When blood sugar stays too high for too long, it damages blood vessels, nerves, and organs throughout your body. The effects range from mild symptoms you might barely notice, like increased thirst and frequent urination, to life-threatening emergencies and permanent organ damage. How much harm occurs depends on how high your levels climb and how long they stay elevated.
For context, the American Diabetes Association recommends most adults with diabetes aim for 80 to 130 mg/dL before meals and under 180 mg/dL one to two hours after eating. Numbers consistently above these ranges set the stage for problems.
Short-Term Symptoms You Might Notice First
Mildly elevated blood sugar often produces no symptoms at all, which is part of what makes it dangerous. As levels climb higher, you may start to feel unusually thirsty, urinate more frequently, and feel fatigued. Blurred vision, headaches, and difficulty concentrating are common. These symptoms develop because excess glucose in your bloodstream pulls water from your tissues and forces your kidneys to work overtime filtering it out.
If levels continue rising, you might notice unintentional weight loss, slow-healing cuts or sores, and frequent infections. Some people experience numbness or tingling in their hands and feet even before they receive a diabetes diagnosis.
When High Blood Sugar Becomes an Emergency
Two conditions represent the most dangerous short-term consequences of very high blood sugar. Both require emergency medical treatment.
Diabetic ketoacidosis (DKA) occurs when blood sugar reaches 200 mg/dL or higher and the body, unable to use glucose properly, starts breaking down fat for fuel at a rapid rate. This produces acids called ketones that build up in the blood, making it dangerously acidic. DKA develops most often in people with type 1 diabetes, sometimes over just hours. Warning signs include nausea, vomiting, abdominal pain, fruity-smelling breath, and confusion.
Hyperglycemic hyperosmolar state (HHS) is more common in type 2 diabetes and involves blood sugar climbing to 600 mg/dL or higher. At those levels, the blood becomes extremely concentrated. HHS develops more gradually, often over days or weeks, and can cause severe dehydration, seizures, and loss of consciousness. Mortality rates for HHS are higher than for DKA, partly because it tends to affect older adults and is often recognized late.
How High Sugar Damages Blood Vessels
The long-term damage from chronically elevated blood sugar comes down to what excess glucose does to your blood vessels. When there’s too much sugar circulating in your bloodstream, glucose molecules attach to proteins throughout your body in a process that produces compounds called advanced glycation end-products. These compounds accumulate over time, cross-linking with structural proteins like collagen and elastin in your vessel walls. The result is stiffer, less flexible blood vessels that can’t dilate properly.
This process also reduces the availability of nitric oxide, a molecule your blood vessels rely on to stay relaxed and open. Without enough of it, blood flow decreases and vessel walls become prone to inflammation. The lining of blood vessels shifts from its normal state to one that promotes clotting, raising the risk of blockages. This is the underlying mechanism behind nearly every long-term complication of high blood sugar.
Heart Disease and Stroke Risk
Damage to large blood vessels is the leading cause of death in people with chronically high blood sugar. A systematic review pooling data from 17 studies found that for every one-percentage-point increase in A1C (a measure of average blood sugar over three months), the risk of total cardiovascular disease rose by 18% in people with type 2 diabetes. Stroke risk increased by 17%, and the risk of peripheral artery disease, where blood flow to the legs becomes restricted, went up by 28%.
These aren’t small numbers. They mean that someone with an A1C of 9% faces substantially higher cardiovascular risk than someone at 7%, and the relationship is continuous. There’s no safe threshold where high blood sugar stops affecting your heart and arteries.
Nerve Damage
High blood sugar damages the small blood vessels that supply your nerves, starving them of oxygen and nutrients. This leads to diabetic neuropathy, which most commonly starts in the feet and legs before progressing upward. Symptoms include pain, burning, tingling, and eventually loss of feeling. Losing sensation in your feet is particularly dangerous because injuries and infections can go unnoticed, sometimes progressing to ulcers that are difficult to heal. In severe cases, this chain of events leads to amputation.
Nerve damage isn’t limited to your extremities. It can also affect the nerves that control digestion, causing constipation, diarrhea, or difficulty emptying the stomach. It can impair bladder function and sexual response. Men with diabetic neuropathy commonly experience erectile dysfunction.
Kidney Damage
Your kidneys contain millions of tiny blood vessel clusters that filter waste from your blood. Chronic high blood sugar damages these filtering units over time, gradually reducing your kidneys’ ability to do their job. Early kidney damage often shows up as small amounts of protein leaking into the urine, something you wouldn’t notice without a lab test.
Kidney disease progresses through stages defined by how well your kidneys are filtering. In the earliest stage, your filtration rate is still normal but damage has begun. As the disease advances through moderate and severe stages, waste products build up in your blood and your kidneys lose the ability to balance fluids and electrolytes. At the final stage, the kidneys can no longer sustain life without dialysis or a transplant.
Vision Loss
The blood vessels in your retina are especially vulnerable to sugar-related damage. Diabetic retinopathy starts when small vessels in the back of the eye weaken, bulge, and leak fluid. In more advanced stages, new abnormal blood vessels grow on the retina’s surface, and these are fragile and bleed easily. Left untreated, this can lead to blindness.
A related condition, diabetic macular edema, occurs when fluid leaks into the central part of the retina responsible for sharp, detailed vision. Even with aggressive treatment, macular edema takes a median of about 33 weeks to resolve. After one year of treatment, roughly 59% of patients see resolution. By two years, that number reaches only 74%, meaning a quarter of patients still have persistent swelling despite consistent care.
Slower Wound Healing and Infection Risk
High blood sugar impairs your body’s ability to heal in multiple ways. Immune cells called macrophages, which are critical for clearing debris and fighting infection at a wound site, become locked into an inflammatory state that prevents them from transitioning to the repair phase of healing. At the same time, the fibroblasts responsible for producing collagen and rebuilding tissue don’t function properly. In people with chronically elevated blood sugar, the compounds produced by glucose binding to proteins actively interfere with collagen cross-linking, weakening the structural scaffolding that new tissue needs to form.
This is why diabetic foot ulcers are notoriously difficult to treat and why even minor cuts and scrapes take longer to close when blood sugar is poorly controlled. The combination of nerve damage (so you don’t feel the injury), poor blood flow (so healing nutrients can’t reach the site), and impaired immune response creates a perfect storm for chronic wounds and infections.
Effects on the Brain
Chronically high blood sugar affects cognitive function in ways that become measurable over time. A Johns Hopkins study tracking participants over 20 years found that people with poorly controlled diabetes experienced 19% more cognitive decline than expected. Even people with pre-diabetes or well-managed diabetes showed smaller but still significant declines compared to those with normal blood sugar.
The damage starts in midlife. That’s an important detail because it means the cognitive effects of high blood sugar aren’t just a concern for older adults. Vascular damage in the brain follows the same patterns as in the rest of the body: reduced blood flow, increased inflammation, and structural changes in small vessels that supply brain tissue with oxygen.
Skin and Foot Complications
High blood sugar makes your skin more susceptible to bacterial and fungal infections. Dry, cracked skin is common because dehydration from excess urination reduces moisture levels. Darkened patches of skin, particularly around the neck and armpits, can develop as a result of insulin resistance.
Foot problems deserve special attention because they represent the intersection of several complications at once. Nerve damage reduces sensation, poor circulation slows healing, and impaired immunity makes infection more likely. Serious skin infections and ulcerations on the feet are one of the most common reasons people with uncontrolled diabetes end up in the hospital, and in severe cases, amputation becomes necessary to prevent life-threatening infection from spreading.