When your blood sugar stays above normal levels, it triggers a chain of effects throughout your body, from mild symptoms you might barely notice to serious organ damage that builds over months and years. A fasting blood sugar of 126 mg/dL or higher, or a random reading of 200 mg/dL or higher with symptoms like excessive thirst and frequent urination, crosses into the range the American Diabetes Association defines as diabetes. What happens next depends on how high your levels go and how long they stay elevated.
What High Blood Sugar Feels Like Day to Day
Mildly elevated blood sugar often produces no obvious symptoms at all, which is part of what makes it dangerous. Many people walk around with readings in the 150 to 250 mg/dL range for weeks or months before anything feels wrong. When symptoms do appear, they tend to creep in gradually: you urinate more often, feel unusually thirsty, get tired more easily, or notice your vision blurring slightly. These happen because your kidneys are working overtime to flush out the excess glucose, pulling water from your tissues in the process.
You might also notice you’re hungrier than usual even though you’re eating enough. That’s because your cells aren’t efficiently absorbing the glucose in your blood, so your body signals that it needs more fuel. Unintentional weight loss can follow for the same reason, particularly in type 1 diabetes. Headaches, difficulty concentrating, and slow-healing cuts round out the picture.
When Blood Sugar Climbs to Dangerous Levels
There are two acute emergencies that can develop when blood sugar spikes severely, and both can be life-threatening without treatment.
The first is diabetic ketoacidosis, or DKA. It happens when your body, unable to use glucose properly, starts breaking down fat for energy at a rapid pace. That process produces acidic byproducts called ketones that accumulate in your blood. DKA typically involves blood sugar above 250 mg/dL, though it can occasionally occur at lower levels. The warning signs are distinct: fruity-smelling breath, nausea and vomiting, abdominal pain, shortness of breath, confusion, and in severe cases, loss of consciousness. DKA is most common in type 1 diabetes but can happen in type 2 as well.
The second is hyperosmolar hyperglycemic state, or HHS, which tends to develop more slowly over days or weeks. Blood sugar in HHS typically exceeds 600 mg/dL. The hallmark is profound dehydration: weeks of increased urination gradually drain your body of fluid until blood pressure drops, heart rate climbs, and mental function deteriorates. Confusion, stupor, and coma are common. HHS carries a higher mortality rate than DKA and is more typical in older adults with type 2 diabetes.
Both emergencies share physical signs like dry mouth, poor skin turgor (when you pinch the skin on your hand and it doesn’t snap back quickly), rapid heartbeat, and low blood pressure. If you experience persistent vomiting, can’t keep fluids down, have blood sugar above 240 mg/dL with symptoms of ketones, or notice confusion or fruity breath, that warrants emergency care.
How High Blood Sugar Damages Your Blood Vessels
The long-term consequences of sustained high blood sugar come down to one central problem: glucose damages blood vessels. It does this by reacting with proteins in your vessel walls, creating harmful compounds that stiffen and injure the lining of arteries and capillaries. This damage unfolds in two categories, small vessels and large vessels, and each produces a different set of complications.
Small Vessel Damage
The tiny capillaries in your eyes, kidneys, and nerves are especially vulnerable. In the eyes, excess glucose damages the small blood vessels in the retina. Those vessels develop weak spots that can bulge and leak fluid, causing swelling in the part of the retina responsible for sharp central vision. As damage progresses and blood flow to the retina decreases, the eye responds by growing new, fragile blood vessels that are prone to bleeding. This progression, from early leaking to abnormal vessel growth, is the path from mild diabetic eye disease to the advanced stage that threatens vision.
In the kidneys, high blood sugar alters the filtering units (glomeruli) by thickening their walls and causing the surrounding tissue to expand. Over time, these structural changes impair the kidney’s ability to filter waste while keeping useful proteins in the blood. Early kidney damage often shows up as small amounts of protein leaking into urine, long before you feel any symptoms. Left unchecked, this can progress to kidney failure.
Nerve damage follows a similar pattern. Prolonged high glucose triggers a cascade of metabolic stress in nerve fibers, including oxidative damage and reduced blood flow to the nerves themselves. The longest nerve fibers are affected first, which is why symptoms typically start in the feet and hands: numbness, tingling, burning pain, or a loss of sensation. Losing feeling in your feet is particularly concerning because you may not notice injuries, which ties directly into another consequence of high blood sugar.
Large Vessel Damage
High blood sugar also accelerates the buildup of cholesterol-laden plaque inside larger arteries, a process called atherosclerosis. Glucose damages artery walls, and that damage creates spots where LDL cholesterol can accumulate and harden. Over time, plaque narrows the arteries and restricts blood flow. When this happens in the arteries supplying the heart, it’s coronary artery disease. When it happens in the arteries supplying the brain, it sets the stage for a stroke. People with diabetes are roughly twice as likely to develop heart disease or have a stroke compared to people without diabetes, and cardiovascular disease is the leading cause of death among people with type 2 diabetes.
Weakened Immune Function and Slow Healing
High blood sugar puts extra stress on your immune system in two ways. First, it directly impairs white blood cells, making them less effective at finding and destroying bacteria and other pathogens. Second, it triggers chronic low-grade inflammation throughout your body. Your immune system is essentially working harder while performing worse, which means infections are more likely to take hold and slower to resolve.
This is why wound healing becomes a real concern. A small cut on your foot that would normally close up in a few days can linger for weeks when blood sugar is poorly controlled. Combine that with nerve damage that prevents you from feeling the wound and reduced blood flow that limits the delivery of oxygen and immune cells to the area, and you have the recipe for serious infections. Diabetic foot ulcers, which develop through exactly this combination, are one of the most common reasons for hospital admissions related to diabetes.
How Quickly Does Damage Develop
The timeline varies depending on how high your blood sugar runs and how consistently it stays elevated. Acute emergencies like DKA can develop within hours to days. The subtle small-vessel complications typically take years of poorly controlled blood sugar to become clinically apparent, but the underlying damage starts much earlier than symptoms suggest. Many people already have early signs of eye or kidney changes at the time they’re first diagnosed with type 2 diabetes, because their blood sugar was elevated for years before anyone checked it.
The relationship between blood sugar control and complications is well established and dose-dependent: the higher your average blood sugar over time (reflected in your A1C level), the greater your risk. Bringing levels closer to normal slows or even halts progression of small-vessel damage. This is true even if damage has already started, which is why blood sugar management matters at every stage.
What Your Numbers Mean
Normal fasting blood sugar is below 100 mg/dL. Between 100 and 125 mg/dL is considered prediabetes. At 126 mg/dL or above on two separate tests, you meet the threshold for diabetes. After eating, blood sugar naturally rises, but a reading of 200 mg/dL or higher two hours after a meal (during a glucose tolerance test) also indicates diabetes.
If you’re monitoring at home and consistently seeing readings above 180 mg/dL after meals or above 130 mg/dL fasting, those numbers are high enough to contribute to long-term complications over time. The target ranges your provider sets for you may differ depending on your age, how long you’ve had diabetes, and other health conditions, but the general principle holds: the closer to normal, the lower the risk of every complication described above.