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High blood sugar or diabetes

Also called: Hyperglycemia, diabetes, prediabetes, high glucose

High blood sugar (hyperglycemia) is when there's too much sugar in your blood, usually from diabetes. It often causes no symptoms at first. Over time, it can cause thirst, peeing a lot, and tiredness, and damage the heart, kidneys, eyes, and nerves. Very high blood sugar with vomiting, confusion, or fast breathing is an emergency.

Common causesType 2 diabetes, type 1 diabetes, prediabetes, steroids, illness
Usually serious?Yes, over time; can be an emergency
Get checked ifYou have symptoms, or your test shows high blood sugar
If you think it's an emergency, call 911.Thinking about suicide? Call or text 988.

Overview

Blood sugar rises when the body doesn't make enough insulin or doesn't use it well. Type 2 diabetes is the most common cause and often develops slowly, without symptoms. Type 1 diabetes can come on quickly, often in children and young adults.

Signs of high blood sugar include thirst, peeing often, blurred vision, tiredness, slow-healing sores, and weight loss. Very high levels can cause diabetic ketoacidosis (DKA) or a hyperosmolar state, both emergencies. Prediabetes means blood sugar is higher than normal but not yet diabetes; lifestyle changes can often prevent diabetes.

When to get medical care

Call 911High blood sugar with vomiting, confusion, trouble breathing, fruity-smelling breath, or extreme drowsiness. A person with diabetes who is unconscious or having a seizure.
Get care todayBlood sugar above 300 mg/dL, or above your provider's limit, that won't come down. Ketones in your urine. Vomiting or not able to keep fluids down if you have diabetes. Extreme thirst and peeing a lot, especially in a child.
See a doctor soonThirst, peeing often, blurred vision, or tiredness. A blood sugar test that was high. Sores that heal slowly or frequent infections. You have risk factors and haven't been screened.

Causes

Common causes include type 2 diabetes, type 1 diabetes, prediabetes, gestational diabetes during pregnancy, and missing diabetes medicine or insulin. Blood sugar can also rise with illness or infection, stress, surgery, eating more carbohydrates, less activity, and medicines like steroids, some antipsychotics, and some water pills. Pancreas disease and Cushing syndrome are less common causes.

OwieClear has pages on 5 conditions that can cause high blood sugar or diabetes. Each shows how soon to get care.

Common causes

Less likely causes

  • PrediabetesSee a doctor if it doesn't get better

    Blood sugar that's higher than normal but not yet diabetes, usually found on an A1C or fasting blood test.

  • Chronic kidney diseaseSee a doctor if it doesn't get better

    Kidneys that slowly lose their ability to filter the blood.

  • Gastroparesis (slow stomach emptying)See a doctor if it doesn't get better

    The stomach empties too slowly, causing early fullness, bloating, nausea, and vomiting food eaten hours before.

What you can do at home

  • Check your blood sugar as often as your provider recommends, and more when you're sick.
  • Take diabetes medicines exactly as prescribed.
  • Drink water, and avoid sugary drinks.
  • Eat balanced meals, watching portions of carbohydrates.
  • Be active most days of the week.
  • Follow your sick-day plan, including checking ketones if you have type 1 diabetes.

Medicines

Treatment may include metformin, other pills or injections, and insulin. Your provider will set blood sugar targets. Don't skip insulin when you're sick; call your provider instead.

At the doctor

Blood tests confirm high blood sugar and diabetes.

Questions you may be asked

  • Symptoms like thirst, peeing, and weight loss
  • Family history of diabetes
  • Your weight, diet, and activity
  • Your medicines
  • Past high blood sugar in pregnancy

The exam

A physical exam, including weight, blood pressure, feet, and skin.

Tests

  • Fasting blood sugar
  • A1C test
  • Glucose tolerance test
  • Urine tests for sugar and ketones
  • Kidney function and cholesterol tests

Prevention

You can lower your risk of type 2 diabetes.

  • Keep a healthy weight
  • Be active at least 150 minutes a week
  • Eat more vegetables and whole grains and fewer sugary foods
  • Get screened if you have risk factors

Check your symptoms

Most symptoms have more than one possible cause. The symptom checker looks at all of your symptoms together, plus your age, and shows how soon to get care.

Start the symptom checker

Sources

Last updated 2026-11-13.