When your blood sugar climbs too high, your body sends a series of warning signals that escalate from mild discomfort to life-threatening emergencies. A normal fasting blood sugar is below 100 mg/dL, while 126 mg/dL or higher on two separate tests indicates diabetes. What happens at each stage above normal depends on how high your levels go, how quickly they rise, and how long they stay elevated.
The First Signs You’ll Notice
The earliest symptoms of high blood sugar are easy to dismiss as nothing serious. You’ll feel unusually thirsty, need to urinate more often, and may develop a headache or blurred vision. These happen because your kidneys are working overtime to filter the excess glucose out of your blood, pulling water from your tissues in the process. That’s why thirst and frequent urination go hand in hand: your body is literally flushing sugar out through your urine, and you’re losing fluid fast.
You might also feel hungrier than usual. Even though there’s plenty of glucose in your bloodstream, your cells can’t absorb it properly without enough insulin, so your body signals that it needs more fuel. This combination of increased thirst, urination, and hunger is the classic triad that often leads to a diabetes diagnosis.
When High Blood Sugar Becomes Dangerous
If blood sugar keeps climbing without treatment, two acute emergencies can develop. Both require hospitalization and can be fatal without intervention.
Diabetic ketoacidosis (DKA) happens most often in people with type 1 diabetes. Without insulin, your body can’t use glucose for energy, so it starts breaking down fat instead. That fat breakdown produces acidic compounds called ketones, which build up in the blood and make it dangerously acidic. Symptoms include nausea, vomiting, abdominal pain, deep labored breathing, and a fruity smell on the breath caused by acetone. In severe cases, it leads to confusion, drowsiness, and loss of consciousness.
Hyperosmolar hyperglycemic syndrome (HHS) is more common in type 2 diabetes and involves blood sugar levels above 600 mg/dL. At that concentration, blood becomes thick and syrupy, causing severe dehydration and altered mental status. Unlike DKA, ketone production is minimal, so the breath doesn’t smell fruity and the blood doesn’t become as acidic. But the dehydration is often more profound, and the confusion or drowsiness can progress to coma. Both DKA and HHS rarely allow for safe discharge from the emergency room. Patients typically need to be admitted with hourly blood sugar and neurological monitoring until they stabilize.
Red Flags That Need Immediate Help
Certain symptoms signal that high blood sugar has crossed into a medical emergency:
- Confusion, extreme drowsiness, or difficulty staying awake
- Deep, rapid, labored breathing
- Persistent vomiting that prevents you from keeping fluids down
- Fruity-smelling breath
- Signs of severe dehydration like dry mouth, sunken eyes, or very little urine output
Any of these warrants a trip to the emergency department, not a wait-and-see approach.
The Silent Damage You Don’t Feel
Perhaps the most unsettling thing about high blood sugar is that it can cause serious harm without producing any obvious symptoms. Research on elderly adults with asymptomatic hyperglycemia found that men had 1.5 times the prevalence of chest pain from heart disease compared to those with normal blood sugar, along with significantly more signs of silent heart attacks on their EKGs. The study concluded that even without symptoms, elevated blood sugar is not a benign condition.
This is why diabetes screening matters. Many people walk around with blood sugar in the prediabetic range (fasting levels of 100 to 125 mg/dL) or even diabetic range for years before they’re diagnosed, accumulating damage to blood vessels and nerves the entire time.
How High Blood Sugar Damages Your Body Over Time
Chronic hyperglycemia injures the body through a specific process. When blood sugar stays elevated, glucose molecules latch onto proteins in your blood and tissues, forming compounds that stiffen and damage blood vessel walls. These compounds trigger inflammation, generate free radicals, and interfere with nitric oxide, the molecule your blood vessels rely on to relax and expand. The result is blood vessels that are stiffer, more prone to blockage, and leakier than they should be.
Certain cells are especially vulnerable. The cells lining your blood vessels can’t regulate how much glucose enters them, so when blood sugar is high, they absorb too much and bear the brunt of the damage. This is why high blood sugar preferentially harms the smallest blood vessels first, particularly in the eyes, kidneys, and nerves.
Eyes
Diabetic retinopathy develops when damaged blood vessels in the retina begin to leak or become blocked. The risk is directly tied to how severe and prolonged the hyperglycemia is, especially when high blood pressure is also present. Over time, this can progress to vision loss.
Kidneys
The filtering units of the kidneys contain dense networks of tiny blood vessels that are highly susceptible to glucose-driven damage. Protein starts leaking into the urine, a sign the filters are breaking down. This occurs in 15 to 40% of people with type 1 diabetes and 5 to 20% of those with type 2.
Nerves
Diabetic neuropathy affects roughly half of all people with diabetes, and the risk rises in proportion to both the duration and severity of high blood sugar. Excess glucose damages the structural proteins inside nerve fibers, interfering with the transport of signals along the nerve. It also reduces blood flow to nerves, starving them of oxygen. The result is numbness, tingling, burning pain, or loss of sensation, typically starting in the feet and hands. Autonomic nerves that control digestion, heart rate, and blood pressure can be affected as well.
Heart and Large Blood Vessels
Beyond small-vessel disease, chronically elevated blood sugar also accelerates atherosclerosis in larger arteries, increasing the risk of heart attack and stroke. The inflammatory cascade triggered by glucose-damaged proteins plays a central role, promoting the buildup of plaques and making existing plaques more likely to rupture.
What “Normal” and “Too High” Actually Mean
Blood sugar targets for people managing diabetes are more nuanced than a single cutoff. The American Diabetes Association’s 2025 guidelines recommend these goals for most nonpregnant adults with diabetes:
- Before meals: 80 to 130 mg/dL
- After meals (at peak): below 180 mg/dL
- A1C: below 7%, which reflects a roughly three-month average of blood sugar levels
For people using continuous glucose monitors, the goal is to spend more than 70% of the day in the 70 to 180 mg/dL range, with less than 5% of the day above 250 mg/dL. Numbers above 250 on a regular basis signal that blood sugar is high enough to accelerate the kind of organ damage described above. Readings above 600 mg/dL put you in the territory of hyperosmolar syndrome, a medical emergency.
For people without diabetes, a fasting blood sugar between 100 and 125 mg/dL falls in the prediabetic range, meaning blood sugar is already higher than ideal and the risk of progressing to diabetes is real. An A1C between 5.7% and 6.4% tells the same story. At this stage, lifestyle changes like increased physical activity and dietary shifts can often bring numbers back to normal and prevent or delay the onset of diabetes.