The essentials

High blood sugar causes thirst, frequent urination, tiredness and blurred vision. Low blood sugar causes shakiness, sweating, hunger and confusion. A glucose below 70 mg/dL (3.9 mmol/L) is considered low, and confusion or seizures are more likely below 54 mg/dL (3.0 mmol/L). A meter or a lab test tells you which one you have; symptoms alone can mislead. Very high sugar with fruity breath, vomiting or fast breathing, or low sugar with drowsiness or a seizure, is an emergency.

What are the symptoms of high blood sugar?

High blood sugar, or hyperglycemia, is a common effect of uncontrolled diabetes. Over time it damages the nerves and blood vessels[6]. Symptoms may come on suddenly, or take years to be noticed in type 2 diabetes[3][6].

  • Urinary and thirst: frequent urination, increased thirst and increased hunger[3][4].
  • General: unusual tiredness, unexplained weight loss, irritability or mood changes[3][4].
  • Eyes: blurred vision[3][4][6].
  • Skin and infections: slow-healing cuts or sores, and repeated urinary tract, skin or yeast infections[3][4].
  • Nerves: numbness or tingling in the hands or feet, and dark skin patches on the neck, armpits or groin in type 2 diabetes[3].

Many people with type 2 diabetes have no symptoms at all, which is why a blood test is the only reliable way to know[3].

What are the symptoms of low blood sugar?

For many people with diabetes, a glucose reading below 70 mg/dL (3.9 mmol/L) is low[1][2]. Your doctor may set a different number for you[2]. Symptoms often come on quickly[2].

  • Mild to moderate: shakiness, hunger, tiredness, dizziness, lightheadedness, confusion, irritability, a fast or irregular heartbeat, headache, and trouble seeing or speaking clearly[2].
  • Severe: the brain may stop working properly, with loss of consciousness or a seizure[2]. Confusion or seizures are more likely below 54 mg/dL (3.0 mmol/L)[1].
  • During sleep: crying out, nightmares, sweating through the sheets, or waking up tired, irritable or confused[2].

When should I see a doctor?

When to see a doctor

  • Emergency: call 911 if a person with low blood sugar cannot be woken, becomes less alert, has a seizure or does not improve after a sugary snack[1][2]. Also call for high blood sugar with fruity-smelling breath, nausea and vomiting, trouble breathing, abdominal pain or fainting: these are signs of diabetic ketoacidosis, a medical emergency[4][5].
  • Within 24 to 48 hours for repeated high readings with thirst, frequent urination and weight loss, for repeated episodes of low sugar, or for ketones in the urine or blood if you have diabetes[5].
  • At your next appointment for blurred vision, slow-healing sores, repeated infections, numbness or tingling, or any symptom of high blood sugar, even mild[3][4].

What causes high or low blood sugar?

  • Common High: diabetes that is undiagnosed or not controlled. Without enough insulin, glucose builds up in the blood[1][6].
  • Common Low, in people with diabetes: insulin or pills that make the pancreas release insulin (sulfonylureas and meglitinides), skipped or delayed meals, too few carbohydrates, fasting while on glucose-lowering medicine, extra physical activity (it can lower glucose for up to 24 hours), alcohol without enough food, and illness that reduces eating[2].
  • Possible Low, in people without diabetes: alcohol, hormone deficiencies, severe heart, kidney or liver failure, sepsis, some weight-loss surgeries and certain medicines[1].
  • Rare Low: an insulin-producing pancreatic tumor (insulinoma)[1].

Which tests show whether my blood sugar is too high or too low?

A fingerstick meter, a continuous glucose monitor or a blood test gives the number. Here is a fictitious lab report for someone with symptoms of high blood sugar.

Fictitious example of a lab report line (United States format)
TestResultReference rangeFlag
Glucose, fasting172 mg/dL70-99 mg/dLH
Hemoglobin A1c8.1 %Less than 5.7 %H
Urine glucosePositiveNegativePOS
Urine ketonesNegativeNegative-
  1. Fasting glucose: the blood sugar after several hours without food. Ranges shown are typical examples: your laboratory's range prevails.
  2. Hemoglobin A1c: reflects the average over about three months, not the value at this moment.
  3. Urine glucose and ketones: glucose in the urine fits high blood sugar. Ketones, when present, call for contact with your provider right away[5].

