Blood sugar below 70 mg/dL (3.9 mmol/L) is considered low and requires prompt attention. A reading below 55 mg/dL (3.0 mmol/L) is severely low, while blood sugar above 240–250 mg/dL (13.3–13.9 mmol/L) can become dangerous when it occurs with illness, ketones, vomiting, or breathing problems.
There is no single dangerous blood sugar level that applies to everyone. The reading, symptoms, ketone level, duration, medications, and personal diabetes plan all help determine whether urgent treatment is necessary.
Dangerous Blood Sugar Levels
| Blood sugar level | What it may mean | What to do |
| Below 55 mg/dL | Severely low blood sugar | Use glucagon if prescribed and seek emergency help |
| 55–69 mg/dL | Low blood sugar | Treat immediately with fast-acting carbohydrates |
| 70–180 mg/dL | Common target range for many people with diabetes | Continue following your personal care plan |
| Above 180 mg/dL | High blood sugar | Recheck and follow your prescribed correction plan |
| 240–250 mg/dL or higher | Possible risk of ketone buildup | Check ketones, especially during illness |
| Around 600 mg/dL or higher | Possible hyperosmolar hyperglycemic state | Seek emergency medical care |
These numbers are general guidelines. Children, pregnant people, older adults, and people with other medical conditions may have different target and emergency ranges.
Blood Sugar Ranges and What They Mean?
Blood sugar, also called blood glucose, is the main sugar circulating in the bloodstream. It provides energy to the brain, muscles, and other tissues. Insulin helps move glucose from the blood into cells.
For many nonpregnant adults with diabetes, common glucose targets are:
- Before meals: 80–130 mg/dL
- About two hours after a meal begins: Below 180 mg/dL
- Common CGM target range: 70–180 mg/dL
These targets may be adjusted according to age, pregnancy, diabetes type, medications, other health conditions, and the risk of hypoglycemia.
An abnormal result is not always an emergency. For example, a fasting glucose level of 126 mg/dL may meet the diagnostic threshold for diabetes, but it is not usually an immediately dangerous reading.
What Blood Sugar Levels May Indicate Diabetes?
Healthcare professionals use laboratory tests to diagnose prediabetes and diabetes. A home glucose meter can identify concerning patterns, but it cannot confirm a diagnosis by itself.
| Test | Normal | Prediabetes | Diabetes range |
| Fasting plasma glucose | 99 mg/dL or lower | 100–125 mg/dL | 126 mg/dL or higher |
| Two-hour glucose tolerance test | 139 mg/dL or lower | 140–199 mg/dL | 200 mg/dL or higher |
| A1C | Below 5.7% | 5.7–6.4% | 6.5% or higher |
| Random plasma glucose with symptoms | Not established | Not established | 200 mg/dL or higher |
A healthcare provider will usually repeat an abnormal test to confirm diabetes unless symptoms and laboratory findings clearly support the diagnosis.
When Is Low Blood Sugar Dangerous?
A blood sugar level below 70 mg/dL is considered hypoglycemia for most people with diabetes. It should be treated promptly because the glucose level may continue to fall.
A reading below 55 mg/dL is considered severely low. At this level, the brain may not receive enough glucose to function normally. The person may become confused, lose coordination, have a seizure, faint, or become unable to treat themselves.
Some people experience severe symptoms at a higher reading, while others may have few warning signs despite a dangerously low level. This condition is called hypoglycemia unawareness.
Symptoms of Dangerously Low Blood Sugar
Common warning signs include shaking or trembling, sweating, hunger, a fast heartbeat, dizziness, anxiety, irritability, headache, blurred vision, weakness, and confusion. As the glucose level falls further, a person may have difficulty speaking clearly or walking normally.
Severe hypoglycemia may cause unusual behavior, loss of consciousness, seizures, coma, and, in rare cases, death.
What Causes Low Blood Sugar?
Low blood sugar commonly occurs when a person:
- Takes too much insulin
- Takes certain diabetes medicines
- Misses or delays a meal
- Eats fewer carbohydrates than expected
- Exercises more than usual
- Drinks alcohol without enough food
- Has kidney or liver disease
- Becomes ill or loses their appetite
The risk may also increase when a person changes their schedule, medication, diet, or physical activity without adjusting their diabetes plan.
How to Treat Low Blood Sugar?
If the person is conscious and can swallow safely, use the 15-15 rule:
- Consume 15 grams of fast-acting carbohydrates.
- Wait 15 minutes.
- Check the glucose level again.
- Repeat if it remains below 70 mg/dL.
- Eat a balanced snack or meal once the level improves.
Fast-acting options include glucose tablets, glucose gel, four ounces of fruit juice, regular soda, or one tablespoon of sugar or honey.
Chocolate and high-fat foods are not ideal for immediate treatment because fat may slow glucose absorption.
If the person is unconscious, having a seizure, or unable to swallow, do not give food or drink by mouth. Administer glucagon if available and call emergency services immediately.
