Michelle Fennell was sitting at her desk when her right eye started feeling heavy. Suddenly, it was blurry and her peripheral vision waned. It felt like she was looking through a fishbowl, she could see straight ahead but not to her right or down.

Two weeks later, a doctor diagnosed Fennell, 53 years old, with nonarteritic anterior ischemic optic neuropathy, or NAION, a condition that leads to sudden, often permanent vision loss.

The doctor asked what medicines she was taking. "He said right away, stop taking Ozempic," to protect her remaining vision, said Fennell, who started taking the drug to treat her Type 2 diabetes six months prior. "I was shocked."

Some emerging scientific research points to an increased risk for patients taking certain GLP-1s to develop NAION. But the evidence is mixed, and not clear enough to say whether or how the drugs cause the condition, doctors and researchers said. Fennell is one of dozens of people suing Novo, maker of the popular weight-loss and diabetes drugs Wegovy and Ozempic, claiming the medication caused this side effect. There are also a similar but smaller number of lawsuits against Eli Lilly, maker of Zepbound. Both companies are fighting the litigation, saying there isn't enough evidence to support the idea that drugs are to blame.

Still, some countries have compelled Novo and Eli Lilly to include warnings or information about NAION on the drugs' labels. There is no such warning in the U.S., though the Food and Drug Administration is investigating the potential link after first identifying a possible connection to NAION in late 2024.

An FDA spokesperson said the agency will decide whether to take action after its assessment, but didn't say when that might be.

"FDA is usually pretty conservative in terms of what warnings are worth mentioning in the label," said Dr. Aaron S. Kesselheim, a Harvard Medical School professor who studies FDA regulation. "They would want there to be a credible association that they believe provides some basis for saying that there's causation," meaning evidence that the drug does, in fact, cause the side effect.

While some evidence suggests semaglutide could roughly double patients' risk, that would still lead to very few losing vision, doctors said. Even at that elevated rate, it would mean two per every 10,000 people taking the drug on average would develop the condition.

A spokesperson for Novo said the personal-injury lawsuits are without merit and that the company is vigorously defending against them. A large, company-funded study analyzed randomized-controlled trials of patients on the drugmaker's GLP-1s and found the drugs didn't increase patients' risk of developing the condition.

"There are no indications that we should really be worried about NAION," said Dr. Tina Vilsbøll, senior medical officer at Novo and lead author on the study. "I do not see causality." Eli Lilly said patient safety is the company's top priority, and they will "continue to review data, including any data regarding ophthalmic issues." The company didn't comment specifically on the litigation but in a court filing, Lilly said studies don't prove any GLP-1s cause NAION, and said its medicines are safe and effective.

While Novo's and some other studies found no increased risk, another from a specialized eye center reported nearly an eightfold increase. Researchers reach different conclusions in part because of different methods they use to classify NAION, which lacks its own specific diagnosis code. The link is also difficult to study because many patients have other health conditions that independently raise their risk of NAION, like diabetes, heart disease, high blood pressure, or sleep apnea. The condition is caused by decreased blood flow to the optic nerve.

Doctors should help patients assess their own risks and benefits, said Dr. John J. Chen, a doctor who specializes in diseases of the optic nerve and a professor at Mayo Clinic. For patients who need Ozempic to control their diabetes or heart disease, the benefits probably outweigh the risk, he said.

The calculations are different for people who don't have the same obesity-related risk factors. "If they want to lose 5 pounds, that's probably not worth the possible risk," he added. "If you're that one patient with NAION, that's devastating." In the U.S., personal-injury firm Weitz & Luxenberg has filed more than 90 lawsuits since 2025 in state court in New Jersey on behalf of patients, including Fennell, who developed NAION while taking Ozempic or Wegovy. They are seeking compensation from Novo. NAION usually affects one eye, but sometimes both, and there is no approved treatment for it.

A separate group of lawsuits filed in federal district court in Philadelphia around the same time also allege that Novo and Lilly's GLP-1 drugs caused NAION, and that the companies failed to adequately warn patients. The federal and state cases are pending.

In June 2025, the European drug regulator, the European Medicines Agency, concluded that NAION is a very rare side effect of semaglutide, the main ingredient in Ozempic and Wegovy, and recommended adding a warning to the European label for the Novo drugs. The medications' labels also carry warnings or information about NAION in the U.K., Japan and Australia. In Australia and New Zealand, the information appears on other GLP-1 labels, too, including Eli Lilly's Mounjaro.

In Denmark, where Novo is based, 27 patients have been awarded upward of $1.5 million in compensation for NAION that they developed after taking Ozempic or Wegovy. An additional 38 cases are pending, according to the independent authority Danish Patient Compensation, which evaluates medical claims that are paid out by the Danish government for patients who experience rare and serious side effects from drugs. Novo said it isn't involved in those cases.

Novo's Dr. Vilsbøll said the payouts aren't an indicator of causality.

Jay Vandegriff, a 69-year-old plaintiff in one of the U.S. lawsuits, started taking Ozempic to treat his Type 2 diabetes in November 2023. On Christmas the following year, he noticed what he thought was a smudge on his glasses. He cleaned them and realized that the mark, which looked like it was from a pencil eraser, was in his own field of vision. He was diagnosed with NAION, which eventually claimed two-thirds of his vision in his left eye.

Vandegriff, a software startup founder, has to stop working on his computer by noon each day because his right eye tires out. An avid skier, he can no longer go out in fog or snowstorms. He gave up fly fishing altogether because of difficulty tying a fly to his line.

"It's affecting every minute of every day," he said of the vision loss. He wishes he had known about the possible risk. "I'd almost pay anything to get my eyesight back."

For those that do develop the condition, it often appears within the first six to 18 months after starting a GLP-1, according to study findings.

In 2025, the American Optometric Association recommended that any patient starting a GLP-1 get an eye exam to check their optic nerve health. Having a physically small or crowded optic nerve, also called a "disc at risk," is the single strongest anatomical risk factor for developing NAION, optometrists and neuro-ophthalmologists said.

If a patient has a disc that "does look scary, then that's a conversation," said Dr. Teri Geist, an optometrist and president of the association. "But I don't think every person being on a GLP-1 needs to worry about going blind."