What Blood Sugar Level Is Dangerous?

Learn when high or low blood sugar may require urgent or emergency medical care.

Educational content written by Dr. Albana Greca, MD, MMedSc
Specialist review by Dr. Ruden Cakoni, MD, Endocrinologist

Blood sugar levels become dangerous when they are too high or too low for the body to function safely. Extremely abnormal glucose levels can lead to dehydration, loss of consciousness, organ damage, or life-threatening emergencies. 

On this page, Dr. Albana Greca explains which blood sugar numbers may be dangerous, which symptoms to watch for, and when urgent or emergency medical care may be needed.

What Blood Sugar Level Is Dangerous?

From a medical perspective, blood sugar becomes dangerous not only because of the number itself, but because of what that number is doing to the body.

A blood sugar level becomes dangerous when it is:

• Low enough to impair brain function, or
• High enough to cause acute complications such as dehydration, diabetic ketoacidosis (DKA), or hyperosmolar hyperglycemic state (HHS).

Danger is not defined by a number alone. Symptoms, how quickly levels change, illness, medications, and individual health factors all matter. Doctors act urgently when glucose levels cross critical thresholds, especially if symptoms are present.

Dangerous HIGH blood sugar levels (hyperglycemia)

When blood sugar remains high, the body begins to lose more water and minerals through urine. This can lead to dehydration, weakness, and in more serious cases, dangerous metabolic complications. For this reason, high blood sugar should be taken seriously, especially when symptoms are present.

Blood sugar consistently above 250 mg/dL

Blood sugar readings that stay above 250 mg/dL need careful attention. At this level, the risk of dehydration, electrolyte imbalance, and acute diabetic complications begins to increase.

Warning signs may include:

• extreme thirst
• frequent urination
• fatigue
• blurred vision
• nausea

As I explain to my patients, one high reading may happen, but persistent readings above 250 mg/dL should not be ignored. If the level does not improve, medical evaluation is recommended.

Blood sugar above 300–400 mg/dL

Blood sugar in the 300–400 mg/dL range is considered high-risk. At this stage, the body may be moving toward serious emergencies such as diabetic ketoacidosis or a hyperosmolar hyperglycemic state.

Symptoms may include:

• vomiting
• abdominal pain
• rapid or deep breathing
• confusion
• drowsiness
• worsening weakness

At this level, you should seek urgent medical advice, especially if symptoms are present or the blood sugar continues to rise.

Blood sugar over 400 mg/dL

Blood sugar above 400 mg/dL is very high and may become a medical emergency, especially if it does not come down, ketones are present, or symptoms such as vomiting, confusion, drowsiness, difficulty breathing, severe weakness, or dehydration occur.

Immediate medical care is required, especially if the person has vomiting, confusion, drowsiness, difficulty breathing, severe weakness, or if ketones are present. This is not a situation to manage alone at home, because the body may quickly become dehydrated and metabolically unstable.

Dangerous Low Blood Sugar Levels: Hypoglycemia

Low blood sugar can become dangerous quickly because the brain depends on glucose for energy. When glucose drops too low, a person may first feel shaky or weak, but if the level continues to fall, brain function can be affected and the situation may become an emergency.

Blood sugar below 70 mg/dL

A blood sugar level below 70 mg/dL is considered low and should be treated promptly to prevent it from dropping further.

Symptoms may include:

• shakiness
• sweating
• hunger
• dizziness
• weakness
• difficulty concentrating
• fast heartbeat
• anxiety or irritability

As I explain to patients, mild hypoglycemia is often treatable if recognized early. The important step is to act quickly and recheck the blood sugar after treatment.

Blood sugar below 54 mg/dL

A blood sugar level below 54 mg/dL is considered clinically significant hypoglycemia. At this stage, the brain may not receive enough glucose to function properly.

Symptoms may include:

• confusion
• slurred speech
• poor coordination
• unusual behavior
• severe weakness
• seizures

This level requires immediate treatment. The person should not be left alone, especially if they are confused, drowsy, or unable to swallow safely.

Blood sugar below 40–45 mg/dL

A blood sugar level below 40–45 mg/dL is very dangerous and may be life-threatening.

At this level, hypoglycemia can cause:

• loss of consciousness
• seizures
• coma
• serious injury from collapse or falls

Emergency medical services should be contacted immediately. If the person is unconscious, having a seizure, or unable to swallow, do not give food or drink by mouth. This situation requires urgent medical care.

You can get more treatment and management options for Blood Sugar Level ranging 45-69 mg/dl. 

Medical emergencies linked to dangerous blood sugar

Dangerous blood sugar levels can affect the whole body, especially the brain, the heart, circulation, hydration, and the body’s acid–base balance. Both very high and very low blood sugar can become medical emergencies if they are not treated quickly.Diabetic Coma

A diabetic coma can happen when blood sugar becomes extremely high or dangerously low. In this condition, the person may lose consciousness and become unable to respond.

Warning signs may include:

• severe confusion
• extreme drowsiness
• unresponsiveness
• deep, rapid, or labored breathing
• severe weakness
• inability to drink, eat, or follow instructions

As I explain to patients and families, diabetic coma is not something to “wait and see.” If a person with diabetes becomes confused, very sleepy, unconscious, or difficult to wake, emergency medical help is needed immediately.

You can get more informed on Diabetic coma: warning signs and blood sugar levels.

Diabetic Ketoacidosis: DKA

Diabetic ketoacidosis, or DKA, is a serious condition that occurs when the body does not have enough insulin. Because glucose cannot enter the cells properly, the body begins to break down fat for energy. This produces ketones, which can build up in the blood and make it too acidic.

