Peginterferon alfa-2a plus ribavirin gave significantly higher sustained virological response than standard interferon alfa-2b plus ribavirin.
[2] Peginterferon-alpha2a and ribavirin combination therapy in chronic hepatitis C: a randomized study of treatment duration and ribavirin dose — Hadziyannis SJ et al., Annals of Internal Medicine 2004. PMID: 14996676. View sourceStudy: Randomized controlled trialGenotype 1 required 48 weeks and weight-based ribavirin, whereas genotypes 2 and 3 responded well to shorter, lower-dose regimens.
[3] Peginterferon alfa-2a alone, lamivudine alone, and the two in combination in patients with HBeAg-negative chronic hepatitis B — Marcellin P et al., The New England Journal of Medicine 2004. PMID: 15371578. View sourceStudy: Randomized controlled trialPeginterferon alfa-2a produced higher sustained off-treatment response than lamivudine in HBeAg-negative chronic hepatitis B, with no added benefit from combination.
[4] Peginterferon Alfa-2a, lamivudine, and the combination for HBeAg-positive chronic hepatitis B — Lau GK et al., The New England Journal of Medicine 2005. PMID: 15987917. View sourceStudy: Randomized controlled trialPeginterferon alfa-2a, with or without lamivudine, achieved higher HBeAg seroconversion than lamivudine alone in HBeAg-positive chronic hepatitis B.
[5] Suicide risk in hepatitis C and during interferon-alpha therapy: a review and clinical update — Sockalingam S et al., Journal of Viral Hepatitis 2011. PMID: 21070503. View sourceStudy: Narrative reviewInterferon-alfa therapy for hepatitis C is associated with depression and elevated suicide risk, supporting the boxed warning and structured psychiatric monitoring.

This article is for educational and research purposes only. Peptides discussed here are not approved for human consumption by the FDA, EMA, or equivalent regulators outside of specific clinical contexts. Always consult a licensed medical professional before any therapeutic use.