Mounjaro Under the Microscope: What Growing NAION Lawsuits and the Latest Safety Evidence Really Show

Mounjaro (tirzepatide) has become part of a growing international safety and legal debate over a rare eye disorder that can cause sudden, potentially permanent vision loss. In the United States, lawsuits alleging that GLP-1 medicines caused non-arteritic anterior ischaemic optic neuropathy (NAION) have been consolidated into a federal multidistrict litigation (MDL), with Mounjaro among the products named. But the existence of lawsuits does not establish that tirzepatide caused the injuries alleged. The scientific evidence remains mixed, and regulators have reached different conclusions about the strength of the signal for tirzepatide compared with semaglutide.

The distinction matters. There is now substantial evidence that a safety question exists; there is not yet equivalent evidence that Mounjaro causes NAION. As of 23 September 2026, the US court is still examining general causation, while new epidemiological research published as recently as 22 September has reported no increased overall ophthalmic risk with tirzepatide during one year of follow-up.

The Lawsuits Are Real — But They Are Not a Finding That Mounjaro Causes Blindness

The US litigation is centred on MDL No. 3163, In re: Glucagon-like Peptide-1 Receptor Agonists (GLP-1 RAs) Non-Arteritic Anterior Ischemic Optic Neuropathy Products Liability Litigation, before Judge Karen Spencer Marston in the Eastern District of Pennsylvania. The court's July 2026 Case Management Order states that the litigation involves personal-injury claims relating to Ozempic, Wegovy, Rybelsus, Saxenda, Trulicity, Mounjaro and Zepbound.

The court recorded that plaintiffs allege that one or more of these medicines caused NAION and that the cases include substantially similar allegations involving failure to warn, design defect and breach of warranty.

The litigation has grown considerably. The US Judicial Panel on Multidistrict Litigation (JPML) reported 216 pending actions in MDL 3163 as of 1 September 2026, up from 200 in August, 146 in July and 110 in June.

However, 216 is the total number of NAION-related federal actions in the MDL — not 216 Mounjaro lawsuits. The proceeding covers products from both Eli Lilly and Novo Nordisk, and the court's July order specifically noted that, as of 15 June, only 19 of 137 cases at that point had been brought against the Lilly defendants.

Eli Lilly itself disclosed in its 2026 regulatory filing that plaintiffs have brought product-liability lawsuits alleging injuries following use of Mounjaro, Trulicity and Zepbound, with most US cases coordinated in two federal MDLs, including the NAION litigation.

What Is NAION?

Non-arteritic anterior ischaemic optic neuropathy is an uncommon disorder involving reduced blood flow to the optic nerve. It typically presents as sudden, painless visual loss, usually affecting one eye. The resulting damage can be permanent. WHO describes NAION as a serious condition that generally produces irreversible visual loss, while Australia's Therapeutic Goods Administration (TGA) describes it as a rare but serious disorder that may result in permanent visual impairment, including blindness.

NAION is not a new condition created by GLP-1 medicines. Diabetes, hypertension, dyslipidaemia, obstructive sleep apnoea and certain optic-disc characteristics are among recognised or reported risk factors, making the investigation of any medicine-related association particularly difficult in populations already at increased baseline risk.

This is one reason why observing NAION after a patient starts a medicine does not, by itself, demonstrate that the medicine caused the condition.

The Court Has Not Yet Decided Whether the Drugs Cause NAION

The July 2026 court order is particularly important because it shows where the litigation actually stands.

Judge Marston ordered early discovery and motion practice on general causation and federal pre-emption/warning adequacy.

The court distinguishes general causation from specific causation. General causation asks whether a substance is capable of causing a particular condition in the general population; specific causation asks whether that substance caused the condition in a particular individual.

That distinction means the court is still dealing with a fundamental scientific question:

Can the medicines covered by this litigation cause NAION?

The answer has not yet been established by the court.

The order also requires the parties to exchange plaintiffs' fact sheets and medical records capable of confirming proof of drug exposure and proof of diagnosis.

