High blood sugar causes problems on two timescales: immediate symptoms that show up within hours or days, and slow, cumulative damage to blood vessels and nerves that builds over months and years. Symptoms typically don’t appear until blood sugar rises above 180 to 200 mg/dL, but damage to tissues can accumulate even at levels below that threshold if they stay elevated consistently.
Early Symptoms You’d Notice First
The first signs of high blood sugar are frequent urination, increased thirst, blurred vision, and unusual fatigue or weakness. These happen because your kidneys work harder to filter out the excess glucose, pulling more water with it. That fluid loss makes you dehydrated and thirsty, and the shifting fluid levels in your eyes temporarily distort your vision.
These symptoms can be easy to dismiss individually. Feeling tired and drinking more water doesn’t immediately scream “blood sugar problem.” But when they show up together and persist, they’re a reliable signal that glucose levels have been running too high.
When High Blood Sugar Becomes an Emergency
If blood sugar stays very high without treatment, two dangerous conditions can develop. Both are medical emergencies.
Diabetic ketoacidosis (DKA) happens when the body can’t use glucose for energy and starts breaking down fat instead, producing toxic acids called ketones. It’s most common in people with type 1 diabetes but can also occur in type 2. Warning signs include fruity-smelling breath, abdominal pain, nausea and vomiting, shortness of breath, and confusion. Without treatment, it can lead to loss of consciousness.
Hyperosmolar syndrome is more common in type 2 diabetes and develops when blood sugar climbs extremely high, sometimes more than 10 times the normal amount. The body loses massive amounts of fluid through urination, leading to severe dehydration. Symptoms progress from increased thirst and weakness to fever, confusion, speech problems, seizures, and coma. This condition develops more slowly than DKA, often over days or weeks, but it carries a high risk of death if untreated.
How Glucose Damages Your Body Over Time
The long-term harm from high blood sugar comes down to a chemical reaction between glucose and the proteins that make up your tissues. When there’s too much glucose in the blood, it latches onto proteins in blood vessel walls, nerves, and organs. These sugar-coated proteins, once formed, are nearly irreversible. They stiffen and distort the structures they’re part of, particularly collagen and elastin, the proteins that keep blood vessels flexible.
This stiffening is why high blood sugar damages so many different parts of the body. Anywhere blood flows, glucose can slowly degrade the vessels that carry it. The smallest blood vessels are the most vulnerable, which is why the eyes, kidneys, and nerve endings are hit first and hardest. But large vessels feeding the heart and brain suffer too.
Damage to Your Eyes
High blood sugar injures the tiny blood vessels in the retina, the light-sensitive layer at the back of your eye. In the earlier stage, called nonproliferative retinopathy, those small vessels start to leak fluid into the retina. Your body tries to repair them, but the damaged vessels eventually close off entirely.
When enough vessels shut down, the retina becomes starved for blood flow. In response, your body grows new blood vessels to compensate. This sounds helpful, but these replacement vessels are fragile and poorly formed. They leak blood and fluid into the gel-like interior of the eye. Over time, scar tissue forms around them and can pull the retina away from the back of the eye, leading to blindness. This progression from leaking vessels to abnormal new growth to potential retinal detachment can take years, and early stages often have no symptoms at all.
Damage to Your Kidneys
Your kidneys filter your blood through millions of tiny blood vessel clusters. High blood sugar damages these filters, and the first sign is a protein called albumin leaking into your urine. A healthy kidney keeps almost all albumin in the blood, with less than 30 mg/g showing up in a urine test. Anything above that level can indicate kidney disease, even when overall kidney function still appears normal by other measures.
This is important because kidney damage from high blood sugar is silent for a long time. By the time you notice symptoms like swelling in your legs or fatigue, significant damage has already occurred. Routine urine tests are the only way to catch it early, which is why they’re a standard part of diabetes care.
Damage to Your Nerves
High blood sugar harms nerves in two ways: directly, through the same glucose-protein reaction that damages blood vessels, and indirectly, by weakening the tiny capillaries that supply nerves with oxygen and nutrients. The result is diabetic neuropathy, which affects roughly half of all people with diabetes over time.
The most common form hits the feet and legs first, then the hands and arms. Symptoms are often worse at night and include numbness, tingling or burning sensations, sharp pains or cramps, and muscle weakness. Some people become so sensitive that even the weight of a bedsheet causes pain. Others lose sensation entirely, which creates its own danger: injuries to the feet go unnoticed, leading to infections and, in severe cases, amputations.
Other patterns of nerve damage can cause weakness in the hands that makes you drop things, deep pain in the hip or thigh with shrinking thigh muscles, or pain in the chest and abdomen. The type and location depend on which nerves are affected.
Damage to Your Heart and Blood Vessels
Heart disease is the leading cause of death for people with diabetes, and the connection is direct. Research from Johns Hopkins found that each 1-percentage-point increase in A1c (a measure of average blood sugar over three months) was associated with a 14 percent increase in coronary heart disease risk. That means someone with an A1c of 9% faces meaningfully higher cardiac risk than someone at 7%, even though both numbers fall in the “diabetic” range.
The mechanism is the same vessel-stiffening process that affects small blood vessels, just applied to larger ones. Glucose-damaged proteins in artery walls make them rigid and more prone to plaque buildup. This raises the risk of heart attack, stroke, and peripheral artery disease, where reduced blood flow to the legs causes pain with walking and slow-healing wounds. The cardiovascular effects of high blood sugar compound with other common risk factors like high blood pressure and high cholesterol, which is why managing all three together matters so much.
How Quickly Does Damage Happen
The acute emergencies, DKA and hyperosmolar syndrome, can develop in hours to days. Long-term complications typically take years of poorly controlled blood sugar to become noticeable, but the underlying damage begins much sooner. Small vessel changes in the retina, for example, can start within a few years of consistently elevated glucose.
The encouraging side of this is that lowering blood sugar slows or stops the progression of most complications. Nerve symptoms may partially improve, kidney decline can be slowed significantly, and the risk of eye disease progression drops substantially with better glucose control. The damage from glucose binding to proteins is nearly irreversible once it happens, but preventing new damage makes a real difference in outcomes. Earlier control matters more than later control, because the accumulated damage from years of high blood sugar can’t be fully undone.