Mosunetuzumab side effects: what 913 FDA adverse event reports show

Mosunetuzumab · Lunsumio · 17 reaction terms · 716 drug–reaction co-reports · data 2026-09-14

Drug class

Bispecific cd20-directed cd3 t-cell engager

FDA label, Indications wording

Brand names

Lunsumio

openFDA label

Who reports

913 reports · 85% serious · 140 with death outcome · 49% female

openFDA FAERS counts, all years, deduplication not applied

Used for

LUNSUMIO is a bispecific CD20-directed CD3 T-cell engager indicated for the treatment of adult patients with relapsed or refractory follicular lymphoma after two or more lines of systemic therapy. This indication is approved under accelerated approval based on response rate. Continued approval for this indication may be contingent upon verification and description of clinical benefit in a confirmatory trial(s).

FDA label, Indications and Usage

What it is

Mosunetuzumab-axgb is a bispecific CD20-directed CD3 T-cell engager. It is a humanized monoclonal anti-CD20xCD3 T-cell-dependent bispecific antibody of the immunoglobulin G1 (IgG1) isotype. Mosunetuzumab-axgb is produced in Chinese Hamster Ovary (CHO) cells by recombinant DNA technology.

FDA label, Description section

Boxed warning

CYTOKINE RELEASE SYNDROME Cytokine release syndrome (CRS), including serious or life-threatening reactions, can occur in patients receiving LUNSUMIO. Initiate treatment with the LUNSUMIO step-up dosing schedule to reduce the risk of CRS.

FDA label

How to read this page. FAERS is a spontaneous-reporting system: counts are not incidence rates and a signal is not proof of causation. Each reaction below is run through an automated screen — is the term in the FDA label (known), in the indications (the disease being treated), an administrative/coding term, or none of these (unlabeled)? That screen is mechanical text matching, not a causality assessment. The 30 hand-written Bradford Hill assessments are on the causality assessment page. Not medical advice.

Causality screen of the 16 strongest signals

Labeled — boxed warning 2Labeled adverse reaction 3Labeled (related wording) 1Likely indication confounding 1Reporting / coding artifact 3Not in label — unassessed 6

What the badges mean

Labeled — boxed warning The term (or its clinical synonym) appears in the FDA boxed warning: causality is regulator-recognised and considered serious.
Labeled adverse reaction The exact term appears in the label's warnings or adverse-reactions sections: a known, expected effect. The FAERS signal confirms the label rather than discovering anything.
Labeled (related wording) The label describes this effect under different wording (e.g. 'neutropenia' for 'neutrophil count decreased'). Treated as known.
Likely indication confounding The term appears in the label's indications: reports probably describe the disease being treated (or its progression), not a drug effect. Classic confounding by indication.
Reporting / coding artifact An administrative, monitoring or exposure term (product issue, off-label use, 'test normal', overdose...). Not a biological reaction; a high PRR here reflects how reports are coded.
Not in label — unassessed Not found in the label text, not an artifact pattern. This is a genuine candidate for a formal causality assessment (Bradford Hill / WHO-UMC); it has NOT had one here.

Strongest disproportionality signals

Flagged terms (PRR ≥ 2, χ² ≥ 4, n ≥ 10), ranked by PRR; infinite PRRs (no comparator reports) excluded. The EBGM column is a second, shrinkage-based estimate (MGPS) with its 5th percentile EB05 and the BCPNN IC025; a pair flagged by PRR alone is marked PRR-only and should be read as fragile. The strata column recomputes the PRR on serious reports only and flags pairs whose reports are unusually skewed by sex or come mostly from consumers rather than clinicians. 1 further terms are in the interactive explorer. Reaction links open the term's cross-drug page.

