The US Food and Drug Administration has told patients and prescribers that GLP-1 drugs used for weight loss and diabetes carry a potential increased risk of a serious eye condition, and is requiring the drugs to carry new label language about it.

The drugs covered include Eli Lilly's Mounjaro and Zepbound and Novo Nordisk's Ozempic and Wegovy. The addition goes into the "warnings and precautions" section of the labeling and names non-arteritic anterior ischemic optic neuropathy, or NAION, a condition the FDA describes as a common cause of sudden, painless vision loss that can leave permanent sight impairment in some cases.

What NAION actually is

NAION is caused by decreased blood flow to the front part of the optic nerve. The underlying cause is not completely understood, which is worth holding onto: a label warning about a mechanism nobody has fully mapped is a flag, not a verdict.

The FDA said it reached the decision after evaluating clinical trial data, animal data, and post-marketing data including observational studies and case reports. That is a broader evidence base than a single signal, and it is also why the agency framed this as a potential increased risk rather than an established cause.

The numbers behind the warning

The strongest figure in the file is not the FDA's. A large study published in 2025 tracked nearly 175,000 diabetic patients treated with semaglutide against an equal number taking non-GLP-1 medications, and found the risk of NAION more than doubled after two years of treatment compared with diabetes drugs from other classes.

Europe got there first. Last year the European Medicines Agency's safety committee said NAION may affect up to 1 in 10,000 people taking semaglutide for at least one year, the first time a regulator confirmed the side effect. Four patients in Denmark who experienced vision loss after using Novo's GLP-1 drugs were granted compensation, according to the Danish Patient Compensation.

What the manufacturers say

Novo says it continues to believe the data do not establish a cause-and-effect relationship and remains confident in the benefit-risk profile of its GLP-1 medicines. A Lilly spokesperson said the FDA's safety communication applies to all medicines with GLP-1 receptor agonist activity and does not reference findings specific to Lilly products, and that the company is evaluating the update.

Bottom line

Citi analyst Geoff Meacham expects the action to raise patient and investor concern temporarily, but argued the rarity of NAION alongside the weight, glycemic, cardiovascular, renal and sleep apnea benefits should preserve a favorable benefit-risk profile and limit changes to prescribing. What remains unsettled is causation, and on that point the regulator and the manufacturers are still some distance apart.