When your blood sugar is too high, your body gives you signals: increased thirst, frequent urination, headaches, blurred vision, and fatigue. A normal fasting blood sugar is below 100 mg/dL, while 126 mg/dL or higher on a fasting test indicates diabetes. What happens next depends on how high it goes, how long it stays elevated, and whether you take steps to bring it down.
What Counts as “Too High”
Blood sugar levels fall into clear ranges. A fasting reading (nothing to eat or drink except water for at least 8 hours) below 100 mg/dL is normal. Between 100 and 125 mg/dL is considered prediabetes. At 126 mg/dL or above, you’re in the diabetes range.
After eating, the picture shifts. A reading below 140 mg/dL two hours after a meal is normal. Between 140 and 199 mg/dL is prediabetes territory, and 200 mg/dL or higher signals diabetes. If you’re checking your sugar at home and consistently seeing numbers above these thresholds, your blood sugar is too high.
Your A1C, a blood test that estimates your average blood sugar over roughly three months, puts these daily numbers into longer perspective. An A1C of 6% corresponds to an average blood sugar around 126 mg/dL. At 7%, it’s about 154 mg/dL. By the time you reach an A1C of 9%, your average blood sugar is running around 212 mg/dL, and at 12%, it’s nearly 300 mg/dL.
Early Symptoms You’ll Notice First
The earliest signs of high blood sugar are hard to miss once you know what to look for. Increased thirst is one of the most common, often paired with frequent urination. Your kidneys work harder to filter excess sugar out of your blood, pulling more water with it. That’s why you pee more often and then feel dehydrated and thirsty shortly after.
Blurred vision happens because excess sugar changes the fluid balance in the lenses of your eyes, temporarily distorting your focus. Headaches, fatigue, and increased hunger round out the early warning signs. Many people also feel mentally foggy or irritable when their sugar is running high, though they may not connect the two right away.
Why Your Sugar Might Be High in the Morning
If you’re waking up with unexpectedly high readings, you may be experiencing what’s called the dawn phenomenon. Overnight, your body naturally releases hormones like cortisol, growth hormone, and glucagon. These hormones increase insulin resistance, which can push blood sugar up in the early morning hours, sometimes producing a high reading before you’ve eaten anything. This is common and doesn’t necessarily mean your management plan is failing, but it’s worth tracking and discussing with your care team if the pattern persists.
What Happens Inside Your Body Short-Term
When blood sugar spikes well above 240 mg/dL, the risks escalate. At this level, your body may start breaking down fat for energy instead of glucose, producing acidic byproducts called ketones. Small amounts of ketones are normal during fasting, but when they build up rapidly alongside very high blood sugar, you can develop diabetic ketoacidosis (DKA). This is a medical emergency. Blood sugar in DKA typically runs between 350 and 500 mg/dL, and the blood becomes dangerously acidic.
Warning signs of DKA include shortness of breath, breath that smells fruity, nausea and vomiting, and a very dry mouth. DKA is most common in people with type 1 diabetes, but it can happen in type 2 as well. If you’re experiencing these symptoms, you need emergency care immediately.
A separate emergency, called hyperosmolar hyperglycemic state (HHS), tends to occur in people with type 2 diabetes. Blood sugar often climbs above 800 mg/dL, causing severe dehydration and confusion. Ketones are usually absent, so it looks different from DKA, but it’s equally dangerous. HHS develops more slowly, often over days or weeks, which means it can sneak up on people who aren’t monitoring regularly.
Long-Term Damage to Small Blood Vessels
Chronically elevated blood sugar damages the tiny blood vessels throughout your body. Think of it this way: sugar in the bloodstream is sticky, and over time it narrows and clogs the smallest vessels, reducing blood flow to the organs that depend on them most.
Your eyes are especially vulnerable. High sugar over months and years harms the delicate blood vessels in the retina, which can lead to diabetic retinopathy and, eventually, vision loss. This is why regular eye exams are important for anyone with diabetes or prediabetes.
Your kidneys work like a filter, keeping what your body needs and removing waste. They’re packed with tiny blood vessels. When those vessels get damaged by chronic high sugar, the kidneys become less efficient. Less waste and fluid gets filtered out, which over time can progress to kidney disease.
Nerve damage is another common consequence. High blood sugar damages the blood vessels that supply oxygen to your nerves, particularly in your feet and hands. As nerves deteriorate, you may lose the ability to feel pain in those areas. That sounds like it might be a relief, but it’s actually dangerous: you might not notice a cut, blister, or infection on your foot until it becomes a serious problem.
Long-Term Damage to Large Blood Vessels
Beyond the small vessels, high blood sugar accelerates atherosclerosis, the buildup of fatty deposits inside your larger arteries. Elevated glucose promotes this process through chemical changes that increase oxidative stress and damage artery walls. Over time, this significantly raises your risk of heart attack, stroke, and peripheral artery disease.
The numbers are striking. Each 1-point increase in A1C raises the risk of a major cardiovascular event (heart attack, stroke, or blocked arteries in the legs) by 18%. Bringing your A1C below 7% reduces cardiovascular risk by 37% over 11 years. Even in the prediabetes range (fasting glucose of 100 to 125 mg/dL), cardiovascular risk climbs meaningfully. At fasting glucose of 126 mg/dL or higher, the risk of heart disease increases three to four times over 30 years.
Effects on Your Brain
Your brain depends on healthy blood flow, and high blood sugar erodes it. Over time, damaged blood vessels in the brain deliver less oxygen-rich blood, and brain cells can die as a result. This leads to problems with memory and thinking that, in severe cases, can progress to a form of dementia called vascular dementia.
In the shorter term, high blood sugar affects mood and mental sharpness. Many people report difficulty concentrating, slower thinking, and mood swings when their sugar is elevated. These effects tend to improve when blood sugar comes back into range, but repeated or sustained highs can cause lasting cognitive changes.
What to Do When Your Sugar Is High
If you check your blood sugar and it’s elevated, drinking water is one of the simplest and most effective first steps. Water helps your kidneys flush excess sugar through urine. Choose water over juice, soda, or other sweetened drinks, which will push your sugar higher.
If your blood sugar is above 240 mg/dL, check for ketones using an over-the-counter urine test kit before doing anything else. If ketones are high, call your doctor or go to an emergency room. Do not exercise when ketones are present. Exercise normally helps lower blood sugar, but with high ketones, it can actually drive your sugar even higher and push you toward DKA.
If ketones are negative and your sugar is moderately high, light physical activity like walking can help bring it down. Your muscles use glucose for fuel during movement, pulling it out of the bloodstream. For people on insulin, a correction dose as prescribed by your doctor is another tool, but the specific amount depends entirely on your individual plan.
Persistent high readings, even if they’re not in the emergency range, signal that something in your routine needs adjusting. Possible culprits include meals higher in carbohydrates than usual, missed or mistimed medication, illness or infection (which naturally raises blood sugar), stress, and inadequate sleep. Tracking your readings alongside what you eat, how you sleep, and how you feel can reveal patterns that aren’t obvious day to day.