In February 2026 the MHRA updated product information for semaglutide regarding the very rare risk of non-arteritic anterior ischaemic optic neuropathy — NAION.
What NAION Is
NAION is a sudden loss of vision caused by reduced blood flow to the optic nerve, the cable carrying signals from the eye to the brain. Starved of blood, part of the nerve is damaged.
The features that distinguish it:
- Sudden — often noticed on waking
- Painless, which is why it is not treated as an emergency by everyone who gets it
- Usually affects one eye
- Often a loss of part of the visual field, particularly the upper or lower half, rather than complete blindness
Vision loss from NAION is often permanent, which is what makes it worth knowing about despite its rarity.
How Rare Is "Very Rare"?
In medicines regulation, terms like this have defined meanings. Very rare means fewer than 1 in 10,000 people. It is the lowest frequency category in use.
Some context is fair here. NAION occurs in the general population, is more common with age, and is associated with diabetes, high blood pressure and a particular optic disc anatomy — conditions that overlap heavily with the population prescribed semaglutide. Establishing how much of the risk is attributable to the medicine is genuinely difficult, and the regulatory response has been to add the information rather than to restrict the medicine.
What to Do
The action is simple and worth committing to memory: sudden changes in vision while taking a GLP-1 need urgent medical attention. Not an optician appointment next week — same-day assessment, through NHS 111, an urgent eye care service, or A&E.
This applies to:
- Sudden loss or blurring of vision in one eye
- A missing area of the visual field
- A sudden change in colour perception
Tell whoever assesses you which medicine you take.
The Other Eye Issue Worth Knowing
Separately, people with type 2 diabetes and existing diabetic retinopathy can experience a temporary worsening of retinopathy when blood glucose falls rapidly. This is a recognised effect of rapid glycaemic improvement generally, not unique to GLP-1 medicines, and is a reason for eye monitoring in that group. If you have diabetic eye disease, raise it before starting.
Keeping It in Proportion
It would be wrong to read a very rare risk as a reason not to treat obesity, which carries substantial and well-established risks of its own. The purpose of a warning like this is not to change the decision for most people. It is to make sure the small number who experience it recognise what is happening and get help quickly enough to matter.