Patients taking Wegovy have nearly five times the risk of sudden sight loss of those on Ozempic, a large-scale study has found.

Glucagon-like peptide-1 receptor agonist (GLP-1 RA) medicines such as semaglutide (sold as Wegovy, Ozempic and Rybelsus) and tirzepetide (sold as Mounjaro) help reduce blood sugar levels, slow digestion and reduce appetite, and have been linked to reduced risks of heart attack, fewer drug overdoses and other health benefits.

But a study, published in the British Journal of Ophthalmology, found that patients taking Wegovy for weight loss had a fivefold greater chance of developing non-arteritic anterior ischemic optic neuropathy (Naion) than the diabetes drug Ozempic, while men had a three times greater risk than women.

These “eye strokes” cause sudden and usually permanent vision loss (pdf) due to reduced blood flow to the optic nerve.

Although rare, affecting around one in 10,000 people who take semaglutide, there was “a potential dose-dependent safety concern” for the drug, the study found.

Wegovy, Ozempic and Rybelsus – made by Novo Nordisk – all contain semaglutide, but have different dosages and formulations.

Dr Edward Margolin, from the department of ophthalmology at the University of Toronto and one of the authors of the research, said Naion was likely to be “a real side-effect” of semaglutide, and faster, quicker or more aggressive weight loss would be likely to “increase the risk” of Naion.

The study examined reports of side-effects submitted to the US medicines regulator, the Food and Drug Administration, through its adverse event reporting system between December 2017 and December 2024.

Canadian researchers compared any reports of Naion associated with up to 2mg of weekly injectable Ozempic for type 2 diabetes; up to 2.4mg of weekly injectable Wegovy for obesity – the highest approved dose – and a daily Rybelsus tablet for type 2 diabetes. They also examined reported side-effects from tirzepatide (Mounjaro).

The study found that Wegovy had the strongest association with sudden vision loss. In contrast, the authors found no increased risk with Rybelsus tablets or tirzepatide.

The authors said that the high doses of Wegovy and the fact injections were faster-acting could explain why the association was higher. In contrast, the limited absorption and slower uptake of Rybelsus tablets probably explained the absence of a detectable link, they said.

The findings came after the UK Medicines and Healthcare products Regulatory Agency issued a drug safety update in February, warning about the risk of Naion. It followed similar warnings by the European medicines regulator.

Dr Alison Cave, the MHRA chief safety officer, said: “The risk of Naion in patients prescribed semaglutide is extremely low. However, as with all medicines, patients and prescribers need to be aware of the symptoms of potential side-effects, even if the risk is very small, to ensure patients receive the appropriate treatment promptly should they occur.”

Samantha Mann, a consultant ophthalmologist and diabetic eye screening lead at the Royal College of Ophthalmologists, said: “This study relied on reported side-effects and so cannot prove causation or determine how common this problem truly is. An increase in this form of optic nerve ‘stroke’ has however not been widely observed in routine clinical practice at St Thomas’ in London, where I work. Further studies are therefore needed to clarify if this increased risk is indeed real.”

A spokesperson for Novo Nordisk said: “Patient safety is our top priority, and we take any reports about adverse events from the use of our medicines very seriously. We work closely with authorities and regulatory bodies from around the world to continuously monitor the safety profile of our products.”

The EU patient leaflets for Wegovy, Ozempic and Rybelsus had been updated to include Naion, they added, but “based on the totality of evidence, we concluded that the data did not suggest a reasonable possibility of a causal relationship between semaglutide and Naion and Novo Nordisk believes that the benefit-risk profile of semaglutide remains favourable”.