What the label lists and what reports show
Mepolizumab side effects
Mepolizumab's FDA label: “Most common adverse reactions (incidence ≥5%): • Asthma: Headache, injection site reaction, back pain, and fatigue.” FAERS holds 49,399 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 dyspnoea (10,576 reports).
- No generic listed
Check it at the source: the mepolizumab FDA label on DailyMed (effective August 6, 2025); 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 NUCALA?NUCALA can cause serious side effects, including: • allergic (hypersensitivity) reactions, including anaphylaxis. Serious allergic reactions can happen after you get your NUCALA injection. Allergic reactions can sometimes happen hours or days after you get a dose of NUCALA. Tell your healthcare provider or get emergency help right away if you have any of the following symptoms of an allergic reaction: o swelling of your face, mouth, and tongue o fainting, dizziness, feeling lightheaded (low blood pressure) o hives o breathing problems o rash • herpes zoster infections. Herpes zoster infections that can cause shingles have happened in people who received NUCALA. The most common side effects of NUCALA include: headache, injection site reactions (pain, redness, swelling, itching, or a burning feeling at the injection site), back pain, and tiredness (fatigue). […]
Source: FDA drug label, GlaxoSmithKline LLC, effective August 6, 2025 (SPL set fefb887c-e4ac-431e-8893-e9d1a5a63fea), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsMost common adverse reactions (incidence ≥5%): • Asthma: Headache, injection site reaction, back pain, and fatigue.
Table 2. Adverse Reactions with NUCALA with ≥3% Incidence and More Common than Placebo in Patients with Severe Asthma (MENSA and SIRIUS)
| Adverse Reaction | NUCALA (Mepolizumab 100 mg Subcutaneous) (n = 263) % | Placebo (n = 257) % |
|---|---|---|
| Headache | 19 | 18 |
| Injection site reaction | 8 | 3 |
| Back pain | 5 | 4 |
| Fatigue | 5 | 4 |
| Influenza | 3 | 2 |
| Urinary tract infection | 3 | 2 |
| Abdominal pain upper | 3 | 2 |
| Pruritus | 3 | 2 |
| Eczema | 3 | <1 |
| Muscle spasms | 3 | <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 adverse reactions are described in greater detail in other sections: • Hypersensitivity reactions [see Warnings and Precautions ( 5.1 )] • Opportunistic infections: herpes zoster [see Warnings and Precautions ( 5.3 )] Most common adverse reactions (incidence ≥5%): • Asthma: Headache, injection site reaction, back pain, and fatigue. ( 6.1 ) • CRSwNP: Oropharyngeal pain and arthralgia. ( 6.1 ) • COPD: Back pain, diarrhea, and cough. ( 6.1 ) • EGPA and HES: Most common adverse reactions are similar to asthma. ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact GlaxoSmithKline at 1-888-825-5249 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. 6.1 Clinical Trials Experience Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared with rates in the clinical trials of another drug and may not reflect the rates observed in practice. Adverse Reactions in Adult and Adolescent Patients Aged 12 Years and Older with Severe Asthma A total of 1,327 patients with severe asthma were evaluated in 3 randomized, placebo-controlled, multicenter trials of 24 to 52 weeks’ duration (Severe Asthma Trials DREAM, MENSA, and SIRIUS).
Source: FDA drug label, GlaxoSmithKline LLC, effective August 6, 2025 (SPL set fefb887c-e4ac-431e-8893-e9d1a5a63fea), via openFDA; the full label on DailyMed
What do FAERS reports show?
49,399 reports list mepolizumab among the medicines taken, each a report of something that happened, not proof the medicine caused it; 3,989 are coded as a death and 15,956 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 17 reactions below, 7 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?
- Mepolizumab: 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