What is ‘eye stroke’ and why has it been linked to weight loss injections?

The phrase “eye stroke” has recently appeared in news reports about a very rare side-effect of weight-loss injections. It’s not a formal medical diagnosis, but a shorthand used to describe a condition in which reduced blood flow damages the optic nerve and causes sudden vision loss.

The phrase might be misleading. Unlike a conventional stroke – which can cause someone to lose the ability to move their limbs or speak – an eye stroke can be harder to recognise at first. Vision can be lost entirely or partially, in one or both eyes, with no numbness or paralysis.

The word “stroke” is used because, as with the more familiar condition, the underlying cause is a loss of blood supply that leads to cell death and tissue damage. The correct medical term for an eye stroke is non-arteritic anterior ischaemic optic neuropathy (Naion).

The recent connection between Naion and weight-loss treatments has made headlines following a large study examining semaglutide, the active ingredient in several popular weight-loss drugs.

Researchers analyse more than 30 million side-effects reported to the US Food and Drug Administration and found that 31,774 involved semaglutide. One drug in particular stood out: Wegovy was found to have a far stronger association with Naion than other semaglutide-based treatments.

The study suggested the risk of eye stroke from Wegovy was almost five times greater than from Ozempic, despite Wegovy being linked to fewer overall reported side-effects.

Understanding why semaglutide might reduce blood flow to the eye requires a little background. Semaglutide is a synthetic version of a naturally occurring hormone called GLP-1, which helps regulate blood sugar. It does this by stimulating insulin production, reducing the release of a sugar-raising hormone called glucagon, and slowing digestion.

Semaglutide has been used to treat type 2 diabetes, heart disease and obesity. Wegovy is administered by injection at a higher maximum dose than Ozempic, another injectable medication. Injected drugs enter the bloodstream faster and in greater concentrations than tablets – and notably, no link was found between Naion and Rybelsus, the tablet form of semaglutide.

The speed at which Wegovy causes weight loss – faster than other treatments – may itself be part of the explanation. The human body is a finely balanced system in which no single organ or process works in isolation. The autonomic nervous system, which controls involuntary functions like heart rate and digestion, relies on a careful balance of hormones to keep things in check. When an external drug significantly alters how those hormones behave, it can affect the rest of the body in unexpected ways.

The relatively high doses used with Wegovy may cause blood pressure to fluctuate beyond normal ranges. A notable drop in blood pressure reduces the rate at which blood flows through the body, and the eye is particularly vulnerable to this. The retina is served by some of the tiniest blood vessels anywhere in the body, and it depends on those small vessels for its oxygen supply. Any significant change in blood pressure can seriously disrupt this delicate circulation.

Men face a much higher risk than women

This does not, however, fully explain why a drug that is broadly beneficial for heart health and blood sugar control might have such a specific harmful effect on eyesight. Nor does it explain another surprising finding from the study: men taking these weight-loss treatments appeared to face three times the risk of vision loss compared to women.

The study did not provide enough detail about the differences between male and female participants. For instance, whether more severely obese men than women were included. In addition, large-scale data of this kind does not always capture the finer details needed to fully understand cause and effect.

It is important to keep all this in perspective: while a link between semaglutide and vision loss has been identified, this side-effect remains rare.

More research is needed to establish safe dosage levels and to understand whether certain factors – such as sex, age, weight, or existing health conditions – make some people more vulnerable than others. Semaglutide is being prescribed for a growing range of conditions and increasingly to younger patients. To ensure that these treatments do not lead to life-changing sight loss, properly designed clinical trials that assess the level of risk are essential.

A spokesperson for Novo Nordisk told the Guardian: “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”.

Barbara Pierscionek does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.