When your blood sugar rises too high, your body starts showing signs within hours, and if levels stay elevated over months or years, serious damage accumulates in your blood vessels, nerves, eyes, kidneys, and brain. A fasting blood sugar of 126 mg/dL or higher on two separate tests indicates diabetes, while a random reading above 200 mg/dL suggests diabetes regardless of when you last ate. What happens at each stage, from a mild spike to a life-threatening emergency, depends on how high your levels go and how long they stay there.
The First Signs Your Body Sends
The earliest symptoms of high blood sugar revolve around your body trying to flush out the excess glucose. Your kidneys filter more sugar out of the blood and pull water along with it, which is why frequent urination is one of the first things people notice. That fluid loss triggers intense thirst as your body tries to compensate. You may also feel hungrier than usual because despite having plenty of glucose in your bloodstream, your cells can’t use it properly, so your body signals for more fuel.
Beyond those three hallmark signs, high blood sugar commonly causes extreme fatigue, blurry vision, slow healing of cuts and bruises, and tingling or numbness in the hands and feet. Blurry vision happens because excess glucose changes the shape of the lens in your eye by drawing fluid into it. These symptoms can develop gradually enough that people live with them for weeks or months before realizing something is wrong.
What Excess Glucose Does Inside Your Body
High blood sugar doesn’t just sit harmlessly in your bloodstream. When too much glucose enters your cells, it overwhelms the normal energy-producing machinery inside your mitochondria. The result is an overproduction of harmful molecules called superoxide, a type of free radical that damages tissues from the inside out.
This triggers a chain reaction. The excess superoxide jams up a key step in how cells break down glucose, causing sugar byproducts to pile up and spill into several destructive pathways. One pathway creates sticky compounds called advanced glycation end products (AGEs), which essentially “caramelize” proteins in your blood vessel walls, stiffening and damaging them. Another pathway activates signaling molecules that increase inflammation. A third depletes an important antioxidant your cells need for protection. All of these processes hit the delicate lining of your blood vessels hardest, which is why so many complications of high blood sugar are ultimately problems of circulation.
Short-Term Emergencies
If blood sugar climbs high enough without treatment, two dangerous conditions can develop. Both require emergency care.
Diabetic Ketoacidosis (DKA)
DKA happens most often in people with type 1 diabetes, though it can occur in type 2 as well. When the body can’t get glucose into cells (usually because of insufficient insulin), it starts burning fat for fuel at an accelerated rate. This produces acids called ketones, which build up in the blood and make it dangerously acidic. DKA is diagnosed when blood sugar is 200 mg/dL or higher, ketone levels are significantly elevated, and the blood becomes too acidic. In severe cases, it can cause confusion, stupor, or coma. Symptoms often include nausea, vomiting, abdominal pain, and a distinctive fruity smell on the breath.
Hyperosmolar Hyperglycemic Syndrome (HHS)
HHS is more common in type 2 diabetes and involves blood sugar levels that can soar to more than 10 times the normal amount, sometimes exceeding 600 mg/dL. Unlike DKA, there’s little or no ketone buildup. Instead, the extreme blood sugar causes massive dehydration as the kidneys work overtime to expel glucose. Early symptoms include increased thirst and urination, but as dehydration worsens, it can progress to fever, confusion, seizures, and coma. HHS develops more slowly than DKA, often over days or weeks, which means it’s sometimes caught late.
Damage to Your Eyes
Chronically high blood sugar weakens the tiny blood vessels in the retina, the light-sensitive tissue at the back of your eye. In the earliest stage, small balloon-like bulges form in these capillaries (called microaneurysms), which can leak fluid or blood. As the damage progresses, the retina responds by growing new, fragile blood vessels that are even more prone to bleeding. This advanced stage, called proliferative retinopathy, can cause severe vision loss. Swelling in the central part of the retina (macular edema) is another common consequence. Diabetic retinopathy is the leading cause of new blindness in working-age adults, and it often has no symptoms until significant damage has already occurred.
Damage to Your Kidneys
Your kidneys contain millions of tiny filtering units, and high blood sugar gradually thickens and scars the membranes inside them. Over time, the filters become less effective, allowing protein to leak into the urine (an early warning sign) while the overall filtering rate declines. This progressive damage is called diabetic nephropathy, and if it advances far enough, the kidneys can no longer clean the blood adequately, eventually requiring dialysis or a transplant. Regular urine and blood tests can catch early kidney damage years before symptoms appear.
Damage to Your Nerves
Nerve damage from high blood sugar, called diabetic neuropathy, results from a combination of poor blood flow to the nerves and the direct toxic effects of excess glucose on nerve cells. The most common form affects the feet and hands in a “stocking-glove” pattern, starting at the toes and fingers and gradually moving inward. People typically feel tingling, burning, numbness, or pain. Some lose the ability to sense temperature or detect injuries, which is why foot wounds in diabetes can go unnoticed and become seriously infected.
When larger nerve fibers are affected, muscle weakness, loss of balance, and diminished reflexes can develop. Foot muscles may weaken enough to change the shape of the foot, creating pressure points that lead to ulcers. Neuropathy can also strike the nerves that control automatic body functions like heart rate, digestion, bladder emptying, and blood pressure regulation, leading to a wide range of symptoms from chronic nausea to dizziness when standing up.
Heart, Brain, and Blood Vessel Risk
People with diabetes have twice the risk of heart disease compared to people without it. The blood vessel damage caused by high glucose accelerates the buildup of fatty plaques in arteries throughout the body. Two protective enzymes that normally keep arteries flexible and resistant to clots are directly shut down by the superoxide that high blood sugar generates. This makes heart attack and stroke significantly more likely.
The brain is particularly vulnerable. High blood sugar damages the small blood vessels that deliver oxygen to brain tissue. When parts of the brain receive too little blood, cells die, causing problems with memory and thinking that can worsen over time into vascular dementia. Diabetes is also linked to an increased risk of Alzheimer’s disease, mood changes, and difficulty learning new information. Frequent episodes of hyperglycemia appear to compound this stress on the brain even in the short term.
What You Can Do When Blood Sugar Is High
If you check your blood sugar and find it elevated, the most effective responses are straightforward. Physical activity helps because working muscles pull glucose out of the bloodstream for energy. Even a 15 to 30 minute walk can make a meaningful difference. Drinking water helps your kidneys flush excess glucose through urine and prevents the dehydration that high blood sugar causes. If you take insulin or other diabetes medication, following your prescribed correction plan is essential.
Food choices matter more than most people realize. Eating more than your body can process at one sitting is one of the most common triggers for a blood sugar spike. Coordinating meals with medication timing, choosing foods that release glucose slowly (fiber-rich vegetables, whole grains, proteins), and watching portion sizes all help prevent highs from happening in the first place.
If your blood sugar stays stubbornly above your target range despite these measures, your medication may need adjustment. Persistent readings above 200 mg/dL, especially paired with symptoms like nausea, vomiting, confusion, or an inability to keep food down, signal that something more serious may be developing and needs prompt medical attention.