A blood sugar level above 250 mg/dL is the point where serious short-term danger begins, and anything above 300 mg/dL needs urgent medical attention. But “dangerous” depends on the timeframe you’re thinking about. A single spike to 200 mg/dL after a large meal is very different from a fasting level that stays at 200 mg/dL for months. Both carry risks, just on different timelines. Here’s how to understand the numbers and know when to act.
Normal, Elevated, and Diabetic Ranges
Blood sugar levels are measured in two main ways: fasting (no food for at least 8 hours) and after eating. The numbers that matter are different for each.
For fasting blood sugar, below 100 mg/dL is normal. Between 100 and 125 mg/dL falls into the prediabetes range. If your fasting blood sugar hits 126 mg/dL or higher on two separate tests, that meets the diagnostic threshold for diabetes.
After eating (or during a glucose tolerance test), the cutoffs are higher because your body is actively processing sugar. Below 140 mg/dL is normal. Between 140 and 199 mg/dL indicates prediabetes. A reading of 200 mg/dL or above means diabetes.
These ranges describe chronic risk rather than immediate danger. A fasting level of 130 mg/dL won’t send you to the emergency room today, but if it stays there for years, it quietly damages blood vessels, nerves, and organs.
The 240–250 mg/dL Warning Zone
Once blood sugar climbs above 240 to 250 mg/dL, you’ve crossed into territory where your body may start producing ketones. Ketones are acids that build up when your cells can’t get enough glucose for energy and start burning fat instead. In small amounts they’re harmless, but in large amounts they make your blood dangerously acidic.
The CDC recommends checking for ketones with an over-the-counter urine or blood test kit whenever your blood sugar is 240 mg/dL or above, especially if you’re feeling sick. The Joslin Diabetes Center advises testing for ketones anytime your blood sugar reads above 250 mg/dL on two consecutive checks, or before exercising when levels are above 250 mg/dL. If you’re pregnant, check for ketones any time your reading exceeds 250 mg/dL.
High ketone levels at this blood sugar range are an early warning sign of diabetic ketoacidosis (DKA), which can become life-threatening within hours if left untreated.
Above 300 mg/dL: When It Becomes an Emergency
Blood sugar above 300 mg/dL is a medical emergency in the making, particularly if it’s not coming down with your usual management. At this level, your body is under significant metabolic stress, and the risk of two dangerous complications rises sharply.
The first is diabetic ketoacidosis, or DKA. This typically occurs in people with type 1 diabetes but can happen with type 2 as well. Blood sugar in DKA usually runs between 350 and 500 mg/dL. The blood becomes acidic, and without treatment, it can progress to a diabetic coma. Early symptoms include fruity-smelling breath, nausea, vomiting, abdominal pain, and confusion. If you notice these, call 911 or get to an emergency room.
The second is hyperglycemic hyperosmolar state, or HHS, which is more common in type 2 diabetes and tends to develop gradually over days or weeks. Blood sugar in HHS often exceeds 600 mg/dL and can climb past 800 mg/dL. The extreme blood sugar causes severe dehydration as the kidneys work overtime to flush out glucose. HHS doesn’t always produce ketones, which means you may not get the early warning signs that come with DKA. Instead, the main symptoms are extreme thirst, very frequent urination, confusion, and eventually loss of consciousness. Both DKA and HHS can be fatal without hospital treatment.
Symptoms That Signal Trouble
Moderately high blood sugar (in the 180 to 250 mg/dL range) often announces itself with increased thirst, frequent trips to the bathroom, blurred vision, and unusual fatigue. These are your body’s early signals that glucose is building up faster than your system can handle it.
The symptoms that indicate a true emergency are different and more severe:
- Fruity-smelling breath, which signals ketone buildup
- Persistent nausea or vomiting, especially if you can’t keep fluids down
- Abdominal pain
- Shortness of breath
- Confusion or difficulty staying alert
- Loss of consciousness
If you’re experiencing any of these alongside a blood sugar reading above 240 mg/dL, that combination warrants emergency care. The inability to keep food or fluids down is especially dangerous because it accelerates dehydration and makes it impossible to manage blood sugar on your own.
How Sustained High Blood Sugar Damages the Body
You don’t need to hit 300 mg/dL for high blood sugar to be “dangerous.” Levels that stay chronically elevated, even in the 150 to 200 mg/dL range, cause slow but real damage over months and years. The mechanism is straightforward: excess glucose in the bloodstream injures the walls of small blood vessels throughout the body. The organs that depend most on tiny blood vessels take the biggest hit.
Your eyes are particularly vulnerable. High blood sugar damages the small vessels in the retina, leading to diabetic retinopathy. Over time this can cause fluid buildup in the central part of the retina (affecting sharp, detailed vision), increase the risk of cataracts, and raise pressure inside the eye. All of these can lead to vision loss.
Your kidneys filter blood through millions of tiny vessels, and sustained high glucose damages them gradually. Early kidney damage often has no symptoms at all. It’s usually caught through a urine test that detects small amounts of protein leaking through damaged filters. Without intervention, this can progress to kidney failure.
Nerve damage is another common consequence. High blood sugar impairs the nerves in your feet, legs, hands, and arms, causing tingling, numbness, or pain. It can also affect the nerves that control digestion, heart rate, and other automatic body functions. Once nerve damage is established, it’s difficult to reverse.
The general target for long-term blood sugar management is an HbA1c of 7% or lower for most adults with diabetes. HbA1c reflects your average blood sugar over the previous two to three months, so it captures the sustained exposure that drives these complications.
What to Do at Each Level
If your blood sugar is between 180 and 250 mg/dL, this is the time for your standard management plan: check your medication timing, drink water, avoid additional carbohydrates, and recheck in a few hours. If you don’t have a diabetes diagnosis and you’re seeing numbers in this range, that’s worth a call to your doctor.
At 250 mg/dL or above on two consecutive readings, test for ketones using an over-the-counter kit (available at most pharmacies). If ketones are negative and you feel okay, continue monitoring every three to four hours. If ketones are positive, contact your healthcare provider right away.
At 300 mg/dL or above, especially with any emergency symptoms like vomiting, confusion, or fruity breath, go to the emergency room. Don’t wait to see if it comes down on its own. At this level, the window between “manageable” and “life-threatening” can close quickly.