When Is High Blood Sugar Dangerous?
Blood sugar above 180 mg/dL is high for many people with diabetes, especially when measured about two hours after eating. One elevated reading is not necessarily an emergency, but repeated or rising levels require attention.
A reading of 240–250 mg/dL or higher becomes more concerning when the person is ill or has ketones. People with diabetes may be advised to test for ketones at this level and follow their sick-day plan.
Blood glucose around 600 mg/dL or higher may occur with hyperosmolar hyperglycemic state, or HHS. This life-threatening condition causes severe dehydration and changes in brain function and requires immediate hospital care.
Symptoms of High Blood Sugar
Early symptoms of high blood sugar may include increased thirst, frequent urination, dry mouth, blurred vision, headache, fatigue, weakness, and unexplained weight loss.
As glucose continues to rise, the person may develop severe dehydration, nausea, confusion, drowsiness, seizures, or loss of consciousness.
Some people with long-standing type 2 diabetes may have dangerously high blood sugar without obvious symptoms. Regular glucose monitoring remains important even when the person feels well.
Is Blood Sugar Over 200 Dangerous?
Blood sugar over 200 mg/dL is high, but the seriousness depends on when it was measured, how long it remains elevated, and whether symptoms are present.
A random laboratory glucose result of 200 mg/dL or higher with classic diabetes symptoms may support a diabetes diagnosis. However, a single reading over 200 mg/dL is not automatically an emergency.
Contact a healthcare provider if readings above 200 mg/dL occur repeatedly, especially with excessive thirst, frequent urination, blurred vision, fatigue, or unexplained weight loss.
Is Blood Sugar Over 300 Dangerous?
Blood sugar over 300 mg/dL can be dangerous, particularly if it does not decrease after following the prescribed treatment plan.
At this level, check ketones if your healthcare team has instructed you to do so. Contact your provider urgently if the reading persists or occurs with vomiting, abdominal pain, fruity-smelling breath, breathing difficulty, dehydration, or confusion.
Do not take additional insulin beyond your prescribed correction dose. Taking insulin doses too close together can cause delayed hypoglycemia.
Is Blood Sugar Over 400 Dangerous?
A reading over 400 mg/dL is severely elevated and requires prompt action. Wash and dry your hands, repeat the test, check ketones if possible, and follow your healthcare provider’s emergency instructions.
Seek urgent medical help if the reading remains high or is accompanied by illness, dehydration, vomiting, breathing changes, confusion, or drowsiness.
A glucose meter that displays “HI” may indicate that the level exceeds the device’s measurement range. Treat this as potentially serious and seek immediate medical guidance.
Dangerous High Blood Sugar Emergencies
Diabetic Ketoacidosis
Diabetic ketoacidosis, or DKA, occurs when the body does not have enough insulin. Because glucose cannot enter cells properly, the body rapidly breaks down fat and produces acidic substances called ketones.
DKA is most common in type 1 diabetes, but people with type 2 diabetes may also develop it. Common triggers include illness, infection, missed insulin, an incorrect insulin dose, a blocked insulin pump, surgery, injury, or certain medicines.
Emergency symptoms may include nausea or repeated vomiting, stomach pain, fast or difficult breathing, fruity-smelling breath, severe thirst, frequent urination, extreme tiredness, confusion, or fainting.
DKA can sometimes occur when glucose is below 250 mg/dL, particularly in people taking SGLT2 inhibitor medicines. Symptoms and ketones must therefore be considered alongside the glucose reading.
Hyperosmolar Hyperglycemic State
Hyperosmolar hyperglycemic state, or HHS, usually affects people with type 2 diabetes. Glucose may rise to around 600 mg/dL or higher, causing profound fluid loss and dehydration.
HHS symptoms may include extreme thirst, frequent urination, dry mouth, weakness, fever, confusion, speech problems, seizures, and reduced consciousness. Symptoms may worsen gradually over several days or weeks.
Without prompt treatment, HHS can cause shock, blood clots, coma, or death.
Why Dangerous Blood Sugar Changes Happen?
Blood glucose can become dangerously high when the body lacks insulin or cannot use insulin effectively. Illness, infection, emotional stress, dehydration, missed medication, steroid use, increased carbohydrate intake, and insulin-pump failure may contribute.
Low glucose generally occurs when insulin or medication lowers blood sugar more than expected. Skipped meals, prolonged exercise, alcohol, appetite loss, and changes in kidney function can increase this risk.
An unexpected reading should never be ignored, especially when the person feels unwell or the level continues moving in the wrong direction.
Diagnosis and Medical Tests
First, confirm an unexpected home reading. Wash and dry your hands, use an unexpired test strip, and repeat the test. Food or sugar residue on the fingers may produce an inaccurate result.
If a continuous glucose monitor reading does not match your symptoms, confirm it with a finger-stick test when possible. CGM readings may lag behind blood glucose during rapid changes.
