When blood sugar climbs too high, it triggers a chain of problems that starts with symptoms you can feel right away and, if it stays elevated over time, leads to damage in your blood vessels, nerves, kidneys, eyes, and brain. A normal pre-meal blood sugar falls between 80 and 130 mg/dL, and anything consistently above 180 mg/dL two hours after eating is considered too high. What happens next depends on how high it goes and how long it stays there.
Why High Blood Sugar Makes You Thirsty and Exhausted
The earliest symptoms of high blood sugar are frequent urination and intense thirst, and they’re directly connected. Your kidneys filter glucose out of the blood, but they can only reabsorb so much. When blood sugar exceeds their capacity, the excess glucose spills into your urine. Because glucose is a solute, it pulls water along with it through osmosis, producing a much higher volume of urine than normal. That fluid loss is what makes you dehydrated and deeply thirsty, sometimes within hours of a spike.
This cycle also explains the fatigue. Your cells rely on glucose for energy, but when blood sugar is high because insulin isn’t doing its job properly, that glucose stays trapped in the bloodstream instead of entering cells. You’re surrounded by fuel you can’t use. The result is persistent tiredness, brain fog, and sometimes irritability, even if you’ve eaten recently.
Other Symptoms You Might Notice
Beyond thirst and fatigue, high blood sugar can cause blurred vision. Excess glucose causes the lenses of your eyes to swell with fluid, physically changing their shape and distorting your focus. This is usually temporary and resolves once blood sugar comes back down, but it can be alarming if you don’t know what’s causing it.
Other common signs include headaches, difficulty concentrating, unexplained weight loss (especially in type 1 diabetes, where the body starts breaking down fat and muscle for energy), slow-healing cuts, and frequent infections. Some people notice dry mouth, nausea, or fruity-smelling breath, which signals the body is producing ketones, a warning sign of a more dangerous situation.
When It Becomes an Emergency
If blood sugar climbs above 240 mg/dL, the risk of a serious complication called diabetic ketoacidosis (DKA) increases significantly. DKA happens when the body, starved for usable glucose, breaks down fat at an accelerated rate. This produces ketones, acidic byproducts that build up in the blood and can poison your organs.
Blood ketone levels between 1.6 and 2.9 mmol/L put you at high risk for DKA and warrant an emergency room visit. Levels above 3.0 mmol/L are very high risk. Symptoms of DKA include vomiting, abdominal pain, rapid breathing, confusion, and that distinctive fruity breath. Without treatment, DKA can lead to coma and death. It develops most often in people with type 1 diabetes but can occur in type 2 as well, particularly during illness or infection.
If your blood sugar is 240 mg/dL or higher and you feel unwell, checking for ketones with an over-the-counter test kit can help you decide how urgently to act. And if your blood sugar exceeds 300 mg/dL, do not exercise to try to bring it down. At that level, exercise can actually raise blood sugar further and accelerate ketone production.
How High Blood Sugar Damages Blood Vessels
Chronically elevated blood sugar doesn’t just cause symptoms you can feel. It quietly damages the lining of your blood vessels through a process driven by oxidative stress and inflammation. When glucose levels stay high, the body produces an excess of reactive oxygen species, unstable molecules that overwhelm your cells’ natural defenses. At the same time, glucose molecules bind to proteins in the blood, forming compounds called advanced glycation end-products. These modified proteins latch onto receptors on blood vessel walls and trigger inflammatory signals, attracting immune cells and promoting the buildup of arterial plaque.
This is why people with poorly controlled diabetes face a dramatically higher risk of heart attack and stroke. The inflammation isn’t limited to large arteries. It affects the tiny blood vessels that supply your kidneys, eyes, and nerves, which is why those organs are especially vulnerable to long-term damage.
Nerve Damage Starts in Your Feet
Peripheral neuropathy is the most common type of nerve damage from high blood sugar, and it follows a predictable pattern: it affects the feet and legs first, then the hands and arms. Early symptoms include tingling, burning, or numbness. Some people develop sharp pains or cramps. Others lose the ability to sense temperature changes or pain altogether.
In some cases, the sensitivity goes in the opposite direction. Touch becomes amplified to the point where even the weight of a bedsheet on your feet is painful. Over time, the loss of sensation in the feet creates a dangerous blind spot. Small cuts or blisters go unnoticed, infections develop, and without treatment, these can progress to ulcers and serious bone and joint damage. This is why foot complications are one of the most common reasons for hospital visits among people with diabetes.
Kidney Damage
Your kidneys filter about 50 gallons of blood per day through millions of tiny blood vessels. Chronic high blood sugar damages those vessels, gradually reducing the kidneys’ ability to do their job. The earliest sign is protein leaking into the urine, something that isn’t normally supposed to happen. When persistent protein levels exceed 300 mg per day on repeated tests, the diagnosis is diabetic nephropathy.
The tricky part is that kidney damage from high blood sugar is painless and produces no obvious symptoms in its early stages. By the time you feel something is wrong, such as swelling in the legs, fatigue, or changes in urination, significant damage has already occurred. This is why routine urine and blood tests are so important for anyone living with elevated blood sugar over time.
Effects on Your Eyes
The temporary blurred vision from a blood sugar spike is one thing. Long-term high blood sugar causes a separate, more serious condition called diabetic retinopathy, where the small blood vessels in the retina become damaged, leak, or grow abnormally. Over years, this can lead to permanent vision loss. Like kidney damage, early retinopathy has no symptoms. It’s detected through dilated eye exams, not by changes in how well you see.
Effects on Your Brain
High blood sugar damages blood vessels in the brain the same way it damages them everywhere else. Over time, reduced blood flow means brain cells receive less oxygen, and some die. This can cause problems with memory and thinking that worsen gradually, eventually progressing to vascular dementia in severe cases. Frequent episodes of hyperglycemia also stress the brain in the short term, contributing to mood shifts, difficulty concentrating, and irritability that many people with poorly managed blood sugar experience day to day.
Bringing Blood Sugar Down Safely
If your blood sugar is elevated but below 300 mg/dL and you have no ketones, physical activity is one of the most effective ways to lower it. Walking for 15 to 30 minutes can make a noticeable difference because working muscles pull glucose directly from the bloodstream. Drinking water helps too, since it supports the kidneys in flushing out excess glucose through urine.
For people on insulin or blood sugar-lowering medications, a correction dose may be appropriate based on the plan you’ve worked out with your care team. The key is knowing your numbers. A blood glucose meter or continuous glucose monitor gives you real-time information so you can act before levels reach dangerous territory. Keeping blood sugar within target range, even imperfectly, dramatically reduces the risk of every complication described above. The damage from high blood sugar is cumulative, which means every hour spent in range counts.