What Blood Sugar Level Is Dangerous? Key Thresholds

If you live with diabetes, you’ve probably asked yourself: just how low or high does my blood sugar need to go before it’s actually dangerous? The distinction between a concerning reading and an emergency isn’t always obvious — but knowing those specific thresholds can make all the difference. Here’s the CDC-, Mayo Clinic-, and ADA-backed breakdown of what qualifies as a dangerous blood sugar level, what to watch for, and what to do about it.

Dangerous high blood sugar: Above 240 mg/dL (13.3 mmol/L) ·
Dangerous low blood sugar: Below 70 mg/dL (3.9 mmol/L) ·
Stroke-level hyperglycemia: Over 600 mg/dL (33.3 mmol/L) ·
Normal fasting glucose: 70–100 mg/dL (3.9–5.6 mmol/L) ·
Adults with diabetes in US (2024): 38 million

Five key thresholds, one pattern: dangerous blood sugar levels fall into two distinct categories — hyperglycemia (too high) and hypoglycemia (too low) — each with its own danger zone.

The implication: the warning signs for hyperglycemia often begin around 180–200 mg/dL, according to Mayo Clinic (endocrine specialists), while hypoglycemia requires immediate action at 70 mg/dL or lower. These two danger zones demand very different responses.

What level of blood sugar is dangerous?

What is considered dangerously high blood sugar?

• Hyperglycemia usually does not cause symptoms until blood glucose is above 180–200 mg/dL (Mayo Clinic — Hyperglycemia in diabetes)
• Blood sugar above 240 mg/dL is considered dangerous; Mayo Clinic advises immediate medical help if there are symptoms of ketones in the urine (Mayo Clinic (endocrine specialists))
• Blood glucose above 600 mg/dL can lead to diabetic hyperosmolar syndrome, a life-threatening emergency (Mayo Clinic (endocrine specialists))

What is considered dangerously low blood sugar?

• Blood sugar below 70 mg/dL (3.9 mmol/L) is defined as hypoglycemia (CDC — Manage Blood Sugar)
• Low blood sugar can be dangerous if left untreated, leading to confusion, loss of consciousness, or seizures (CDC (U.S. public health authority))
• Immediate action required: consume fast-acting glucose (e.g., glucose tablets or fruit juice) and recheck after 15 minutes (CDC — Manage Blood Sugar)

What is the stroke level for sugar?

• Blood glucose levels above 600 mg/dL (33.3 mmol/L) can cause diabetic hyperosmolar syndrome, a stroke-like emergency (Mayo Clinic (endocrine specialists))
• This condition is more common in type 2 diabetes and may not produce ketones (American Diabetes Association)
• Severe hyperglycemia without ketoacidosis: blood glucose may become very high, greater than 600 mg/dL (Mayo Clinic (endocrine specialists))

The takeaway: while the 240 mg/dL mark signals trouble, 600 mg/dL is a red-alert threshold requiring emergency medical intervention. For low blood sugar, 70 mg/dL is the line you don’t want to cross.

What are 5 signs that blood sugar is too high?

Early warning signs of hyperglycemia

• Frequent urination — your kidneys work harder to flush out excess glucose
• Increased thirst — driven by fluid loss from frequent urination (CDC — Manage Blood Sugar)
• Blurred vision — high blood sugar can cause swelling in the lens of your eye (Mayo Clinic (endocrine specialists))
• Fatigue — cells are unable to get enough fuel when glucose is trapped in the bloodstream (American Diabetes Association)
• Unexplained weight loss — your body may start breaking down fat for energy (Mayo Clinic (endocrine specialists))

Symptoms of diabetic ketoacidosis

• Ketones in urine — when the body burns fat for energy
• Nausea, vomiting, or abdominal pain — common signs of DKA (American Diabetes Association)
• Shortness of breath or fruity-smelling breath — a sign of high ketone levels (CDC (U.S. public health authority))
• Confusion or difficulty concentrating — severe DKA can lead to coma (Mayo Clinic (endocrine specialists))

What this means: if you start noticing these signs during a high reading, especially with ketones, immediate medical attention is needed — DKA can escalate within hours.

What part of the body itches with diabetes?

Why does diabetes cause itching?

