People with type 2 diabetes taking GLP-1 medicines may experience potentially blinding eye conditions as soon as the day after starting the drugs, and eye doctors are now asking their patients to be vigilant in speaking up about any vision changes while taking them.

A new study in JAMA Ophthalmology highlights the importance for people with type 2 diabetes who take GLP-1 medicines to contact their health care provider immediately if they experience any vision changes because of the heightened risk for problems such as swelling of the optic nerve, a problem known as an “eye stroke.”

The findings suggest that rapid correction of high blood sugar may be linked to the onset of the new vision problems, the study authors concluded. The new research summarized vision problems of nine people with type 2 diabetes taking either semaglutide or tirzepatide, which are GLP-1 medicines. Semaglutide is known by the brand names Ozempic and Wegovy, and tirzepatide is known by the brand names Mounjaro and Zepbound. Seven people experienced nonarteritic anterior ischemic optic neuropathy (NAION, or eye stroke), one person had papillitis (optic nerve inflammation), and one person had paracentral acute middle maculopathy, which can cause blind spots.

“This review was not conducted in any way that we can say these drugs caused the complications,” said lead author Bradley J. Katz, MD, PhD, a neuro-ophthalmologist at University of Utah Health, in a news release. “Further studies are needed to test our hypothesis. However, this is an important issue for ophthalmologists as we monitor usage of these drugs and how to best be in communication with our patients about them.”

NAION was the most common condition described in the study, and it typically occurs in patients in their 50s and 60s but can occur when people are younger, Katz said during a podcast interview published alongside the paper. People are generally considered at increased risk for NAION if they have high blood pressure or high cholesterol, have diabetes, or if they smoke, he said.

NAION usually occurs in one eye, Katz said, and is typically noticed upon waking. It’s painless, but eye doctors examining people with the condition can see optic nerve swelling, which normally resolves in six weeks. The symptoms can range from mild to severe. “Most patients lose about half of their peripheral vision in a typical situation,” said Katz, who personally saw a patient who had sudden, painless vision loss after using semaglutide. The patient stopped taking the medicine but later restarted it to help control her blood sugar and experienced vision loss in the other eye the next day.

Other people with NAION whose cases were described in the research paper experienced symptoms of the eye condition between three months and 16 months after starting a GLP-1 medicine.