Novo Nordisk's popular weight-loss and diabetes drugs Wegovy and Ozempic may, in very rare cases, cause a serious eye condition that can lead to vision loss, the European Medicines Agency's safety committee said on Friday.

In the past, studies in type-2 diabetes patients have linked Ozempic to the condition called non-arteritic anterior ischemic optic neuropathy (NAION). But this is the first time a regulator has confirmed the side effect.

The condition may affect up to 1 in 10,000 people taking semaglutide, the active ingredient in Wegovy and Ozempic as well as in Novo's other diabetes drug Rybelsus, for at least one year, the regulator said.

NAION is the second-most common cause of blindness due to optic nerve damage, after glaucoma.

"This has been reported as a potential risk for some time, so I think the clinical community is relatively aware of it. I don't see this as making any major difference to prescribing patterns," said Barclays analyst Emily Field.

US-listed shares of the Danish drugmaker were up nearly 2.5% in early trading.

The EMA, which started its review in December, has asked Novo to add NAION as a side effect of very rare frequency in the product information accompanying drugs that contain semaglutide.

Novo said it would work with the EMA to update the labels, adding clinical trials and after-market studies did not suggest a reasonable possibility that the drugs caused the condition.

The "benefit-risk profile of semaglutide remains favourable," the company said in a statement.

Novo has recently faced investor concerns that it is losing its first-mover advantage in the highly competitive obesity treatment market, leading the company to oust CEO Lars Fruergaard Jorgensen in May.

Wegovy and Eli Lilly's Zepbound currently dominate the weight-loss drug market, potentially worth about $150 billion by the next decade.

The EMA said several large studies in type-2 diabetes patients have suggested that use of Novo's drugs could raise the risk of developing NAION by twofold.

A study of nearly 350,000 diabetes patients published in March showed that the risk of developing NAION more than doubled after two years of treatment with Ozempic, compared to patients taking medicines from other classes.

The US Department of Health and Human Services did not immediately respond to a Reuters request for comment on whether the Food and Drug Administration was conducting a probe into the side effect.