Common warning signs of DKA include:

• nausea and vomiting
• abdominal pain
• fruity-smelling breath
• rapid or deep breathing
• extreme fatigue
• dehydration
• confusion or worsening weakness

DKA requires immediate hospital treatment. It usually needs intravenous fluids, insulin, electrolyte correction, and close medical monitoring.

You can have a few more information on When to seek emergency care when having Diabetic ketoacidosis symptoms.

Hyperosmolar Hyperglycemic State: HHS

Hyperosmolar hyperglycemic state, or HHS, is a serious diabetes emergency that usually involves very high blood sugar and severe dehydration. It is more common in people with type 2 diabetes, especially during illness, infection, dehydration, or missed medication.

Warning signs may include:

very high blood sugar
extreme thirst
very frequent urination
dry mouth
severe weakness
confusion
drowsiness
seizures
loss of consciousness

HHS requires urgent hospital treatment. It is not something to manage at home.

Why Dangerous Blood Sugar Levels Happen

Dangerous glucose levels usually have a cause. Understanding the cause is very important because it helps prevent the same emergency from happening again.

Possible causes include:

• missed insulin or diabetes medication
• infection, fever, or another illness
• dehydration
• prolonged fasting or skipped meals
• taking too much insulin
• taking sulfonylurea medicines without enough food
• alcohol use
• vomiting or poor food intake
• stress, surgery, or serious inflammation
• incorrect insulin dose or timing

From a medical point of view, the goal is not only to correct the blood sugar number, but also to understand why it happened. This helps the patient, family, and healthcare provider adjust the treatment plan safely and reduce the risk of recurrence.

You can have a more profound understanding on low blood glucose causes. 

Preventing Dangerous Blood Sugar Levels

Not every blood sugar emergency can be completely avoided, but good daily diabetes management can greatly reduce the risk. The goal is to recognize changes early, act in time, and prevent glucose levels from becoming dangerously high or dangerously low.

Important prevention steps include:

• monitoring blood sugar regularly
• eating consistent, balanced meals
• taking diabetes medication or insulin at the correct time
• staying well hydrated
• having a clear sick-day plan for illness, fever, vomiting, or poor appetite
• learning the early warning signs of both high and low blood sugar
• knowing when to contact a doctor or seek urgent care

As I often explain to patients, prevention is not only about checking numbers. It is about understanding your body’s signals, following your treatment plan, and responding early before a small glucose change becomes a medical emergency.

You can get more Daily Tips on Blood Sugar Control.

What Are Ketones, and Why Do They Matter?

Ketones appear when the body lacks enough insulin and begins breaking down fat for energy.
Moderate or high ketones together with high blood sugar may indicate diabetic ketoacidosis (DKA), a medical emergency.

You should check ketones if:

• Blood sugar is above 240 mg/dL (13.3 mmol/L)
• You are sick, dehydrated, or not eating normally
• You feel nauseated or have abdominal pain

Positive ketones mean your body is lacking insulin, and urgent medical advice is needed.

Important: People taking SGLT2 inhibitor medicines may sometimes develop diabetic ketoacidosis even when blood sugar is not extremely high. These medicines include drugs such as empagliflozin, dapagliflozin, and canagliflozin. If you take one of these medicines and develop nausea, vomiting, abdominal pain, rapid breathing, unusual fatigue, or positive ketones, seek urgent medical advice.

What to Do If Your Blood Sugar Is Dangerous?

If Blood Sugar Is Low

Use the 15–15 rule:

• Take 15 g of fast-acting carbohydrate
• Recheck blood sugar after 15 minutes
• Repeat if still low

Once stable, eat a balanced snack if advised by your healthcare provider.

If Blood Sugar Is High

• Drink water
• Follow your personal correction plan
• Do NOT exercise if ketones are present
• Seek medical advice if levels do not improve

Never take extra insulin unless it is part of a written correction plan provided by your healthcare provider.

When to Go to the Hospital for Blood Sugar Levels?

🚨 Seek Emergency Care Immediately If

• Blood sugar stays above 300 mg/dL (16.7 mmol/L) and does not come down
• You have vomiting, abdominal pain, or rapid/deep breathing
• You feel confused, drowsy, or unable to stay awake
• Ketones are positive in urine or blood
• Blood sugar rises above 600 mg/dL (33.3 mmol/L)
• There is loss of consciousness or seizures

As I explain to patients, the safest decision in these situations is to act early. It is better to seek urgent medical help before dehydration, acid buildup, or brain-related symptoms become more severe.

Key Medical Message

Dangerous blood sugar levels should be understood medically, not with fear. A glucose level becomes dangerous when it threatens the brain, hydration, circulation, or other vital organs.

Knowing the warning thresholds and acting early can prevent serious complications and may save lives.

If symptoms are severe, or if blood sugar levels are extremely high or dangerously low, do not wait at home. Seek emergency medical care immediately.

Frequently Asked Questions

Take-Home Tips

• Monitor blood sugar regularly and watch for patterns

• Avoid intense exercise during illness or when ketones are present

• Limit alcohol, as it increases hypoglycemia risk

• Ask your doctor to adjust medications if lows or highs occur

• Have an emergency plan for both high and low blood sugar

Helpful tools:

• Convert mmol/L to mg/dL
• HbA1c to average glucose calculator
• Blood sugar level checker
• Insulin correction calculator
• Blood sugar levels by time of day

References:

• Kids too high blood sugar
• Blood Sugar Targets
• Hypoglycemia classification—ADA
• Too Low Blood Sugar Self Care
• General Clinical Guidance on DKA and HHS

Medical Disclaimer

This page is for educational purposes only and does not replace professional medical advice, diagnosis, or treatment.
Always speak with your doctor or qualified healthcare provider before changing your diabetes medication, diet, supplement use, or treatment plan.