What the label lists and what reports show
Peginterferon beta 1a side effects
Peginterferon beta 1a's FDA label: “The most common adverse reactions in clinical trials of subcutaneous PLEGRIDY (incidence ≥10% and at least 2% more frequent on PLEGRIDY than on placebo) were injection site erythema, influenza-like illness, pyrexia, headache, myalgia, chills, injection site …” FAERS holds 13,031 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is influenza like illness (1,464 reports).
- No generic listed
Check it at the source: the peginterferon beta 1a FDA label on DailyMed (effective November 12, 2025, Biogen Inc.); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of PLEGRIDY?See “ What is the most important information I should know about PLEGRIDY? ” PLEGRIDY may cause serious side effects, including: serious allergic reactions. Serious allergic reactions can happen if you take PLEGRIDY. Symptoms may include: itching, swelling of the face, eyes, lips, tongue, or throat, trouble breathing, feeling faint, anxiousness, skin rash, hives, skin bumps. Get emergency help right away if you have any of these symptoms. Talk to your healthcare provider before taking another dose of PLEGRIDY. injection site reactions. PLEGRIDY may commonly cause redness, pain, itching, or swelling at the place where your injection was given. Call your healthcare provider right away if an injection site becomes swollen and painful or the area looks infected. […]
Source: FDA drug label, Biogen Inc., effective November 12, 2025 (SPL set 08f0ea03-4e6d-195d-aef4-886e32befa95), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common adverse reactions in clinical trials of subcutaneous PLEGRIDY (incidence ≥10% and at least 2% more frequent on PLEGRIDY than on placebo) were injection site erythema, influenza-like illness, pyrexia, headache, myalgia, chills, injection site pain, asthenia, injection site pruritus, and arthralgia ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Biogen at 1-800-456-2255 or FDA at 1-800-FDA-1088 or http://www.fda.gov/medwatch.
Table 3: Adverse Reactions in the 48-Week Placebo-Controlled Phase of Study 1 with an Incidence 2% Higher for PLEGRIDY Than for Placebo
| Reaction | PLEGRIDY (N=512) % | Placebo (N=500) % |
|---|---|---|
| Headache | 44 | 33 |
| Nausea | 9 | 6 |
| Vomiting | 5 | 2 |
| Myalgia | 19 | 6 |
| Arthralgia | 11 | 7 |
| Injection site erythema | 62 | 7 |
| Influenza like illness | 47 | 13 |
| Pyrexia | 45 | 15 |
| Chills | 17 | 5 |
| Injection site pain | 15 | 3 |
| Asthenia | 13 | 8 |
| Injection site pruritus | 13 | 1 |
| Hyperthermia | 4 | 1 |
| Pain | 5 | 3 |
| Injection site edema | 3 | 0 |
| Injection site warmth | 3 | 0 |
| Injection site hematoma | 3 | 1 |
| Injection site rash | 2 | 0 |
| Body temperature increased | 6 | 3 |
| Alanine aminotransferase increased | 6 | 3 |
| Aspartate aminotransferase increased | 4 | 2 |
| Gamma-glutamyl-transferase increased | 3 | 1 |
| Pruritus | 4 | 1 |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
From the label: adverse reactions, continuedThe following serious adverse reactions are discussed in more detail in other sections of labeling: Hepatic Injury [see Warnings and Precautions ( 5.1 )] Depression and Suicide [see Warnings and Precautions ( 5.2 )] Anaphylaxis and Other Allergic Reactions [see Warnings and Precautions ( 5.3 )] Injection Site Reactions Including Necrosis [see Warnings and Precautions ( 5.4 )] Congestive Heart Failure [see Warnings and Precautions (Section 5.5 )] Decreased Peripheral Blood Counts [see Warnings and Precautions ( 5.6 )] Thrombotic Microangiopathy [see Warnings and Precautions ( 5.7 )] Pulmonary Arterial Hypertension [see Warnings and Precautions ( 5.8 )] Autoimmune Disorders [see Warnings and Precautions ( 5.9 )] Seizures [see Warnings and Precautions ( 5.10 )] The most common adverse reactions in clinical trials of subcutaneous PLEGRIDY (incidence ≥10% and at least 2% more frequent on PLEGRIDY than on placebo) were injection site erythema, influenza-like illness, pyrexia, headache, myalgia, chills, injection site pain, asthenia, injection site pruritus, and arthralgia ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Biogen at 1-800-456-2255 or FDA at 1-800-FDA-1088 or http://www.fda.gov/medwatch. …
Source: FDA drug label, Biogen Inc., effective November 12, 2025 (SPL set 08f0ea03-4e6d-195d-aef4-886e32befa95), via openFDA; the full label on DailyMed
What do FAERS reports show?
13,031 reports list peginterferon beta 1a among the medicines taken, each a report of something that happened, not proof the medicine caused it; 333 are coded as a death and 3,143 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 21 reactions below, 11 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Peginterferon beta 1a: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions