Diabetes doesn’t directly cause the most common form of glaucoma, but it significantly raises your risk. A large meta-analysis of prospective studies found that people with diabetes are 36% more likely to develop open-angle glaucoma compared to people without diabetes. The relationship is complex: high blood sugar creates conditions in the eye that make glaucoma more likely to develop and harder to control once it does.
How Diabetes Raises Glaucoma Risk
Open-angle glaucoma, the type responsible for most cases, develops when the optic nerve at the back of the eye gradually deteriorates. In people with diabetes, several overlapping processes accelerate that damage.
Chronic high blood sugar triggers inflammation, oxidative stress, and the buildup of harmful sugar-protein compounds in retinal tissue. These processes damage the nerve cells at the back of the eye that transmit visual signals to the brain. Studies of retinal tissue from people with diabetes show elevated markers of cell death in these nerve cells, along with signs that the cells’ internal self-destruct pathways have been activated. There’s also evidence that diabetes disrupts how nerve cells communicate by allowing excess amounts of a chemical messenger called glutamate to accumulate, which can be toxic to the cells at high levels.
Beyond nerve damage, diabetes also raises the fluid pressure inside the eye, which is the most well-known risk factor for glaucoma. A study of 114 people with diabetic eye disease found that those with higher long-term blood sugar levels (measured by HbA1c) had significantly higher eye pressure. People with an HbA1c of 9.0 or above had eye pressures in a range associated with glaucoma risk, while those with lower HbA1c values averaged lower pressures. Notably, among people with high HbA1c levels, fewer than 1% had low eye pressure, suggesting that poorly controlled blood sugar almost always pushes pressure upward.
Blood Sugar Control and Glaucoma Progression
The connection between diabetes and glaucoma isn’t just about whether you develop it. How well you manage your blood sugar also affects how quickly glaucoma worsens once it’s present. Higher baseline HbA1c is a significant predictor of needing glaucoma surgery down the line. For every small increase in average blood sugar, the likelihood of requiring surgical intervention goes up.
The duration of diabetes matters too. The longer you’ve lived with diabetes, the higher your cumulative risk of developing glaucoma. This makes sense given that the damage to retinal nerve cells and the elevation in eye pressure are gradual processes that compound over years. Early and consistent blood sugar management is one of the most meaningful things you can do to protect your vision on multiple fronts, since the same metabolic damage that drives glaucoma also drives diabetic retinopathy.
Neovascular Glaucoma: A More Direct Link
There is one type of glaucoma that diabetes can directly cause. Neovascular glaucoma occurs when advanced diabetic retinopathy triggers the growth of abnormal new blood vessels on the iris and in the eye’s drainage system. These vessels block the normal outflow of fluid, causing eye pressure to spike rapidly. Unlike open-angle glaucoma, which develops slowly and painlessly over years, neovascular glaucoma can cause sudden pain, redness, and vision loss.
This form is less common but more aggressive. In a study of eyes with proliferative diabetic retinopathy that had already developed new blood vessels in the front of the eye, about 8% went on to develop full neovascular glaucoma. Among those with the most advanced changes to the eye’s drainage angle, the rate jumped to 53%. This type of glaucoma is largely preventable with timely treatment of the underlying retinopathy, which is why regular dilated eye exams are critical for anyone with diabetes.
Does Metformin Offer Protection?
One of the more promising findings in recent years involves metformin, the most commonly prescribed medication for type 2 diabetes. A large retrospective study of over 150,000 people with type 2 diabetes found that those taking high doses of metformin had a 25% lower risk of developing open-angle glaucoma compared to those not taking it. Every additional gram of metformin used was associated with a small but measurable reduction in glaucoma risk.
A separate observational study comparing people on metformin to those using other blood sugar medications found that metformin users had significantly lower odds of developing both glaucoma and diabetic retinopathy. Metformin appears to have anti-inflammatory and cell-protective effects in the eye that go beyond simple blood sugar control. No research has yet tested whether metformin would protect against glaucoma in people without diabetes, so these findings apply specifically to people already managing type 2 diabetes.
What This Means for Eye Screening
Because glaucoma develops without symptoms until significant vision loss has already occurred, the 36% increased risk that comes with diabetes makes regular eye exams essential. A standard diabetes eye exam focuses on retinopathy, but it should also include a check of eye pressure and an evaluation of the optic nerve. Many people don’t realize that these are separate conditions requiring separate screening.
If you already have glaucoma and diabetes, keeping your HbA1c as close to your target as possible has direct implications for how your glaucoma progresses. People with HbA1c above 7.5% who were using insulin had a 20% higher rate of eventually needing glaucoma surgery compared to those on oral medications alone, but this difference disappeared when blood sugar was well controlled. The practical takeaway: blood sugar management is not just about preventing diabetic retinopathy. It’s a meaningful lever for glaucoma outcomes too.