Medical evaluation may include plasma glucose and A1C testing, blood or urine ketone tests, electrolyte and kidney-function tests, and measurements of blood acidity or bicarbonate. Doctors may also check for infection, review medicines and insulin delivery, and measure blood osmolality when HHS is suspected.
Treatment for Dangerous High Blood Sugar
Follow the correction plan provided by your diabetes care team. Do not change insulin or medication doses without professional guidance unless your written plan instructs you to do so.
Drink water or sugar-free fluids if you are alert, able to drink, and have not been told to restrict fluids. Avoid strenuous exercise when ketones are present because physical activity may worsen ketoacidosis.
DKA and HHS require hospital treatment. Care may include intravenous fluids, insulin, potassium or other electrolyte replacement, close monitoring, and treatment of the illness or infection that triggered the emergency.
Possible Complications
Severe hypoglycemia can cause falls, injuries, seizures, loss of consciousness, coma, and heart rhythm problems. Repeated episodes may also reduce awareness of future lows.
Dangerously high blood sugar can lead to diabetic ketoacidosis (DKA) or hyperosmolar hyperglycemic state (HHS), both of which are serious high blood sugar emergencies. Over time, uncontrolled hyperglycemia may damage blood vessels and nerves, increasing the risk of heart disease, stroke, kidney disease, vision loss, diabetic nerve damage, foot ulcers, infections, and slow wound healing.
Maintaining glucose near the personal target range can help reduce the risk of these complications.
When to Call Emergency Services?
Call emergency services immediately if someone is unconscious, having a seizure, unable to swallow, severely confused, or behaving unusually.
Emergency care is also needed for trouble breathing, fast and deep breathing, fruity-smelling breath with vomiting or abdominal pain, an inability to keep fluids down, moderate or high ketones, or signs of severe dehydration.
Seek immediate help if the glucose meter shows a reading near 600 mg/dL, displays “HI,” or the person has symptoms of DKA or HHS, regardless of the glucose number.
How to Prevent Dangerous Blood Sugar Levels?
Check your glucose as recommended and understand your personal target and emergency ranges. Take medicines as prescribed, and never stop basal insulin without professional advice.
Keep glucose tablets or another fast-acting sugar nearby, carry medical identification if you use insulin, and have ketone tests available if advised.
Create a written sick-day plan and inspect insulin-pump tubing and the infusion site after an unexpected high reading. Teach family members how to recognize hypoglycemia and use glucagon.
Record your glucose readings, meals, activity, illnesses, and medication doses. Regular A1C, kidney, eye, and foot checks can help identify problems before serious complications develop.
Questions to Ask Your Healthcare Provider
- What is my personal target blood sugar range?
- What glucose level is dangerous for me?
- When should I check for ketones?
- At what reading should I contact your office?
- How should I adjust my diabetes plan during illness?
- Should I keep glucagon at home?
- Could my medicines cause hypoglycemia or DKA?
- When should I confirm a CGM reading with a glucose meter?
Conclusion
Blood sugar below 70 mg/dL requires prompt treatment, while a reading below 55 mg/dL is severely low. High glucose becomes particularly concerning above 240–250 mg/dL when accompanied by illness, ketones, vomiting, or breathing problems.
A reading around 600 mg/dL may indicate a life-threatening emergency. Symptoms, ketones, medications, and the duration of the abnormal reading are just as important as the number. Seek emergency help for unconsciousness, seizures, confusion, breathing difficulty, repeated vomiting, severe dehydration, or signs of DKA or HHS.
FAQS
Blood sugar below 55 mg/dL is severely low. High readings become an emergency when accompanied by ketones, vomiting, breathing problems, confusion, or severe dehydration.
A reading of 250 mg/dL is high. Check ketones if advised, particularly during illness, and contact your healthcare provider if it persists or causes symptoms.
Yes, blood sugar over 300 mg/dL can become dangerous. Follow your treatment plan and seek urgent guidance if it remains elevated or ketones are present.
A glucose level of 400 mg/dL may cause severe dehydration and increase the risk of DKA or HHS. Confirm the result and seek prompt medical guidance.
Yes. Blood sugar around 600 mg/dL may indicate HHS, which can cause seizures, coma, or death without emergency hospital treatment.
There is no universal fainting threshold. Readings below 55 mg/dL are severely low, but unconsciousness may occur at different levels in different people.
Yes. Some people may have very high glucose without clear symptoms, particularly after living with type 2 diabetes for a long time.
Follow your personal diabetes plan. Ketone testing is commonly advised during illness, with DKA symptoms, or when glucose reaches approximately 240–250 mg/dL.
No. Treat the low reading, recheck after 15 minutes, and ensure glucose has recovered. Follow your provider’s instructions for preventing another overnight low.
Yes. Dirty hands, expired strips, temperature extremes, or device problems can affect results. Wash and dry your hands before repeating an unexpected test.
