When your blood sugar climbs too high, your body sends clear warning signals, and ignoring them can lead to serious problems. A fasting blood sugar above 130 mg/dL or a post-meal reading above 180 mg/dL is considered above the target range for most adults with diabetes. What happens next depends on how high it goes, how long it stays elevated, and whether your body can still produce enough insulin to bring it back down.
The First Symptoms You’ll Notice
The earliest signs of high blood sugar tend to involve thirst and urination. When glucose builds up in your bloodstream, your kidneys work harder to filter it out, pulling extra water along with it. That’s why you pee more often, and why you feel thirsty almost immediately after drinking. You might also notice headaches and blurred vision. These symptoms can creep in gradually enough that you dismiss them, especially if you don’t know your blood sugar is elevated.
If blood sugar stays high for days or weeks, the symptoms shift. Fatigue becomes harder to shake because your cells aren’t efficiently converting glucose into energy. You may lose weight without trying, since your body starts breaking down fat and muscle for fuel instead. Slow-healing cuts, recurring skin infections, and vaginal yeast infections are also common with prolonged high blood sugar. These are signs your body has been running above normal for a while, not just a single spike after a meal.
Where the Danger Zones Start
A single high reading after a big meal isn’t usually dangerous on its own. But when blood sugar climbs above 240 mg/dL while you’re feeling sick, the CDC recommends checking for ketones using an over-the-counter urine test kit. Ketones are acids your body produces when it burns fat instead of glucose for energy, and high levels can tip your blood into a dangerous state called diabetic ketoacidosis, or DKA. This is most common in people with type 1 diabetes but can happen in type 2 as well.
DKA develops when your blood becomes too acidic. Symptoms include nausea, vomiting, abdominal pain, fruity-smelling breath, and confusion. It’s a medical emergency. If left untreated, it can lead to coma and death. High ketones on a home test kit, combined with any of those symptoms, mean you need emergency care immediately.
There’s another critical condition called hyperosmolar hyperglycemic state, which tends to affect older adults with type 2 diabetes. Blood sugar can rise to ten times the normal level, sometimes exceeding 600 mg/dL. At that point, the body loses so much water that the blood becomes dangerously concentrated with sodium, glucose, and other substances. Severe dehydration, seizures, and loss of consciousness can follow. This condition develops over days or weeks, often triggered by an illness or infection that goes unmanaged.
What Chronic High Blood Sugar Does to Your Body
The real damage from high blood sugar happens slowly, over months and years. Persistently elevated glucose injures the smallest blood vessels in your body first, which is why the eyes, kidneys, and nerves take the earliest hits.
In your eyes, damaged blood vessels in the retina can leak or swell, gradually distorting your vision. This is diabetic retinopathy, and it’s one of the leading causes of blindness in adults. In your kidneys, the tiny filtering units called glomeruli thicken and scar over time. The filtering membrane swells first, then the surrounding tissue expands and hardens, eventually destroying the kidney’s ability to clean your blood. In advanced stages, this leads to kidney failure requiring dialysis or a transplant.
Nerve damage, or neuropathy, typically starts in your feet and hands. You might feel tingling, burning, or numbness. Over time, losing sensation in your feet makes it easy to miss small injuries that can become serious infections.
The Heart Disease Connection
High blood sugar doesn’t just affect small blood vessels. It significantly raises your risk of heart attack and stroke. A large study from University College London found that people with elevated blood sugar had a 30 to 50 percent greater risk of cardiovascular disease, even when their levels were below the threshold for a diabetes diagnosis. For people with diagnosed diabetes, the risk roughly doubled.
This means blood sugar doesn’t have to be dramatically high to cause cardiovascular harm. Even the moderately elevated range that qualifies as prediabetes (an A1C between 5.7% and 6.4%) carries meaningful risk for your heart and blood vessels over time.
How Blood Sugar Levels Are Classified
Your A1C test measures your average blood sugar over the previous two to three months. A normal A1C is below 5.7%. Prediabetes falls between 5.7% and 6.4%. An A1C of 6.5% or higher indicates diabetes. For most adults already managing diabetes, the American Diabetes Association recommends keeping A1C below 7%, which corresponds to an average blood sugar of about 154 mg/dL.
Day-to-day targets for people with diabetes are a fasting blood sugar between 80 and 130 mg/dL, and a reading below 180 mg/dL one to two hours after eating. These numbers are general guidelines. Your personal targets may be adjusted based on your age, how long you’ve had diabetes, and whether you have other health conditions.
What You Can Do When Blood Sugar Spikes
Drinking water is one of the simplest and most effective responses to a blood sugar spike. Extra fluids help your kidneys flush out excess glucose through urine and prevent the dehydration that makes hyperglycemia worse.
Exercise can also help bring blood sugar down by prompting your muscles to absorb glucose from your bloodstream. But there’s an important cutoff: if your blood sugar is above 250 mg/dL, exercise can actually make things worse by triggering your liver to release even more glucose. Check with your diabetes care team about your specific exercise thresholds. If you do exercise, check your blood sugar 15 to 30 minutes beforehand and roughly every 30 to 60 minutes during the activity. Keep water and a fast-acting carbohydrate source nearby in case your sugar drops too quickly in the other direction.
If you take insulin, you may have a correction dose built into your treatment plan for high readings. Follow whatever protocol your care team has given you. Stacking extra insulin doses without guidance is risky because it can cause a dangerous low blood sugar rebound hours later.
For persistent highs that don’t respond to your usual strategies, or any reading above 240 mg/dL accompanied by illness, test for ketones and contact your care team. Repeated high readings over several days often signal that something in your management plan needs adjusting, whether that’s medication timing, carbohydrate intake, stress, sleep, or an underlying illness your body is fighting.