The Mounjaro Question Is Different From the Semaglutide Question

Much of the scientific evidence that triggered regulatory action concerns semaglutide, the active ingredient in Ozempic, Rybelsus and Wegovy.

Mounjaro contains tirzepatide, a dual glucose-dependent insulinotropic polypeptide (GIP)/GLP-1 receptor agonist. It therefore should not automatically be treated as scientifically interchangeable with semaglutide. The distinction is recognised in the scientific literature and by regulators.

This is arguably the most important point for understanding the current controversy.

The evidence linking semaglutide to NAION is stronger than the evidence linking tirzepatide specifically to NAION.

Why Semaglutide Triggered Regulatory Action

Several observational studies have reported an association between semaglutide and NAION.

A Danish nationwide cohort study involving 424,152 people with type 2 diabetes reported 218 NAION events during more than 1.9 million person-years of observation. Semaglutide exposure was associated with a higher incidence of NAION and an adjusted hazard ratio of 2.19 (95% CI 1.54–3.12).

A separate Danish-Norwegian study identified 32 NAION events among 61,377 semaglutide initiators across the two countries. Its pooled adjusted hazard ratio was 2.81 (95% CI 1.67–4.75), corresponding to an incidence-rate difference of approximately 1.41 additional cases per 10,000 person-years.

These findings contributed to regulatory scrutiny.

In June 2025, the European Medicines Agency's Pharmacovigilance Risk Assessment Committee (PRAC) concluded that NAION should be classified as a very rare adverse effect of semaglutide, potentially affecting up to 1 in 10,000 people taking the medicine.

WHO subsequently issued a medical product alert specifically concerning semaglutide medicines, citing the EMA assessment and noting that its own Advisory Committee on Safety of Medicinal Products had recommended that NAION be included as a potential risk in semaglutide's risk-management plan.

The UK's Medicines and Healthcare products Regulatory Agency (MHRA) also updated semaglutide product information in February 2026 and advised urgent ophthalmological assessment following sudden or rapidly worsening visual loss.

These regulatory conclusions concern semaglutide. They should not be presented as proof that tirzepatide carries the same established risk.

For Tirzepatide, the Evidence Is Much More Conflicted

One study that received attention analysed 1,511,637 eligible patients with type 2 diabetes, ultimately producing 159,398 propensity-score-matched patients. The study compared people prescribed semaglutide or tirzepatide with matched patients receiving other diabetes medicines.

During two years of follow-up, NAION occurred in 35 patients (0.04%) in the semaglutide/tirzepatide group and 19 patients (0.02%) in the comparison group, producing a hazard ratio of 1.76 (95% CI 1.01–3.07).

But there is an important limitation:

The study combined semaglutide and tirzepatide.

It therefore cannot establish that Mounjaro or tirzepatide independently caused the observed association.

A Pharmacovigilance Study Found a Tirzepatide Signal — But That Is Not Proof of Causation

A 2026 pharmacovigilance study analysed reports in the US Food and Drug Administration Adverse Event Reporting System (FAERS) from January 2022 through June 2025.

Researchers identified 28 cases of ischaemic optic neuropathy in which tirzepatide was the primary suspect drug. Their disproportionality analysis produced a reporting odds ratio of 2.599 (95% CI 1.778–3.799).

This represents a safety signal worthy of investigation, not an incidence estimate and not proof that tirzepatide caused the events.

Spontaneous adverse-event reporting systems are designed to identify potential safety signals. They generally cannot establish the denominator of all people exposed to a medicine and cannot, by themselves, establish causality.

The FDA has similarly listed Mounjaro, Zepbound and other GLP-1 receptor agonists under a potential signal for NAION and stated that it was evaluating whether regulatory action was necessary.

The FDA Is Still Investigating the Question

The FDA's Sentinel Initiative has a dedicated study examining the risk of NAION following GLP-1 receptor agonist use in people with type 2 diabetes.

