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    What is high blood sugar?

    On this page
    1. What causes high blood sugar?
    2. High blood sugar test
    3. What are the signs that blood sugar is too high?
    4. Is high blood sugar dangerous?
    5. High blood sugar treatment
    6. Our health hub

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    What is high blood sugar

    High blood sugar (hyperglycaemia) is too much sugar in the blood. People with diabetes will often have high blood sugar. If left untreated, it can become serious. It can sometimes affect people without diabetes, for example, if you’ve recently had a severe infection or have had a heart attack. 

    The information in the table below is taken from It outlines the recommended target blood glucose levels. Blood glucose is measured in mmol/L (millimoles per litre) or mg/DL (milligrams per decilitre). 

    Target levels by type 

    Upon waking    

    Before meals
    At least 90 minutes after meals

    4.0 to 5.9 mmol/L
    Under 7.8 mmol/L
    Type 1 diabetes
    4 to 7 mmol/L
    Under 8.5 mmol/L
    Type 2 diabetes5 to 7 mmol/L
    4 to 7 mmol/L
    5 to 9 mmol/L
    Children with type 1 diabetes4 to 7 mmol/L
    4 to 7 mmol/L
    5 to 9 mmol/L 

    *Non-diabetic information is provided for information purposes, but is not part of NICE guidelines.

    What causes high blood sugar?

    Type 2 diabetes is a common condition, linked with an inactive lifestyle, being overweight, or having a family history of it. It causes the sugar in the blood to become too high. Type one diabetes also causes blood sugar to become too high, and it is usually identified in childhood.

    If you’ve got diabetes, high blood sugar happens when:

    • Your body cannot produce enough insulin to process the sugar in your blood
    • Your body cannot use insulin effectively enough – insulin is a hormone that manages blood sugar levels  

    High blood sugar in people with diabetes can be triggered by a range of things, such as stress, dehydration, or missing a dose of diabetes medication.

    High blood sugar test

    If your GP thinks you might have diabetes, they are likely to carry out a HbA1c (glycated haemoglobin) test by taking a blood sample from a vein, or from a fingerpick. This will measure your average blood sugar levels over a three-month period. Red blood cells live for about three months, which is why the test can provide a measurement for this period. Glucose sticks to the red blood cells if they are alive.

    Alternatively, you can check your blood sugar levels with an at-home blood test. A home blood test will help you understand your risk of pre-diabetes or type 2 diabetes. This will give you the information you need to follow-up with your doctor to get a proper diagnosis.

    If you have symptoms of diabetes, it is very important that you talk to your healthcare provider.

    What are the signs that blood sugar is too high?

    When blood sugar is only a little bit higher than it should be, you may not notice any symptoms. But as they get higher, you might notice:

    • Needing to urinate more than usual
    • Blurred vision
    • Feeling very thirsty
    • Feeling sick
    • Feeling tired
    • Weight loss 

    If you have any of these symptoms you should speak to your GP.

    Is high blood sugar dangerous?

    If high for a short time, high blood sugar is not usually serious. If it stays high for too long, it can lead to:

    • Permanent damage to the eyes and vision problems
    • Permanent damage to the nerves in the hands and feet
    • Diabetic ketoacidosis – this is a serious condition that can happen to people with diabetes, and it needs urgent attention in hospital  

    Other risks for people with diabetes

    • You’re more at risk of cardiovascular disease if you’ve got diabetes. This is because high blood sugar levels can damage blood vessels. More sugar sticks to the red blood cells and it builds up in the blood. The build-up can block and damage vessels carrying blood to and from your heart.
    • People with diabetes should also work to keep cholesterol levels in the normal range. Because diabetes can damage blood vessels, it’s easier for cholesterol to stick to them. Too much cholesterol can increase your risk of stroke and heart disease. Your GP can also test your cholesterol levels with a blood test called a lipid profile. 

    High blood sugar treatment

    If you have been diagnosed with diabetes following a blood sugar test, your doctor may advise:

    • Changes to your diet
    • More exercise
    • An increased water intake
    • Taking insulin to keep levels under control
    • Monitoring blood sugar levels  

    Talk to your doctor if you're unsure about what to do next. 

    You should seek urgent medical attention if you have diabetes and experience any of the following symptoms:

    • Feeling or being sick
    • Abdominal pain
    • Signs of dehydration – for example, dry skin
    • Rapid and deep breathing
    • Difficulty staying awake 

    Sometimes, these symptoms could indicate a lack of insulin causing a harmful substance to build up (diabetic ketoacidosis) or extremely high blood sugar levels (a hyperosmolar hyperglycaemic state).

    Our health hub

    We’re here for you with advice articles from our NHS-experienced clinicians. Visit our healthy lifestyle hub to learn about changes you can start making today.


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