FDA warns GLP-1 drugs may cause sudden vision loss

The Food and Drug Administration is requiring new safety warnings on all GLP-1 drugs after finding evidence of a potential increased risk of a rare eye condition that can cause sudden vision loss, CNBC reported.

The condition, called nonarteritic anterior ischemic optic neuropathy, or NAION, is caused by decreased blood flow to the front part of the optic nerve. It can result in permanent vision loss. The FDA said a causal relationship has not been established, but cases have been reported for all currently marketed GLP-1 products.

The agency’s review analyzed 126 clinical trials involving more than 134,000 patients. It found 6.4 cases of NAION per 100,000 patient-years among GLP-1 users, compared with 3.8 among patients receiving other treatments or placebos. The FDA identified 15 probable NAION events, 10 in patients treated with GLP-1s and five in patients receiving a placebo.

Drugmakers push back

Novo Nordisk, which makes Ozempic and Wegovy, said it “continues to believe the data do not establish a cause-and-effect relationship, and we remain confident in the benefit-risk profile of our GLP-1 medicines.”

Eli Lilly, the company behind Mounjaro, Zepbound, and Foundayo, said it is “carefully evaluating the update and will respond to FDA promptly.” Patient safety is Lilly’s “top priority,” the drugmaker added.

The companies have reason to resist. GLP-1 drugs are the biggest-selling class of pharmaceuticals in the world, generating tens of billions of dollars in annual revenue. A safety warning that dents prescribing could cost both companies billions, and the litigation that follows could cost more.

The defense rests on a confounder: diabetes itself is a known risk factor for NAION. Patients taking GLP-1 drugs for diabetes may develop the eye condition because of their underlying disease, not because of the drug. Separating the two causes is difficult, and the FDA acknowledged that the evidence is mixed when comparing GLP-1 users to patients taking other diabetes medications.

European precedent

European regulators flagged the risk first. The European Medicines Agency’s safety committee concluded in June 2025 that NAION was a very rare side effect of semaglutide, the active ingredient in Ozempic and Wegovy, that may affect up to 1 in 10,000 people taking it. The U.K.’s medicines regulator followed with a similar warning in February.

The European and British warnings focused specifically on semaglutide. The FDA’s action is broader, covering all GLP-1 medications, including Eli Lilly’s tirzepatide-based products. The wider scope increases the exposure for Lilly, which has been gaining market share on Novo Nordisk.

The FDA decision follows a review of clinical trials, real-world patient data, and published studies. The agency found a statistically significant increase in NAION risk among GLP-1 users compared with patients taking DPP-4 inhibitors, another class of diabetes medications, but not compared with those taking SGLT-2 inhibitors.

Existing warnings

GLP-1 drugs already carry the FDA’s strongest safety warning, a boxed warning, over a potential risk of thyroid tumors. Studies have shown that semaglutide and tirzepatide cause thyroid C-cell tumors in rodents, though it remains unknown whether the drugs cause these tumors in humans.

Both drugs also carry warnings about diabetic retinopathy, another eye condition that can lead to blurred vision and sight loss. In a two-year trial, diabetic retinopathy complications occurred in 3% of patients treated with Ozempic, compared with 1.8% of those receiving a placebo.

The NAION warning adds to a growing list of safety signals around GLP-1 drugs, including pancreatitis, gallbladder problems, and muscle loss. None has dented prescribing so far, because the weight loss and cardiovascular benefits are large and the side effects are rare.

What analysts say

Citi analysts said the FDA’s labeling action is “likely to attract attention but should have limited impact on GLP-1 uptake.” The rarity of NAION and the weight, blood sugar, cardiovascular, renal, and sleep apnea benefits should preserve a favorable benefit-risk profile and limit changes to prescribing, they wrote.

The analysis may prove correct. GLP-1 drugs have survived previous safety warnings without a measurable drop in demand, because patients and doctors judge the benefits to outweigh the risks. A rare eye condition with no proven causal link is unlikely to change that calculation for most patients.

But the warning adds to the legal exposure. If patients who develop NAION after taking GLP-1 drugs sue, the FDA warning gives their lawyers a document to point to. Novo Nordisk and Eli Lilly face the prospect of years of litigation, even if the science does not prove causation.

What I think

The FDA made the right call. The agency found a signal in 126 trials and 134,000 patients, and the signal is consistent with observational data. A warning label does not ban the drug; it gives doctors and patients the information to make a choice. Six million Americans are taking these drugs, and they deserve to know about a potential risk to their vision, even if the risk is rare and unproven.

The drugmakers’ response is predictable and not fully reassuring. Novo Nordisk’s statement that the data “do not establish a cause-and-effect relationship” is technically correct but sidesteps the question. The FDA did not say the drugs cause NAION; it said the evidence warrants a warning. Those are different claims, and Novo Nordisk is arguing against the second by disputing the first.

The comparison to diabetic retinopathy is worth noting. GLP-1 drugs already carry a warning for an eye condition that affects 3% of patients in trials. NAION is far rarer, but it causes sudden, permanent vision loss, which is a more severe outcome than the progressive damage of retinopathy. The severity matters more than the frequency when the FDA decides whether to warn.

The pharmaceutical industry has a history of resisting safety warnings until the evidence becomes overwhelming. Vioxx is the cautionary tale: Merck disputed cardiovascular risk signals for years before pulling the drug, and the delay cost lives and billions in settlements. Novo Nordisk and Eli Lilly are not at that stage with GLP-1 drugs, but the pattern of disputing every warning is the pattern that gets companies into trouble.

The prescribing impact will probably be small, as Citi predicts. But the litigation impact could be large, and that is the real risk for the drugmakers. A warning label is a gift to plaintiffs’ lawyers, and the first NAION lawsuits are likely already being drafted.