The study is listed as “In progress” and was designed specifically because earlier observational studies had produced conflicting results. The planned analysis uses real-world claims data and additional electronic-health-record linkages to investigate the potential causal relationship.

This is significant because it demonstrates that the US regulatory question remains open.

The FDA-hosted Mounjaro prescribing information revised in December 2025 does not list NAION among Mounjaro's warnings, precautions or established adverse reactions.

Therefore, as of 23 September 2026, the US position should be described as ongoing safety evaluation, rather than a regulatory determination that Mounjaro causes NAION.

Australia Reached a Particularly Important Conclusion

Australia provides one of the clearest illustrations of the difference between a safety signal and established causality.

In July 2026, the TGA announced class-wide safety-information updates concerning the potential for GLP-1 medicines to cause NAION and included Mounjaro among the medicines marketed in Australia.

The TGA reported 36 cases of optic ischaemic neuropathy in its adverse-event database:

23 involving semaglutide

3 involving liraglutide

10 involving tirzepatide.

But the regulatory interpretation is more nuanced.

The TGA said its independent assessment found the available evidence conflicting. Its Advisory Committee on Medicines concluded that the evidence may support a signal for semaglutide, but not for tirzepatide.

This means that the existence of reported tirzepatide cases did not lead the Australian expert committee to conclude that the evidence established a tirzepatide-specific association.

New Zealand Has Now Added Another Piece to the Regulatory Picture

New Zealand's Medsafe had previously placed GLP-1 receptor agonists, including tirzepatide, under monitoring for acute persistent visual loss and explicitly stated that a monitoring communication does not mean that the medicine causes the adverse event.

The regulatory position has since evolved.

Medsafe's September 2026 Prescriber Update reports a Mounjaro data-sheet update listing non-arteritic ischaemic optic neuropathy (NAION) under section 4.8, adverse effects.

The current Mounjaro data sheet available through Medsafe was revised in August 2026.

This is important evidence of an evolving international regulatory response, but a product-information listing is not equivalent to a declaration that causality has been definitively established.

A New Study Published on 22 September Adds More Uncertainty

Just one day before this report, researchers published a large retrospective cohort study in Diabetology & Metabolic Syndrome examining ocular outcomes among people initiating tirzepatide.

The study used the TriNetX global federated health research network and included adults with type 2 diabetes or overweight/obesity. Patients initiating tirzepatide were propensity-score matched with users of GLP-1 receptor agonists and conventional therapies.

Among people with type 2 diabetes, the one-year composite incidence of visual impairment, diabetic retinopathy or other retinal disorders was 3.5% among both tirzepatide and other GLP-1 receptor agonist users, with a hazard ratio of 1.01 (95% CI 0.88–1.15).

Among people with overweight or obesity without diabetes, the composite incidence was 1.9% with tirzepatide versus 2.1% with other GLP-1 receptor agonists, with a hazard ratio of 0.92 (95% CI 0.81–1.03).

The researchers concluded that tirzepatide was not associated with increased ophthalmic risk over one year, while cautioning that longer follow-up is required.

This study did not establish that tirzepatide protects against NAION; rather, it provides additional evidence against interpreting the current safety signal as proof of a broad increase in ophthalmic risk.

Another Large Study Found Lower NAION Risk With Tirzepatide

A separate 2026 study in Ophthalmology Retina compared 102,590 tirzepatide users with 102,590 users of non-GIP GLP-1 receptor agonists after propensity-score matching.

Over a 36-month follow-up, tirzepatide was associated with a lower observed risk of NAION, with a hazard ratio of 0.45 (95% CI 0.27–0.86). It also found lower observed risks for diabetic retinopathy, diabetic macular oedema, vitreous haemorrhage/retinal detachment and diabetic-retinopathy-related vision-saving interventions.

The authors did not claim that tirzepatide prevents NAION. They concluded that prospective research is required to confirm the findings.