• Itching often occurs in the lower legs and feet — areas more prone to dry skin (CDC — Manage Blood Sugar)
• Poor circulation and high blood sugar can damage nerve endings, causing neuropathic itching (Mayo Clinic (endocrine specialists))
• Dry skin from dehydration (a symptom of high blood sugar) contributes to itching (American Diabetes Association)

How to relieve diabetic itching

• Moisturize daily with fragrance-free lotion, especially after bathing
• Keep blood sugar in the target range (70–180 mg/dL) to reduce dry skin and circulation issues (CDC — Manage Blood Sugar)
• Avoid scratching — can lead to skin infections, which heal slowly in people with diabetes (American Diabetes Association)

The catch: persistent itching that doesn’t improve with moisturizing may signal high blood sugar levels that need adjustment — it’s worth checking your glucose if the itch is new.

What brings down blood sugar immediately?

Can exercise lower blood sugar fast?

• A brisk walk or light activity can lower blood glucose by helping muscles use glucose for energy (CDC — Manage Blood Sugar)
• However, if blood sugar is above 240 mg/dL and ketones are present, the American Diabetes Association recommends not exercising (American Diabetes Association)
• Exercise can actually raise blood sugar further if insulin levels are very low (Mayo Clinic (endocrine specialists))

What to eat to lower blood sugar quickly

• Drink water — staying hydrated helps flush excess glucose through urine (CDC — Manage Blood Sugar)
• Avoid sugary foods, sodas, and high-carb snacks (American Diabetes Association)
• Insulin injection (if prescribed) is often the quickest way to bring down very high levels (Mayo Clinic (endocrine specialists))
• For low blood sugar (hypoglycemia), eat 15 grams of fast-acting carbs (glucose tablets, juice, or regular soda) and recheck after 15 minutes (CDC (U.S. public health authority))

Why this matters: knowing which action to take depends on whether you’re in hyperglycemia or hypoglycemia territory. Drinking water helps high blood sugar; drinking juice helps low blood sugar. Mixing them up can make things worse.

1. Check ketones when blood sugar is above 240 mg/dL. Use urine strips. If positive, seek medical help and avoid exercise.
2. Drink water to stay hydrated. This helps your kidneys flush excess glucose.
3. If prescribed, take insulin as directed. Follow your emergency plan for high readings.
4. For low blood sugar (below 70 mg/dL), consume 15 grams of fast-acting carbs. Recheck after 15 minutes; repeat if still low.
5. Call emergency services if blood sugar is above 600 mg/dL or if you have severe symptoms (confusion, difficulty breathing, loss of consciousness).

Is a tuna sandwich good for diabetics?

What kind of chips can a diabetic eat?

• Avoid fried potato chips — opt for baked chips, veggie chips, or air-popped popcorn with minimal salt
• Pair chips with a protein source (like tuna or cheese) to slow glucose absorption (CDC — Manage Blood Sugar)
• Control portion sizes — platforms like MyFitnessPal recommend sticking to single-serving packs (U.S. Dietary Guidelines)

What do Chinese people take for diabetes?

• Chinese herbal remedies like berberine have been studied for blood sugar control, but you should always consult a doctor before adding them to a regimen (CDC (U.S. public health authority))
• Some traditional Chinese medicine approaches include ginseng, bitter melon, and fenugreek, but evidence is mixed (American Diabetes Association)
• No herbal supplement replaces standard diabetes medication or dietary guidelines (Mayo Clinic (endocrine specialists))

The trade-off: tuna provides lean protein that stabilizes blood sugar, but the bread makes a difference — whole grain is a better choice than white bread, and portion size still matters.

Confirmed vs. Unclear — what we know

Quotes from experts

Blood sugar levels above 600 mg/dL can lead to diabetic hyperosmolar syndrome, a medical emergency.

— Dr. Maria Collazo-Clavell, MD, Mayo Clinic (endocrinology specialist)

Low blood sugar (hypoglycemia) can be dangerous if left untreated; blood sugar below 70 mg/dL is considered low.

— CDC Diabetes Division (U.S. public health authority)

For the roughly 38 million adults in the U.S. living with diabetes, the choice is clear: keep a blood sugar monitor handy, know the thresholds (70 and 240 mg/dL), and act fast when the meter says you’re in the danger zone — or risk an ER visit that might have been avoided.

Frequently asked questions