Show: hide coding/lab/outcome terms hide indication terms unlabeled only hide PRR-only

Reaction	Reports	PRR 95% CI · ROR · χ²/p	EBGM EB05 · IC025	Label screen	Serious-only & strata	Trial cross-check
Cytokine release syndrome	213	258.8 229.9–291.4ROR 336.9 · χ² 53,876 / p <1e-300	250.2 EB05 223.2 · IC025 +7.1 3/3 methods agree	Labeled — boxed warning exact term	serious PRR 165.3 · 93% clinician	no trial data cached
Covid-19 pneumonia	40	77.3 57.0–104.7ROR 80.8 · χ² 2,927 / p <1e-300	74.4 EB05 57.0 · IC025 +4.8 3/3 methods agree	Labeled (related wording) related wording	serious PRR 53.2 · 95% clinician	no trial data cached
Therapy partial responder	28	42.6 29.6–61.4ROR 43.9 · χ² 1,096 / p 1e-240	41.4 EB05 30.1 · IC025 +4.0 3/3 methods agree	Reporting / coding artifact	serious PRR 35.1 · ♀ 100% (drug 49%) · 93% clinician	no trial data cached
Disease progression	124	24.1 20.5–28.4ROR 27.7 · χ² 2,734 / p <1e-300	24.0 EB05 20.7 · IC025 +4.2 3/3 methods agree	Reporting / coding artifact	serious PRR 18.7 · 93% clinician	no trial data cached
Cholecystitis	10	19.3 10.4–35.7ROR 19.5 · χ² 155 / p 6e-36	18.0 EB05 9.5 · IC025 +2.3 3/3 methods agree	Not in label — unassessed	serious PRR 13.3	no trial data cached
Disorientation	32	18.5 13.1–25.9ROR 19.1 · χ² 512 / p 1e-113	18.2 EB05 13.4 · IC025 +3.3 3/3 methods agree	Not in label — unassessed	serious PRR 15.4 · ♀ 100% (drug 49%) · 94% clinician	no trial data cached
Pneumonitis	18	15.2 9.6–24.1ROR 15.5 · χ² 226 / p 3e-51	14.6 EB05 9.3 · IC025 +2.7 3/3 methods agree	Not in label — unassessed	serious PRR 10.8	no trial data cached
Neutropenia (low neutrophils (infection-fighting cells))	82	13.9 11.3–17.0ROR 15.1 · χ² 972 / p 1e-213	13.8 EB05 11.4 · IC025 +3.3 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 9.2 · 98% clinician	no trial data cached
Splenomegaly	7	13.7 6.5–28.6ROR 13.8 · χ² 70 / p 3e-17	10.7 EB05 4.5 · IC025 +1.6 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 10.5	no trial data cached
Cytomegalovirus infection	8	11.0 5.5–22.0ROR 11.1 · χ² 63 / p 9e-16	8.4 EB05 4.0 · IC025 +1.6 3/3 methods agree	Not in label — unassessed	serious PRR 7.6	no trial data cached
Malignant neoplasm progression	45	10.1 7.6–13.5ROR 10.6 · χ² 362 / p 5e-81	9.7 EB05 7.4 · IC025 +2.7 3/3 methods agree	Reporting / coding artifact	serious PRR 7.0 · ♀ 81% (drug 49%) · 96% clinician	no trial data cached
Cytopenia	5	9.6 4.0–23.0ROR 9.6 · χ² 30 / p 2e-8	6.1 EB05 2.6 · IC025 +0.9 3/3 methods agree	Labeled — boxed warning exact term	serious PRR 6.7	no trial data cached
Lymphoma	6	8.9 4.0–19.7ROR 8.9 · χ² 34 / p 2e-9	6.0 EB05 2.8 · IC025 +1.1 3/3 methods agree	Likely indication confounding term in indications; also in warnings	serious PRR 6.1	no trial data cached
Covid-19	68	8.8 7.0–11.1ROR 9.4 · χ² 466 / p 1e-103	8.5 EB05 6.9 · IC025 +2.7 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 10.6 · 97% clinician	no trial data cached
Injection site reaction	19	6.3 4.1–9.9ROR 6.5 · χ² 80 / p 2e-19	5.6 EB05 3.8 · IC025 +1.7 3/3 methods agree	Not in label — unassessed	serious PRR 18.8	no trial data cached
Neoplasm progression	8	5.2 2.6–10.4ROR 5.3 · χ² 23 / p 7e-7	4.1 EB05 2.3 · IC025 +0.9 3/3 methods agree	Not in label — unassessed	serious PRR 4.9	no trial data cached

Trial evidence (ClinicalTrials.gov)

Trial cross-check not yet run for this drug (ClinicalTrials.gov pull covers the 800 most-reported drugs first).