This finding sits in direct tension with the pharmacovigilance signal and some litigation allegations.

That is precisely why the evidence cannot responsibly be reduced to “Mounjaro causes blindness”.

A Clinical Case Report Adds a Signal, Not a Verdict

In June 2026, Case Reports in Ophthalmology published a report of acute unilateral NAION following tirzepatide therapy.

The report described an obese patient who developed progressive, painless visual loss after initiating tirzepatide for weight reduction. The patient had started treatment at 15 mg once weekly without standard dose titration, according to the case report.

A single case report can be important for pharmacovigilance because it can generate or strengthen a safety hypothesis. But it cannot establish population-level incidence or causality.

The Scientific Literature Does Not Speak With One Voice

A 2026 review in Clinical & Experimental Ophthalmology assessed the evidence surrounding GLP-1 receptor agonists and NAION.

The authors concluded that case reports and pharmacovigilance data provide potential signals but are susceptible to confounding and reporting bias. They also noted that observational studies have produced mixed findings, with some Scandinavian studies reporting increased risks while large US and multinational datasets have produced null or weaker associations.

A separate 2026 systematic review and meta-analysis of 28 observational studies found no statistically significant increase in NAION with GLP-1 receptor agonists compared with other diabetes treatments, reporting a pooled relative risk of 1.01 (95% CI 0.62–1.64), although substantial heterogeneity was present.

Taken together, these findings mean that an association has been reported, but causation remains unresolved — particularly for tirzepatide.

What Lilly Says

Eli Lilly has acknowledged the litigation and says it continuously evaluates safety information from clinical trials and post-marketing experience.

In Lilly's medical information material on Mounjaro, the company states that NAION is not listed as an adverse event in the tirzepatide product information and acknowledges publications examining possible NAION risks with semaglutide and tirzepatide. Lilly says it continues to monitor tirzepatide safety data and will evaluate product labelling with regulators if a new potential risk is identified.

Lilly's corporate filing separately confirms that it is defending product-liability litigation involving Mounjaro, Trulicity and Zepbound.

The company's position should be distinguished from the allegations made by plaintiffs in the litigation.

What the Evidence Actually Establishes as of 23 September 2026

Are there federal lawsuits involving GLP-1 medicines and NAION?
Yes.

Is Mounjaro included?
Yes.

Is Eli Lilly a defendant?
Yes.

Are there 216 Mounjaro lawsuits?
No. The 216 figure represents the total pending actions in MDL 3163 across the medicines covered by the litigation.

Has a US court ruled that Mounjaro causes NAION?
No.

Has FDA established that Mounjaro causes NAION?
No. FDA is still evaluating the safety signal.

Has EMA established NAION as a very rare adverse effect of semaglutide?
Yes.

Has EMA made the same determination for tirzepatide?
Not identified in its semaglutide determination.

Did Australia's expert committee support a tirzepatide NAION signal?
No. It said the evidence may support the signal for semaglutide, but not tirzepatide.

Has New Zealand added NAION to Mounjaro safety information?
Yes, in its September 2026 data-sheet update.

Is there evidence of a possible tirzepatide signal?
Yes, including pharmacovigilance reports and case reports.

Is there evidence against an increased tirzepatide risk?
Yes, including large observational studies.

Is causality established for Mounjaro?
No.

Why the Distinction Matters for Patients and Pharmacists

The current evidence does not support telling patients that Mounjaro causes blindness. Equally, the existence of conflicting evidence does not justify dismissing reports of sudden visual loss.

The appropriate safety message is more precise: NAION is a serious and potentially permanent condition under investigation in relation to GLP-1 medicines, with the strongest regulatory evidence currently concerning semaglutide. The evidence specific to tirzepatide remains unsettled.

For pharmacists and other healthcare professionals, the key issue is therefore pharmacovigilance rather than panic. Sudden or rapidly worsening vision in a person taking a medicine under investigation for a potential ocular adverse effect warrants prompt clinical assessment. The MHRA specifically advises urgent ophthalmological assessment for sudden visual loss in patients taking semaglutide, while WHO similarly advises immediate medical attention for sudden or rapidly worsening eyesight.

