The US Food and Drug Administration (FDA) told makers of popular weight-loss GLP-1 drugs that they must add warning labels about a rare eye condition that can cause sudden blindness.
Manufacturers of Ozempic, Wegovy, Mounjaro and Zepbound must warn about nonarteritic anterior ischemic optic neuropathy (NAION). The disorder happens when vessels supplying blood to the optic nerves become blocked, depriving the nerves of oxygen.
The condition can cause painless decreased vision in one eye, which can be permanent, the FDA said.
In a statement to the BBC, the American Academy of Ophthalmology said "any added risk appears small, and it has not been established that these medications cause NAION".
"GLP-1 medications offer important benefits for people with diabetes, obesity, and heart disease, so our message to patients is simple," the group said. "Don't stop your medication without talking to your doctor, and if you notice sudden, painless vision loss in one eye, see an ophthalmologist promptly."
There is no known treatment to reverse vision loss caused by NAION. It is the most common cause of acute optic nerve injury in people over 50 years old, according to the academy.
The FDA said it conducted a detailed review of reports of NAION in GLP-1 users as well as a review of other studies and concluded there is an "increased risk of rare NAION events" in people taking the drugs.
In a statement, Novo Nordisk, the manufacturer of Wegovy, one of the most popular weight-loss medications, said the company believes the data "do not establish a cause-and-effect relationship".
"We recognize that NAION can have a profound impact on patients and their families. Patient safety is a top priority for Novo, and we continue to monitor the safety of our medicines," the manufacturer said.
Some research was challenging to review because people taking GLP-1 drugs for obesity and type 2 diabetes may already have risk factors for NAION separate from the drugs, according to the regulator. The FDA said it was not known why those taking GLP-1 drugs may be more at risk for the eye condition.
The drugs already have warning labels for several other conditions including pancreatitis and gallbladder disease. Some of the most common symptoms for semaglutide - a key ingredient in many GLP-1 drugs - include nausea, vomiting and stomach pain.
Last year, a safety arm of the European Medicines Agency, a part of the European Union, said up to 1 in 10,000 people taking semaglutide may be affected by NAION.
In a study published two years ago analysing 17,000 patients, US researchers found those taking semaglutide were four times more likely to be diagnosed with NAION than those not taking the drug.
But one of the authors of the study, Harvard Medical School ophthalmology professor Dr Joseph Rizzo, said the findings were "significant but tentative", and more research was needed on the relationship between the drug and the eye condition in a more diverse population.