When your blood sugar climbs too high, your body sends clear warning signals: excessive thirst, frequent urination, headaches, and blurred vision. These are the early signs. Left unchecked, persistently high blood sugar damages blood vessels and nerves throughout your body, and in extreme cases, it can trigger life-threatening emergencies. What happens next depends on how high your levels go, how long they stay elevated, and how quickly you respond.
Why High Blood Sugar Causes Symptoms
Your kidneys act as a filter for your blood. When glucose levels rise beyond what your kidneys can reabsorb, the excess sugar spills into your urine. Sugar pulls water along with it, which is why you start urinating far more than usual. That fluid loss triggers intense thirst as your body tries to compensate for the dehydration. You may also feel unusually hungry because your cells aren’t absorbing glucose properly, even though there’s plenty circulating in your bloodstream.
Blurred vision happens because high glucose changes the fluid balance in the lens of your eye, causing it to swell. Headaches follow from dehydration and shifting fluid pressures. These early symptoms can develop gradually over days or even weeks, making them easy to dismiss.
When It Becomes an Emergency
Two dangerous conditions can develop when blood sugar stays very high: diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS). Both require emergency treatment.
DKA typically develops when blood sugar exceeds 250 mg/dL, and readings often land between 350 and 500 mg/dL. Without enough insulin, your body starts burning fat for fuel, producing acidic byproducts called ketones. DKA comes on fast, sometimes within hours, with symptoms including nausea, vomiting, abdominal pain, fatigue, rapid deep breathing, and a distinctive fruity odor on the breath. Mental confusion can progress to coma. DKA is most common in people with type 1 diabetes but can occur in type 2 as well.
HHS is slower and more insidious. It develops over days to weeks, with blood sugar frequently climbing above 800 mg/dL. The hallmarks are severe dehydration, weakness, and a steadily declining mental state. Unlike DKA, the body doesn’t produce large amounts of ketones, but the extreme dehydration itself becomes dangerous. HHS is more common in older adults with type 2 diabetes and carries a high mortality rate when treatment is delayed.
How High Blood Sugar Damages Blood Vessels
The long-term consequences of chronically elevated blood sugar come down to what excess glucose does inside your blood vessels. A high-glucose environment triggers inflammation and increases the production of harmful molecules called reactive oxygen species, which are essentially unstable particles that damage cells. This combination weakens the inner lining of blood vessels, reducing their ability to relax and dilate normally. Over time, the vessels stiffen, narrow, and become prone to blockages.
This process affects both small and large blood vessels, but the timeline and consequences differ depending on which organs are involved.
Small Blood Vessel Damage: Eyes, Kidneys, Nerves
The smallest blood vessels in your body are the most vulnerable to glucose-related damage. Three complications are especially common.
- Eyes (retinopathy): Tiny blood vessels in the retina weaken and leak, or new fragile vessels grow abnormally. Over time this can lead to vision loss. It’s the leading cause of blindness in working-age adults with diabetes.
- Kidneys (nephropathy): The filtering units in your kidneys rely on delicate blood vessels. When those vessels are damaged, the kidneys gradually lose their ability to filter waste, potentially progressing to kidney failure.
- Nerves (neuropathy): High blood sugar causes chemical changes in nerves and damages the small blood vessels that supply them with oxygen and nutrients. This typically starts in the feet and hands as tingling, numbness, or burning pain. Left untreated, nerve damage can spread to the digestive tract, sexual organs, and other areas of the body.
These complications develop over years, not days. But they often progress silently. By the time symptoms appear, significant damage may already be done, which is why regular screening matters for anyone managing diabetes.
Large Blood Vessel Damage: Heart and Brain
Adults with diabetes are nearly twice as likely to have heart disease or a stroke compared to adults without diabetes, according to data from the National Institute of Diabetes and Digestive and Kidney Diseases. High blood sugar damages the blood vessels and nerves that control your heart and cardiovascular system. Combined with the inflammation and vessel stiffening described above, this creates an environment where plaque builds up faster, blood clots form more easily, and arteries lose their flexibility.
Heart attack and stroke are the leading causes of death among people with diabetes. The risk increases the longer blood sugar remains poorly controlled, and it compounds with other factors like high blood pressure and high cholesterol.
What Blood Sugar Numbers Mean
Normal fasting blood sugar falls below 100 mg/dL. A reading between 100 and 125 mg/dL indicates prediabetes, and 126 mg/dL or higher on two separate tests is the diagnostic threshold for diabetes. After meals, blood sugar in a healthy range typically stays below 140 mg/dL.
If your blood sugar is consistently above 200 mg/dL, something beyond diet and lifestyle may need attention. That level often signals the need for a medication adjustment or further evaluation. Readings above 250 mg/dL with symptoms like nausea, vomiting, or confusion suggest a possible emergency, especially if you have type 1 diabetes or are feeling progressively worse.
Bringing High Blood Sugar Down Safely
Water is your first line of defense. Your kidneys filter excess sugar out through urine, so staying well hydrated helps your body flush glucose more efficiently. Stick with water rather than juice, soda, or other drinks that add more sugar to the problem.
Physical activity also lowers blood sugar directly. When you exercise, your muscles pull glucose from your bloodstream and use it for energy, which brings levels down without requiring extra insulin. Aim for at least 150 minutes of moderate exercise per week, broken into smaller sessions if needed, along with strength training twice a week. That said, if your blood sugar is very high (above 250 mg/dL) and you have ketones present, exercise can actually make things worse. Check with your care team about safe thresholds for activity.
Some medications, particularly steroids, can raise blood sugar significantly. If you’re started on a new medication and notice your levels climbing, that connection is worth flagging with your doctor. You may need temporary blood sugar monitoring or an adjustment to your diabetes treatment.
For people already on insulin or blood sugar-lowering medications, a high reading sometimes means the current dose or timing isn’t working well enough. Consistent readings above 200 mg/dL are a signal to reach out to your care team rather than trying to manage the pattern on your own.