Patients should not interpret the existence of litigation as proof that their medicine is responsible for an individual symptom. At the same time, suspected adverse drug reactions should be appropriately evaluated and reported through national pharmacovigilance systems.

The Bigger Question: Can a Lawsuit Move Faster Than the Science?

The Mounjaro-NAION controversy illustrates a recurring challenge in medicine safety.

A case report can generate a signal. A spontaneous-reporting database can strengthen that signal. An observational study can identify an association. Regulatory review can determine whether warnings are warranted. But establishing causation generally requires a much stronger body of evidence.

That hierarchy is particularly important for rare events such as NAION, where randomised trials may contain too few cases to provide precise estimates and observational studies can be affected by underlying disease, differences between treatment groups and other confounding factors.

The US court is now explicitly moving towards this scientific question. Its July order prioritises general causation — whether the medicines are capable of causing NAION — before the litigation progresses further.

Meanwhile, regulators continue to monitor the evidence, and researchers continue to produce new data.

That means the most accurate conclusion on 23 September 2026 is neither “Mounjaro causes blindness” nor “there is nothing to investigate”.

The evidence supports a more careful conclusion:

There is a credible safety question surrounding GLP-1 medicines and NAION, but the strength of evidence varies substantially by medicine. Semaglutide has the clearest regulatory recognition of a very rare NAION risk, while the evidence concerning tirzepatide remains conflicting. Mounjaro is therefore appropriately under continued pharmacovigilance and scientific scrutiny, but causation has not been established.

That is where the evidence stands today

Evidence Base Used for This Report

1. Federal court record

US District Court for the Eastern District of Pennsylvania. In re: Glucagon-like Peptide-1 Receptor Agonists (GLP-1 RAs) Non-Arteritic Anterior Ischemic Optic Neuropathy Products Liability Litigation, MDL No. 3163, Case Management Order No. 12, 2 July 2026.
Primary evidence for the drugs involved, allegations, number of cases at the time, general causation proceedings and litigation timetable.

2. US Judicial Panel on Multidistrict Litigation

US Judicial Panel on Multidistrict Litigation. Pending MDL Dockets by Actions Pending, report dated 1 September 2026.
Primary evidence for the 216 pending actions in MDL 3163.

3. US FDA

US Food and Drug Administration. New Safety Information or Potential Signals of Serious Risks Identified from the FDA Adverse Event Monitoring System, October–December 2024.
Lists Mounjaro, Zepbound, Ozempic, Wegovy and other GLP-1 medicines under the potential NAION signal and states that FDA was evaluating the need for regulatory action.

4. FDA Sentinel Initiative

US FDA Sentinel Initiative. Risk of Non-arteritic Anterior Ischemic Optic Neuropathy Following GLP-1 Receptor Agonist Use in Patients with Type 2 Diabetes Mellitus.
Ongoing FDA Sentinel investigation designed to evaluate the potential association.

5. FDA-approved Mounjaro prescribing information

US Food and Drug Administration. Mounjaro (tirzepatide) Prescribing Information, revised December 2025.
Used to verify the current US product information and absence of NAION from the listed warnings/adverse reactions.

6. European Medicines Agency

European Medicines Agency, Pharmacovigilance Risk Assessment Committee. PRAC concludes eye condition NAION is a very rare side effect of semaglutide medicines Ozempic, Rybelsus and Wegovy, 6 June 2025.
Primary regulatory evidence for the semaglutide-specific conclusion.

7. World Health Organization

WHO. The use of semaglutide medicines and risk of non-arteritic anterior ischemic optic neuropathy (NAION), Medical Product Alert, 27 June 2025.
Primary international safety communication on semaglutide and NAION.

