When blood sugar climbs above roughly 180 to 200 mg/dL, your body starts showing signs of distress. Below that threshold, you may not feel anything at all, which is part of what makes high blood sugar dangerous. What happens next depends on how high it goes, how long it stays elevated, and whether it becomes a pattern.
How High Is Too High
Normal fasting blood sugar sits below 100 mg/dL. Between 100 and 125 mg/dL is considered prediabetes. Anything above 125 mg/dL while fasting meets the threshold for hyperglycemia in someone without a diabetes diagnosis. For people already managing diabetes, blood sugar above 180 mg/dL one to two hours after eating is generally considered too high.
Some people are first diagnosed with Type 1 diabetes only after their blood sugar has climbed above 250 mg/dL. At that level, the body is already under significant metabolic stress, even if the person has been attributing their symptoms to something else for weeks.
Early Symptoms You’ll Notice First
The first signs of high blood sugar tend to be subtle enough that people explain them away. You urinate more frequently because your kidneys are working overtime to filter excess glucose out of your blood. That glucose pulls extra water along with it through a process called osmotic diuresis, which is why you feel increasingly thirsty no matter how much you drink. You may also notice blurred vision and unusual fatigue or weakness.
These four symptoms, frequent urination, excessive thirst, blurred vision, and fatigue, are the classic early warning signs. They can develop gradually over days or weeks, which makes them easy to miss. Many people assume they’re just dehydrated or tired from a busy schedule.
What Happens Inside Your Body
High blood sugar doesn’t just make you feel off. It triggers a cascade of damage at the cellular level, particularly in your blood vessels. Excess glucose ramps up the production of harmful molecules called superoxides, which are a type of free radical. In studies of diabetic blood vessels, the activity of the enzyme system that generates these free radicals was dramatically higher than in healthy controls, with one key component showing a sevenfold increase.
Under normal conditions, your blood vessels produce nitric oxide, a molecule that keeps them relaxed and flexible. When blood sugar is chronically high, superoxides react with nitric oxide and destroy it. Even worse, the enzyme that normally produces nitric oxide gets “uncoupled” and starts producing more superoxides instead. Your blood vessels essentially turn a protective system into a damaging one.
This is why high blood sugar over time leads to stiff, narrowed arteries and poor circulation. It also explains the connection between diabetes and heart disease, stroke, nerve damage, and kidney problems. The damage accumulates quietly. As the CDC notes, the effects of high blood sugar on the brain happen over time and aren’t obvious right away, but repeated episodes stress the brain and can eventually lead to problems with memory, thinking, and even vascular dementia.
When It Becomes an Emergency
If blood sugar stays very high without treatment, two life-threatening conditions can develop. Which one depends largely on the type of diabetes involved.
Diabetic ketoacidosis (DKA) happens most often in Type 1 diabetes. Without enough insulin, your cells can’t use glucose for fuel, so your body starts breaking down fat at an accelerated rate. This produces acids called ketones that build up in the blood and make it dangerously acidic, sometimes dropping blood pH as low as 6.9 to 7.2. Symptoms include fruity-smelling breath, nausea, vomiting, abdominal pain, shortness of breath, and confusion. Left untreated, it leads to loss of consciousness and can be fatal.
Hyperosmolar hyperglycemic state (HHS) is more common in Type 2 diabetes, particularly in older adults. Blood sugar can soar to extreme levels, sometimes above 600 mg/dL. The hallmark of HHS is severe dehydration without the acid buildup seen in DKA. Because ketones are usually absent, the fruity breath doesn’t typically occur. Instead, the extreme dehydration and high blood concentration cause confusion, seizures, and coma. HHS develops more slowly than DKA, often over days to weeks, which can delay recognition.
Both conditions require emergency medical treatment. The key difference: DKA involves acid buildup from ketones, while HHS involves extreme dehydration and blood concentration without significant ketone production.
The Kidney Connection
Your kidneys are on the front line when blood sugar rises. Normally, they filter your blood and reabsorb glucose back into the body. But when glucose levels exceed about 180 mg/dL, the kidneys can’t keep up. The overflow glucose spills into your urine and drags water with it. This is why the first symptom most people notice is frequent urination, sometimes waking up several times a night.
That water loss pulls electrolytes along with it, including sodium and potassium. The result is a cycle of dehydration and electrolyte imbalance that makes you feel progressively worse. If you’re not replacing fluids quickly enough, the dehydration compounds the problem by concentrating your blood sugar even further.
How Doctors Track Long-Term Levels
A single blood sugar reading is a snapshot. To understand what’s been happening over the past two to three months, doctors use the A1C test, which measures how much glucose has attached to your red blood cells over their lifespan. The result comes as a percentage that translates to an estimated average glucose level.
An A1C of 7% corresponds to an average blood sugar of about 154 mg/dL. At 9%, the average jumps to roughly 212 mg/dL, a level where symptoms are likely noticeable and long-term damage accelerates. For most people with diabetes, keeping A1C below 7% significantly reduces the risk of complications. For people without diabetes, a normal A1C falls below 5.7%.
Home glucose monitors give you real-time numbers, while A1C gives you the bigger picture. Both matter. Someone whose blood sugar swings between 70 and 300 throughout the day might have a deceptively “acceptable” A1C because the highs and lows average out, but those spikes are still doing vascular damage with each episode.
What Prolonged High Blood Sugar Does Over Time
Occasional spikes after a large meal are normal and not a cause for alarm. The real danger comes from blood sugar that stays elevated for weeks, months, or years. Chronic hyperglycemia damages small blood vessels first, which is why the earliest complications tend to show up in the eyes, kidneys, and nerves in the feet and hands. These organs depend on tiny, delicate capillaries that are especially vulnerable to glucose-driven damage.
Nerve damage, or neuropathy, often starts as tingling or numbness in the toes and fingers. Over time, it can progress to pain or a complete loss of sensation, which is why people with uncontrolled diabetes are at higher risk for foot injuries they don’t feel and wounds that heal slowly. Kidney damage follows a similar slow progression: the filtering units gradually scar and lose function, sometimes taking years before symptoms appear.
In the brain, repeated hyperglycemia stresses blood vessels and neurons alike. This doesn’t cause sudden, dramatic symptoms. Instead, it contributes to a gradual decline in memory and cognitive processing speed that may not be noticeable for years. The vascular damage in the brain follows the same pattern as everywhere else in the body: excess glucose, free radical production, loss of nitric oxide, and progressive stiffening of blood vessel walls.
The encouraging reality is that much of this damage is preventable or at least slowed dramatically by keeping blood sugar closer to normal ranges. Even modest improvements in blood sugar control reduce the risk of every major complication.