High blood sugar causes a cascade of effects in your body, starting with increased thirst and frequent urination and potentially progressing to serious organ damage over time. A fasting blood sugar above 126 mg/dL on two separate tests indicates diabetes, while anything above 100 mg/dL is already in the prediabetes range. What happens next depends on how high your levels are, how long they stay elevated, and whether you take steps to bring them down.
What Your Body Feels First
The earliest symptoms of high blood sugar are driven by a simple mechanism: when excess glucose builds up in your blood, your kidneys try to flush it out. The sugar in the fluid your kidneys filter pulls extra water along with it, a process called osmotic diuresis. That’s why you urinate more often, and why you feel intensely thirsty afterward. You’re losing more water than usual, and your body is trying to replace it.
Beyond thirst and frequent urination, you may notice fatigue, blurred vision, and headaches. These symptoms can build gradually, which makes them easy to dismiss. Many people chalk up the tiredness to poor sleep or stress, not realizing their blood sugar has been creeping upward for weeks or months.
How High Is Too High
Normal fasting blood sugar is below 100 mg/dL. Between 100 and 125 mg/dL is prediabetes. At 126 mg/dL or higher on two separate fasting tests, you have diabetes. After eating, a normal reading stays below 140 mg/dL. A reading between 140 and 199 mg/dL two hours after a meal signals prediabetes, and anything above 200 mg/dL indicates diabetes.
A random blood sugar reading of 200 mg/dL or higher at any time of day, regardless of when you last ate, also suggests diabetes. Your A1C, a blood test that reflects your average blood sugar over roughly three months, tells a longer story. An A1C of 6% corresponds to an average blood sugar of about 126 mg/dL. At 7%, your average is around 154 mg/dL. By the time A1C reaches 9%, your average blood sugar is roughly 212 mg/dL, a level that accelerates damage throughout the body.
Short-Term Dangers: DKA and HHS
When blood sugar stays very high and the body can’t use glucose for energy, it starts breaking down fat at an accelerated rate. This produces acidic byproducts called ketones, which build up in the blood. The result is diabetic ketoacidosis (DKA), a life-threatening emergency most common in people with type 1 diabetes. DKA comes on quickly, with nausea, vomiting, abdominal pain, fruity-smelling breath, rapid heartbeat, and deep labored breathing. Without treatment, it can lead to coma or death.
People with type 2 diabetes face a different acute risk called hyperosmolar hyperglycemic state (HHS). This develops more slowly than DKA, with extremely high blood sugar levels and severe dehydration that progresses to confusion and altered consciousness. HHS doesn’t always involve the same ketone buildup, but it’s equally dangerous.
If you have high blood sugar along with vomiting, extreme thirst, difficulty breathing, confusion, or fruity-smelling breath, that combination requires emergency care.
How High Blood Sugar Damages Blood Vessels
The long-term harm from high blood sugar comes down to what excess glucose does to your blood vessels. When glucose stays elevated for extended periods, sugar molecules latch onto proteins in your blood vessel walls and form compounds that stiffen and damage those structures. This process affects both collagen and elastin, the proteins that keep blood vessels flexible, making them increasingly rigid over time.
These sugar-protein compounds also trigger inflammation, increase the production of harmful oxygen molecules in vessel walls, and reduce the availability of nitric oxide, a chemical your blood vessels need to relax and dilate properly. The combined effect is blood vessels that are stiffer, more inflamed, more prone to clotting, and less able to deliver oxygen where it’s needed. This is the root cause of nearly every long-term complication of high blood sugar.
Eyes, Kidneys, and Nerves
The smallest blood vessels in your body are the most vulnerable. In the eyes, high blood sugar damages the tiny capillaries of the retina, initially causing small bulges in the vessel walls. Over time, this can progress to the growth of fragile new blood vessels that leak or bleed, leading to blurred vision, retinal detachment, and eventually blindness. Diabetic retinopathy is one of the most common causes of adult blindness in the United States, and it produces no symptoms in its early stages.
In the kidneys, the filtering units gradually thicken and scar under the strain of high blood sugar. This happens in 20 to 40% of adults with diabetes. The damage is silent for years. By the time you notice symptoms like swelling or fatigue from kidney dysfunction, significant filtration capacity has already been lost. High blood pressure accelerates the process.
Nerve damage from high blood sugar typically starts in the feet and hands, causing numbness, tingling, or pain. Losing sensation in your feet is particularly problematic because small injuries can go unnoticed and become serious infections. This is why annual foot exams that test your ability to feel pressure, vibration, and temperature are a standard part of diabetes care.
Heart, Brain, and Larger Vessels
High blood sugar accelerates the buildup of plaque in larger arteries throughout the body. This raises the risk of heart attack, stroke, and peripheral artery disease (reduced blood flow to the legs and feet). Cardiovascular disease is the leading cause of death in people with diabetes, and the risk starts climbing well before blood sugar reaches diabetic levels.
The brain takes a quieter hit. Repeated episodes of high blood sugar damage the blood vessels that deliver oxygen-rich blood to brain tissue. Over time, reduced blood flow can kill brain cells, leading to problems with memory, learning, and mood. This process isn’t obvious right away. The effects accumulate gradually, and prolonged high blood sugar is associated with a higher risk of vascular dementia and Alzheimer’s disease.
Bringing Blood Sugar Down Safely
If your blood sugar is elevated but below 240 mg/dL and you feel well, physical activity is one of the fastest ways to lower it. Your muscles pull glucose from the blood for energy during exercise, which can bring levels down within an hour or two. Walking, cycling, or any moderate movement works.
There’s an important exception: if your blood sugar is above 240 mg/dL, check for ketones first (urine test strips are available at most pharmacies). If ketones are present, do not exercise. Physical activity when ketones are high can push blood sugar even higher. In that situation, hydration and medical guidance are the right next steps.
Drinking water helps your kidneys flush excess glucose more effectively. Adjusting your next meal to reduce carbohydrate intake can also prevent levels from climbing further. If high readings keep happening despite these steps, it usually means your overall treatment plan, whether that involves medication, insulin, or diet, needs adjustment.
Why Sustained Levels Matter More Than Spikes
A single high blood sugar reading after a large meal isn’t cause for alarm in most people. The real damage comes from sustained elevation over weeks, months, and years. That’s what the A1C test captures. Someone with an A1C of 8% has had an average blood sugar around 183 mg/dL, meaning their blood vessels, nerves, and organs have been soaking in excess glucose continuously. At an A1C of 10%, that average jumps to 240 mg/dL, and at 12%, it reaches nearly 300 mg/dL.
Every percentage point reduction in A1C meaningfully lowers the risk of complications. The relationship isn’t subtle. Bringing an A1C from 9% down to 7% dramatically reduces the odds of eye disease, kidney disease, and nerve damage. The earlier you catch and address high blood sugar, the more reversible the damage tends to be. The blood vessel changes that start silently in the background are far easier to prevent than to undo.