8. Australian Therapeutic Goods Administration

Therapeutic Goods Administration. GLP-1 RAs and rare vision disorder, 23 July 2026.
Primary evidence for the Australian adverse-event figures and the expert assessment that supported the signal for semaglutide but not tirzepatide.

9. New Zealand Medsafe

Medsafe. Recent data-sheet updates: Important new safety information, September 2026.
Primary evidence for the September 2026 Mounjaro data-sheet update listing NAION under adverse effects.

10. Wang et al., JAMA Network Open

Wang L, Volkow ND, Kaelber DC, Xu R. Semaglutide or Tirzepatide and Optic Nerve and Visual Pathway Disorders in Type 2 Diabetes. JAMA Network Open. 2025;8:e2526327.
Evidence from 159,398 matched patients; important because semaglutide and tirzepatide were analysed together.

11. Grauslund et al., International Journal of Retina and Vitreous

Grauslund J, Taha AA, Molander LD, et al. Once-weekly semaglutide doubles the five-year risk of nonarteritic anterior ischemic optic neuropathy in a Danish cohort of 424,152 persons with type 2 diabetes. International Journal of Retina and Vitreous. 2024;10:97.

12. Simonsen et al., Diabetes, Obesity and Metabolism

Simonsen E, Lund LC, Ernst MT, et al. Use of semaglutide and risk of non-arteritic anterior ischemic optic neuropathy: A Danish–Norwegian cohort study. Diabetes, Obesity and Metabolism. 2025;27:3094–3103.

13. Ophthalmology Retina — tirzepatide cohort

Ocular Outcomes with Tirzepatide versus Glucagon-like Peptide-1 Receptor Agonists in Type 2 Diabetes. Ophthalmology Retina. 2026;10(6):625–632.
Evidence from 102,590 matched tirzepatide users and 102,590 non-GIP GLP-1 RA users; reported HR 0.45 for NAION.

14. Truong et al., Diabetology & Metabolic Syndrome

Truong NNK, Liao CT, Chen CW, et al. Incident ocular outcomes with tirzepatide versus GLP-1 receptor agonists and conventional therapies in diabetes and obesity: a retrospective cohort study from a global federated health research network. Diabetology & Metabolic Syndrome. Published 22 September 2026. DOI: 10.1186/s13098-026-02299-6.

15. Tirzepatide pharmacovigilance study

Exploring the Potential Link Between Tirzepatide and Non-Arteritic Anterior Ischemic Optic Neuropathy (NAION): Evidence from FAERS and Google Trends. 2026.
Identified 28 ischaemic optic neuropathy reports with tirzepatide as the primary suspect and a statistically significant disproportionality signal.

16. Tirzepatide case report

Valášková J, Bondarchuk D, Kadlic P, et al. Acute Unilateral NAION following Tirzepatide Therapy: A Case Report. Case Reports in Ophthalmology. 2026;17:806–813. DOI: 10.1159/000552569.

17. Clinical evidence review

Danesh-Meyer HV, Rizzo JF III. Is There a Causal Link Between GLP-1 Receptor Agonists and Non-Arteritic Anterior Ischaemic Optic Neuropathy? A Critique of the Clinical Evidence. Clinical & Experimental Ophthalmology. 2026;54:554–564. DOI: 10.1111/ceo.70096.

18. Systematic review and meta-analysis

Ocular disorders during treatment with GLP-1 receptor agonists: a systematic review and meta-analysis of observational studies. Frontiers in Pharmacology. 2026.
Included 28 observational studies and found no statistically significant pooled increase in NAION, while reporting substantial heterogeneity.

19. Eli Lilly regulatory filing

Eli Lilly and Company. 2026 Form 10-Q, reporting product-liability litigation involving Mounjaro, Trulicity and Zepbound and the federal MDL proceedings.

20. Eli Lilly medical information

Eli Lilly and Company. Mounjaro/tirzepatide medical information on NAION. Lilly states that NAION is not listed as an adverse event in the cited Mounjaro product information and that it continues to monitor safety data.