When blood sugar climbs too high, your body starts sending distress signals: excessive thirst, frequent urination, headaches, and blurred vision. Many people with diabetes don’t notice symptoms until their blood sugar reaches 250 mg/dL or higher, which means damage can start building before you feel anything obvious. What happens next depends on how high the sugar goes, how long it stays elevated, and whether your body can still produce some insulin.
The First Signs You’ll Notice
The earliest symptoms of high blood sugar tend to revolve around fluid. When glucose builds up in your bloodstream, your kidneys try to flush the excess out through urine. That pulls water from your body, which is why you pee more often and feel intensely thirsty. You might also feel hungrier than usual, because even though there’s plenty of glucose in your blood, your cells can’t absorb it properly without enough working insulin. It’s fuel your body can’t access.
Headaches and blurred vision often follow. The blurred vision happens because excess glucose changes the fluid balance in the lens of your eye, temporarily warping its shape. These symptoms can come on gradually, especially in type 2 diabetes, making them easy to dismiss as tiredness or dehydration from another cause.
What Happens Inside Your Body
Excess glucose in the bloodstream is surprisingly destructive. Over hours and days of elevated sugar, glucose molecules latch onto proteins in your blood and tissues, creating compounds called advanced glycation end products. These compounds stiffen blood vessel walls and trigger inflammation, particularly in the smallest blood vessels that feed your eyes, kidneys, nerves, and heart.
Your nerves take a direct hit, too. High glucose appears to damage the protective coating around nerve fibers, a process similar to insulation wearing off electrical wiring. This affects both the nerves that control sensation in your hands and feet and the ones that regulate automatic functions like heart rate and digestion. The damage builds silently over months and years of repeated high sugar episodes.
The brain is also vulnerable. Repeated spikes in blood sugar stress the blood vessels that deliver oxygen to brain tissue. When those vessels are damaged, brain cells can die. The CDC notes that these effects happen over time and aren’t obvious right away, meaning people often don’t realize their brain is being affected until problems with memory and thinking emerge.
How Dehydration Escalates Quickly
The cycle of water loss from high blood sugar can accelerate into something dangerous. As your kidneys dump more glucose into your urine, they pull increasing amounts of water along with it. This is called osmotic diuresis, and it can leave you severely dehydrated even if you’re drinking fluids. The dehydration concentrates your blood further, raising the concentration of glucose, sodium, and other substances. That concentrated blood then pulls water out of your organs, including your brain, which can cause confusion, drowsiness, and eventually loss of consciousness.
This is the mechanism behind hyperosmolar hyperglycemic state (HHS), a complication most common in type 2 diabetes. Blood sugar in HHS can climb to more than 10 times the normal level, sometimes exceeding 600 mg/dL. The hallmarks are extreme dehydration, very high blood sugar, and decreased alertness. HHS develops over days or weeks and is a medical emergency.
When the Body Starts Burning Fat for Fuel
In type 1 diabetes, and sometimes in type 2, the body may have so little working insulin that cells can’t use glucose at all. Starved for energy, the body switches to breaking down fat. That process produces acids called ketones, which build up in the blood and make it dangerously acidic. This is diabetic ketoacidosis, or DKA.
DKA typically develops when blood sugar is above 250 mg/dL, though the real danger comes from the acid buildup rather than the glucose number alone. Symptoms include nausea, vomiting, abdominal pain, a fruity smell on the breath, and rapid breathing as the body tries to blow off excess acid. Without treatment, DKA can progress to coma and death within hours.
If you have diabetes and your blood sugar is 250 mg/dL or above, the CDC recommends checking your blood sugar every four to six hours and testing your urine for ketones. The presence of ketones at high blood sugar levels is the signal that your body is tipping toward DKA, and it’s time to get medical help.
The Long-Term Cost of Repeated Highs
A single blood sugar spike won’t cause lasting damage in most cases. But chronically elevated glucose, the kind that shows up as a high A1C over months, steadily wears down your body’s most vulnerable tissues.
- Eyes: High sugar damages the tiny blood vessels at the back of the eye, leading to diabetic retinopathy and eventual vision loss. This is the leading cause of new blindness in working-age adults.
- Kidneys: The filtering units in the kidneys are packed with small blood vessels. Chronic high glucose reduces their ability to filter waste, which can progress to chronic kidney disease over years.
- Nerves: Numbness, tingling, or pain in the feet and hands is one of the most common complications. It can make daily activities difficult and, in the feet, can prevent you from noticing injuries.
- Feet: Reduced blood flow combined with nerve damage means wounds heal slowly and are more likely to become infected. This combination is the primary reason diabetes leads to amputations.
- Brain: Damaged blood vessels in the brain can lead to vascular dementia over time, with gradual problems in memory, attention, and thinking speed.
How Your Body Recovers After a Spike
After a high blood sugar episode, recovery depends on the cause and your treatment approach. If you took a correction dose of insulin or your medication kicked in, blood sugar can start dropping within an hour or two. But the body doesn’t snap back to baseline immediately. You may feel fatigued, foggy, or washed out for several hours after a spike, partly because dehydration takes time to reverse and partly because the rapid glucose shift itself is taxing.
Morning highs are a common pattern that can keep blood sugar elevated for hours if not addressed, creating a daily window of damage. If you’re seeing regular spikes at specific times of day, that’s a pattern worth tracking, because bringing down those predictable highs is one of the most effective ways to protect your blood vessels and nerves over the long term.
The Numbers That Matter
Normal fasting blood sugar is below 100 mg/dL. A fasting level of 126 mg/dL or higher on two separate tests meets the diagnostic threshold for diabetes. After meals, a reading of 200 mg/dL or higher is also diagnostic.
For people already living with diabetes, many won’t feel symptoms until blood sugar passes 250 mg/dL, which is why routine monitoring matters more than waiting for symptoms. By the time you feel thirsty, headachy, and blurry-eyed, your sugar has likely been elevated for a while. Above 250 mg/dL, it’s time to check for ketones. Above 600 mg/dL, you’re in the territory where HHS becomes a real risk, especially if you’re older or fighting an illness. These aren’t sharp cutoffs. They’re ranges where the danger ramps up, and the body’s ability to self-correct becomes increasingly overwhelmed.