Got a glucose or A1c result you do not understand? BloodSense explains each line of your complete report and helps you prepare your questions for the visit.

Get my glucose results explained

Read each test in detail in our guides to fasting glucose, estimated average glucose, insulin and urine glucose. For the full picture of the disease, see diabetes: symptoms, blood tests and treatment.

What should I do when my blood sugar is low?

The steps below are for people with diabetes who have a low reading. Ask your doctor for your own plan[2].

  1. Take 15 to 20 grams of glucose or carbohydrate right away, for example four glucose tablets, 1/2 cup of fruit juice, 1/2 can of regular soda or 1 tablespoon of sugar or honey. If you have kidney disease, avoid orange juice[2].
  2. Wait 15 minutes and check again. If it is still low, repeat step 1[2].
  3. When you are back in range, eat a snack if your next meal is more than an hour away[2].

Glucose tablets or gel work fastest. Some diabetes medicines slow the digestion of carbohydrate, so other sources may not raise glucose quickly enough[2]. If you cannot treat yourself, glucagon by injection or nasal spray is the best treatment, and family, friends and coworkers should know how to give it and to call 911 right after[2]. A medical alert ID helps others recognize that you have diabetes[2].

What should I do when my blood sugar is high?

  • Follow the plan given by your diabetes team; do not change a medicine on your own.
  • Check for ketones if you have type 1 diabetes or if your plan says so, and contact your provider right away if they are present[5].
  • Go to the emergency room or call 911 for fruity breath, nausea and vomiting, trouble breathing or reduced alertness[5].
  • If you have never been diagnosed and have symptoms, book a blood test: early diagnosis relies on relatively inexpensive blood glucose testing[6].

Questions to ask your doctor

  1. What are my own target and my own "low" number?
  2. Which symptoms should make me check my glucose or my ketones?
  3. Do I need a glucagon kit, and who should be trained to use it?
  4. Could any of my medicines be causing low or high readings?
  5. How often should I have an A1c and a fasting glucose?

Frequently asked questions

What blood sugar level is considered low?

A reading below 70 mg/dL (3.9 mmol/L) is considered low, and severe symptoms are more likely below 54 mg/dL (3.0 mmol/L)[1].

Can you feel high blood sugar?

Not always. Many people with type 2 diabetes have no symptoms for years[3].

Can low blood sugar happen without diabetes?

Yes, but it is less common. Causes include alcohol, hormone deficiencies, severe organ failure and some medicines[1].

Why do I wake up tired and sweaty after a bad night?

Low blood sugar during sleep can cause nightmares, sweating and waking tired, irritable or confused[2].

What are the signs of diabetic ketoacidosis?

Extreme tiredness, trouble breathing, fruity-smelling breath, nausea, vomiting, abdominal pain and fainting. It is a medical emergency[4][5].

  1. MedlinePlus Medical Encyclopedia. Low blood sugar. Reviewed January 25, 2026. US National Library of Medicine. medlineplus.gov (accessed October 24, 2026)
  2. National Institute of Diabetes and Digestive and Kidney Diseases. Low blood glucose (hypoglycemia). Last reviewed July 2021. National Institutes of Health. niddk.nih.gov (accessed October 24, 2026)
  3. Centers for Disease Control and Prevention. Diabetes symptoms. Reviewed May 15, 2024. cdc.gov (accessed October 24, 2026)
  4. National Institute of Diabetes and Digestive and Kidney Diseases. Symptoms and causes of diabetes. Last reviewed October 2024. National Institutes of Health. niddk.nih.gov (accessed October 24, 2026)
  5. MedlinePlus Medical Encyclopedia. Diabetic ketoacidosis. Reviewed January 10, 2025. US National Library of Medicine. medlineplus.gov (accessed October 24, 2026)
  6. World Health Organization. Diabetes (fact sheet). November 14, 2024. who.int (accessed October 24, 2026)

This page is general information. It does not replace the advice of a doctor, who interprets your results with your symptoms, history and examination.