Most frequently reported reactions

By raw co-report count, regardless of disproportionality. Common terms here are often the illness itself or generic outcomes.

Reaction	Reports	PRR 95% CI · ROR · χ²/p	EBGM EB05 · IC025	Label screen	Serious-only & strata	Trial cross-check
Cytokine release syndrome	213	258.8 229.9–291.4ROR 336.9 · χ² 53,876 / p <1e-300	250.2 EB05 223.2 · IC025 +7.1 3/3 methods agree	Labeled — boxed warning exact term	serious PRR 165.3 · 93% clinician	no trial data cached
Disease progression	124	24.1 20.5–28.4ROR 27.7 · χ² 2,734 / p <1e-300	24.0 EB05 20.7 · IC025 +4.2 3/3 methods agree	Reporting / coding artifact	serious PRR 18.7 · 93% clinician	no trial data cached
Neutropenia (low neutrophils (infection-fighting cells))	82	13.9 11.3–17.0ROR 15.1 · χ² 972 / p 1e-213	13.8 EB05 11.4 · IC025 +3.3 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 9.2 · 98% clinician	no trial data cached
Covid-19	68	8.8 7.0–11.1ROR 9.4 · χ² 466 / p 1e-103	8.5 EB05 6.9 · IC025 +2.7 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 10.6 · 97% clinician	no trial data cached
Malignant neoplasm progression	45	10.1 7.6–13.5ROR 10.6 · χ² 362 / p 5e-81	9.7 EB05 7.4 · IC025 +2.7 3/3 methods agree	Reporting / coding artifact	serious PRR 7.0 · ♀ 81% (drug 49%) · 96% clinician	no trial data cached
Covid-19 pneumonia	40	77.3 57.0–104.7ROR 80.8 · χ² 2,927 / p <1e-300	74.4 EB05 57.0 · IC025 +4.8 3/3 methods agree	Labeled (related wording) related wording	serious PRR 53.2 · 95% clinician	no trial data cached
Disorientation	32	18.5 13.1–25.9ROR 19.1 · χ² 512 / p 1e-113	18.2 EB05 13.4 · IC025 +3.3 3/3 methods agree	Not in label — unassessed	serious PRR 15.4 · ♀ 100% (drug 49%) · 94% clinician	no trial data cached
Therapy partial responder	28	42.6 29.6–61.4ROR 43.9 · χ² 1,096 / p 1e-240	41.4 EB05 30.1 · IC025 +4.0 3/3 methods agree	Reporting / coding artifact	serious PRR 35.1 · ♀ 100% (drug 49%) · 93% clinician	no trial data cached
Injection site reaction	19	6.3 4.1–9.9ROR 6.5 · χ² 80 / p 2e-19	5.6 EB05 3.8 · IC025 +1.7 3/3 methods agree	Not in label — unassessed	serious PRR 18.8	no trial data cached
Pneumonitis	18	15.2 9.6–24.1ROR 15.5 · χ² 226 / p 3e-51	14.6 EB05 9.3 · IC025 +2.7 3/3 methods agree	Not in label — unassessed	serious PRR 10.8	no trial data cached
Cholecystitis	10	19.3 10.4–35.7ROR 19.5 · χ² 155 / p 6e-36	18.0 EB05 9.5 · IC025 +2.3 3/3 methods agree	Not in label — unassessed	serious PRR 13.3	no trial data cached
Neoplasm progression	8	5.2 2.6–10.4ROR 5.3 · χ² 23 / p 7e-7	4.1 EB05 2.3 · IC025 +0.9 3/3 methods agree	Not in label — unassessed	serious PRR 4.9	no trial data cached
Cytomegalovirus infection	8	11.0 5.5–22.0ROR 11.1 · χ² 63 / p 9e-16	8.4 EB05 4.0 · IC025 +1.6 3/3 methods agree	Not in label — unassessed	serious PRR 7.6	no trial data cached
Splenomegaly	7	13.7 6.5–28.6ROR 13.8 · χ² 70 / p 3e-17	10.7 EB05 4.5 · IC025 +1.6 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 10.5	no trial data cached
Lymphoma	6	8.9 4.0–19.7ROR 8.9 · χ² 34 / p 2e-9	6.0 EB05 2.8 · IC025 +1.1 3/3 methods agree	Likely indication confounding term in indications; also in warnings	serious PRR 6.1	no trial data cached

