What the label lists and what reports show
Mosunetuzumab side effects
Mosunetuzumab's FDA label: “The most common adverse reactions (≥ 20%) are injection site reactions, fatigue, rash, CRS, COVID-19 infection, musculoskeletal pain, and diarrhea.” FAERS holds 916 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 cytokine release syndrome (213 reports).
- No generic listed
- Boxed warning
Check it at the source: the mosunetuzumab FDA label on DailyMed (effective June 29, 2026); 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 LUNSUMIO VELO?LUNSUMIO VELO can cause serious side effects, including: See " What is the most important information I should know about LUNSUMIO VELO? " Neurologic problems. LUNSUMIO VELO can cause serious and life-threatening neurologic problems. Your healthcare provider will check you for neurologic problems during treatment with LUNSUMIO VELO. Your healthcare provider may also refer you to a healthcare provider who specializes in neurologic problems. […]
Source: FDA drug label, Genentech, Inc., effective June 29, 2026 (SPL set 6a2b6dc2-48bd-4da7-a611-bc6bae2893c8), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common adverse reactions (≥ 20%) are injection site reactions, fatigue, rash, CRS, COVID-19 infection, musculoskeletal pain, and diarrhea.
Table 7. Adverse Reactions (≥ 10%) in Patients with Relapsed or Refractory FL Who Received LUNSUMIO VELO Subcutaneous Injection in GO29781
| Reaction | Adverse Reaction All Grades (%) | LUNSUMIO VELO (N = 94) Grade 3 or 4 (%) |
|---|---|---|
| Cytokine release syndrome | 30 | 2.1 |
| Injection site reactions Injection site reactions includes injection site reaction,… | 69 | 0 |
| Fatigue Fatigue includes fatigue, asthenia, and lethargy. | 39 | 0 |
| Edema Edema includes edema, edema peripheral, face edema, pulmonary edema, fluid… | 13 | 0 |
| Pyrexia | 11 | 1.1 |
| Chills | 11 | 0 |
| Rash Rash includes rash, injection site rash, erythema, dermatitis, palmar-plantar… | 35 | 3.2 |
| Dry skin | 11 | 0 |
| Headache | 17 | 0 |
| Peripheral neuropathy Peripheral neuropathy includes peripheral neuropathy, peripheral… | 11 | 0 |
| Dizziness Dizziness includes dizziness and vertigo. | 10 | 0 |
| Musculoskeletal pain Musculoskeletal pain includes musculoskeletal pain, back pain,… | 20 | 0 |
| Arthralgia | 13 | 0 |
| Cough | 13 | 0 |
| Dyspnea | 11 | 0 |
| Diarrhea | 20 | 0 |
| Nausea | 14 | 0 |
| Constipation | 14 | 0 |
| Abdominal pain | 13 | 0 |
| COVID-19 Adverse reaction with fatal outcome. , Grade 5 COVID-19 occurred in 3.2% of… | 27 | 4.3 |
| Upper respiratory tract infection Upper respiratory tract infection includes upper… | 15 | 2.1 |
| Pneumonia Pneumonia includes lung consolidation and specific types of pneumonia including… | 13 | 4.3 |
| Insomnia | 15 | 0 |
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 elsewhere in the labeling: Cytokine Release Syndrome [see Warnings and Precautions (5.1) ] Neurologic Toxicity, including Immune Effector Cell-Associated Neurotoxicity Syndrome [see Warnings and Precautions (5.2) ] Infections [see Warnings and Precautions (5.3) ] Hemophagocytic Lymphohistiocytosis [see Warnings and Precautions (5.4) ] Cytopenias [see Warnings and Precautions (5.5) ] Tumor Flare [see Warnings and Precautions (5.6) ] The most common adverse reactions (≥ 20%) are injection site reactions, fatigue, rash, CRS, COVID-19 infection, musculoskeletal pain, and diarrhea. The most common Grade 3 to 4 laboratory abnormalities (≥ 15%) are decreased lymphocyte count, decreased neutrophil count, and increased uric acid. ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Genentech at 1-888-835-2555 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 to rates in the clinical trials of another drug and may not reflect the rates observed in practice. …
Source: FDA drug label, Genentech, Inc., effective June 29, 2026 (SPL set 6a2b6dc2-48bd-4da7-a611-bc6bae2893c8), via openFDA; the full label on DailyMed
What do FAERS reports show?
916 reports list mosunetuzumab among the medicines taken, each a report of something that happened, not proof the medicine caused it; 140 are coded as a death and 415 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 20 reactions below, 13 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?
- Mosunetuzumab: 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