When blood sugar climbs too high, it can cause problems that range from mild and reversible to severe and life-threatening. In the short term, you may notice increased thirst, frequent urination, blurred vision, and fatigue. Over months and years, persistently elevated blood sugar quietly damages blood vessels, nerves, kidneys, and eyes. Symptoms often don’t appear until blood sugar exceeds 180 to 200 mg/dL, which means damage can accumulate before you feel anything wrong.
Early Symptoms You Might Notice
The first signs of high blood sugar are easy to dismiss. You urinate more often because your kidneys are working overtime to flush excess glucose out of your blood. That fluid loss makes you unusually thirsty. You may feel weak or tired even after a full night’s sleep, and your vision can temporarily blur as shifting fluid levels change the shape of your eye’s lens.
These symptoms tend to build gradually. Many people attribute them to stress, aging, or not drinking enough water. That’s part of what makes high blood sugar dangerous: it can stay elevated for weeks or months without prompting a trip to the doctor. For reference, the American Diabetes Association defines diabetes as a fasting blood sugar of 126 mg/dL or higher, or a random reading of 200 mg/dL or higher.
When It Becomes an Emergency
If blood sugar stays very high without treatment, two serious conditions can develop. Both require emergency medical care.
The first is diabetic ketoacidosis, or DKA. This happens most often in people with type 1 diabetes. When your body can’t use glucose for energy because it lacks insulin, it starts breaking down fat at a rapid pace. That process floods the bloodstream with acids called ketones, which make the blood dangerously acidic. DKA typically involves blood sugar above 250 mg/dL, along with nausea, vomiting, abdominal pain, fruity-smelling breath, and confusion. Without treatment, it can lead to coma.
The second is hyperosmolar hyperglycemic syndrome, which is more common in people with type 2 diabetes. Blood sugar can soar to ten times the normal level. The hallmark is extreme dehydration: dry mouth, rapid heart rate, low blood pressure, and fever. This condition develops over days or weeks and is especially dangerous in older adults who may not recognize their thirst signals or have difficulty staying hydrated.
How High Blood Sugar Damages Blood Vessels
The long-term consequences of high blood sugar come down to what excess glucose does to the walls of your blood vessels. When glucose stays elevated, it sticks to proteins in your blood and tissues through a chemical reaction that creates compounds called advanced glycation end products, or AGEs. These modified proteins accumulate over time, cross-linking with collagen and elastin in vessel walls. The result is stiffer, less flexible arteries.
At the same time, high glucose triggers a chain reaction inside cells that produces reactive oxygen species, essentially unstable molecules that damage the delicate inner lining of blood vessels. This damage makes vessel walls stickier and more prone to inflammation, which accelerates plaque buildup. A self-reinforcing loop develops: the initial damage generates more of these harmful molecules, which causes further damage. This is why people with diabetes develop atherosclerosis (hardening of the arteries) earlier and more extensively than people without it, raising the risk of heart attack and stroke.
Nerve Damage and Loss of Sensation
Nearly half of all people with diabetes develop some form of peripheral neuropathy, which is damage to the nerves in the hands and feet. About 7.5% already have detectable nerve damage at the time they’re first diagnosed. Among younger people diagnosed with type 2 diabetes during childhood or adolescence, roughly 18% develop peripheral neuropathy.
The damage typically starts in the longest nerves first, which is why tingling, numbness, or burning pain in the feet is often the earliest sign. Over time, you may lose the ability to feel cuts, blisters, or pressure sores on your feet. This is a serious problem because unnoticed wounds can become infected and, in severe cases, lead to amputation. High blood sugar can also affect the nerves that control digestion, heart rate, bladder function, and blood pressure, though these effects are harder to quantify. Early signs of autonomic nerve dysfunction can appear within one to two years of a diabetes diagnosis.
Kidney Damage Over Time
Your kidneys contain millions of tiny filtering units, each fed by delicate blood vessels. High blood sugar damages these vessels gradually, reducing the kidneys’ ability to filter waste. One of the earliest detectable signs is albumin leaking into the urine. A healthy kidney lets less than 30 mg/g of albumin through; anything above that, confirmed by two tests over three months or more, signals kidney disease.
The progression from early leakage to significant kidney disease usually takes years, but it’s not always a straight line. Some people’s kidneys deteriorate faster depending on blood pressure, genetics, and how well blood sugar is managed. People with higher levels of albumin in their urine face a greater risk of eventually progressing to kidney failure, which requires dialysis or a transplant.
Vision Changes and Retinopathy
The retina at the back of your eye is packed with small blood vessels that are particularly vulnerable to glucose damage. Diabetic retinopathy progresses through two main stages. In the earlier stage, called nonproliferative retinopathy, the small blood vessels in the retina weaken and leak fluid or blood. Your body tries to repair the damage by closing off the affected vessels, but this reduces blood flow to the retina.
In the more advanced proliferative stage, the retina responds to reduced blood flow by growing new blood vessels. These new vessels are fragile and bleed easily, which can cause sudden vision loss. At any point during either stage, fluid can leak into the central part of the retina (the macula), causing blurry vision that affects reading, driving, and recognizing faces. Retinopathy can be diagnosed as early as five years after diabetes onset, which is why regular eye exams matter even when your vision seems fine.
Increased Vulnerability to Infections
High blood sugar puts extra stress on the immune system. White blood cells, which are your body’s primary defense against bacteria and viruses, function less effectively when blood sugar is elevated. The CDC notes that people with diabetes experience more frequent urinary tract infections and skin infections than the general population.
Healing slows down too. Cuts, scrapes, and surgical wounds take longer to close because the same vascular damage that affects your organs also reduces blood flow to your skin. Combined with nerve damage that prevents you from feeling injuries, this creates a situation where small wounds can quietly become serious infections.
How Quickly Damage Develops
The timeline varies depending on how high blood sugar runs and how consistently it stays elevated. Acute emergencies like DKA can develop in hours to days. Chronic complications follow a slower but relentless course. Subtle nerve dysfunction can appear within one to two years. Retinopathy may be detectable within five years. Kidney disease, heart disease, and advanced neuropathy generally take longer but progress faster when blood sugar, blood pressure, and cholesterol are all poorly controlled.
The encouraging part of this timeline is that blood sugar management at any point can slow or even halt the progression of many of these complications. The damage from high glucose is cumulative, so every period of better control reduces the total burden on your blood vessels, nerves, and organs.