Unlabeled candidate signals

These flagged terms were not found in the FDA label text and do not match an artifact pattern. They are the pairs that would merit a structured assessment (temporality, dose-response, biological plausibility, consistency across databases, trial or cohort evidence). The screen has not done that; string-matching misses label wording, and a strong PRR can still be stimulated reporting, co-medication, or the disease itself.

Disorientation — n=32, PRR 18.5

Want to assess one of these properly? The runbook describes the Bradford Hill / trial-evidence workflow used on the assessment page, and handoff_top1000_signals.csv lists the strongest unassessed pairs across all drugs.

Common questions

What are the most reported side effects of Mosunetuzumab?

In FDA FAERS, the most frequently co-reported reaction terms for Mosunetuzumab are cytokine release syndrome, neutropenia, covid-19, covid-19 pneumonia, disorientation, injection site reaction. These are report counts, not rates: FAERS has no denominator, reporting is voluntary and uneven, and a term can be reported because of the illness being treated rather than the drug.

Does Mosunetuzumab have an FDA boxed warning?

Yes. The current FDA label carries a boxed warning: CYTOKINE RELEASE SYNDROME Cytokine release syndrome (CRS), including serious or life-threatening reactions, can occur in patients receiving LUNSUMIO. Initiate treatment with the LUNSUMIO step-up dosing schedule to reduce the risk of CRS.

Do FAERS signals prove that Mosunetuzumab causes these side effects?

No. A disproportionality signal (PRR, ROR) means a reaction is reported more often with this drug than with others in the database. That can reflect a real effect, but also the disease being treated, stimulated reporting after news or litigation, co-medications, or how reports are coded. Causality needs temporality, dose-response, biological plausibility, consistency across data sources and, ideally, trial or cohort evidence. This page screens each term against the FDA label and ClinicalTrials.gov results; it does not establish causation.

Sources & methods

FDA label via openFDA (set ID 2ef0cf38-101c-4681-98fe-c05dc9ead443, effective 20260629; matched on openfda.generic_name.exact:"MOSUNETUZUMAB")
Bayesian shrinkage measures computed offline over all 1,317,300 cached pairs: BCPNN information component (IC, with Norén closed-form IC025) and MGPS empirical-Bayes geometric mean (EBGM, EB05) with a two-component gamma prior fitted by maximum marginal likelihood (α₁=0.57, β₁=0.02, α₂=1.42, β₂=0.70, w=0.17); unstratified
FAERS pair statistics from the OpenPV build of 2026-09-14 (openFDA count= aggregation, field patient.drug.openfda.generic_name.exact, 17 reaction terms, min 3 co-reports); PRR/ROR with 95% CI, chi-square with Yates correction
Serious-only and stratified counts (serious:1, patient.patientsex, primarysource.qualification) from openFDA count queries; serious-only PRR uses the drug serious report total and the term global serious total (N_serious=11,882,979); strata shown only when n ≥ 20
Seriousness and sex counts from openFDA count=serious, patient.patientsex, seriousnessdeath

Page generated 2026-10-08. Label and PubChem text are retrieved automatically and may lag the current label; always check the current prescribing information. OpenPV is a beta research tool of the Open Source Medicine Foundation.