High blood sugar: symptoms and causes

If your blood sugar levels become too high you might experience symptoms like feeling thirsty all the time, blurred vision, or recurrent infections that take a long time to heal. It’s important to familiarise yourself with the signs and symptoms of high blood sugar levels so you can safely manage and lower your levels should you need to do so.
Louise Carleton
min read
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Quick summary

  • There are many different reasons why your blood sugar levels might be too high, such as illness, stress, an unhealthy diet, and not taking type 2 diabetes medication correctly
  • Symptoms of high blood sugar levels include having a dry mouth or feeling thirsty, blurred vision, fruity-smelling breath, feeling tired, and cuts and infections that take a long time to heal
  • Fortunately, it is possible to lower and manage your blood sugar levels through things like diet and exercise, and making sure you take your diabetes medication correctly

Hyperglycaemia is the medical term for when blood sugar levels in the body are too high. This is a condition that affects many people with diabetes including those that suffer from type 1, type 2, and gestational diabetes. There are many reasons why your blood sugar levels might be too high including: 

  • Illness
  • Stress
  • Poor eating habits including snacking, eating too much, or eating lots of processed food
  • Not getting enough exercise
  • Forgetting to take type 2 diabetes medication or taking the wrong dose
  • Certain medications, such as steroids

High blood sugar symptoms

Everyone’s blood sugar levels will fluctuate throughout the day, especially after we’ve eaten. If you suffer from type 2 diabetes it’s important to pay close attention to your blood sugar levels to ensure they stay at a healthy level for you. You should familiarise yourself with the signs and symptoms of high blood sugar so you can act quickly to safely bring them down if they become too high. Let’s explore what high blood sugar levels might feel like.

Keeping your blood sugar at healthy levels will help reduce the risk of hyperglycaemia

Forouhi et al., BMJ

Feeling thirsty

If your blood sugar levels are too high you might find that you’re suddenly feeling very thirsty all the time or that your mouth feels dry, no matter how much you drink.[1] This happens when there is too much glucose in the blood—to remove it, our bodies try to flush it out via the bladder and the kidneys by going to the toilet. This increased need to urinate means we lose lots of fluids and we become dehydrated, meaning we feel thirsty as a result.

Feeling tired

Hyperglycaemia can also make you feel tired and worn out even if you’re getting plenty of rest and sleep. When our bodies aren’t processing insulin correctly the glucose that normally goes to our cells and gives them energy instead stays in the bloodstream.[2] Without the energy to function properly, we start to feel weak and fatigued. When our blood sugar levels are too high it’s normal to feel tired all the time and it’s not uncommon that even the smallest of tasks require a lot of energy.

Blurred vision

Another sign of hyperglycaemia is blurry or distorted vision. Too much glucose in the blood can affect the small capillaries and tissues around the eyes which can distort the lens, making things appear unclear and out of focus.[3] As well as affecting your eyesight this can also cause headaches and dizziness, which can also be signs of dehydration.

Recurring infections

Another obvious sign of hyperglycaemia is recurring infections that take a long time to heal. High blood sugar levels and needing to go to the toilet more often can increase the chances of urinary tract infections and disrupt the natural balance of yeast in the body, resulting in thrush. Diabetes can also aggravate such conditions as fluctuating blood sugar levels impact blood flow reaching inflamed areas, as well as affecting our nerves and sensory functions.[4]

Changes to your skin that take longer to heal

As well as recurrent infections you might also notice cuts, bruises, and infections to your skin that take longer to heal. When blood sugar levels are too high or left uncontrolled, this can cause damage to your nerves, particularly in areas like the hands, feet, and lower legs. High blood sugar levels also affect the flow of blood around the body which causes delays in the healing process. You might also notice changes to your skin, such as changes in colour or texture, with skin appearing thicker or having a waxy-looking texture, all of which suggest insulin resistance and high blood sugar levels.[5]

Fruity smelling breath

Another high blood sugar symptom is having fruity-smelling breath, sometimes called acetone breath; this is caused by a high level of ketones (an acid made in the liver) in your blood. When our bodies can’t get enough energy from glucose they start burning fat instead, which causes a build-up of ketones. In some cases, this can lead to a condition called diabetic ketoacidosis, or DKA, where such high levels of ketones become toxic, a condition that can be life-threatening if it’s not treated quickly.[6]

Sore, bleeding gums

As well as having fruity-smelling breath you might notice painful changes to your mouth: bleeding gums, ulcers, or cuts that don’t seem to heal are all signs of having high blood sugar. Glucose is found naturally in our saliva but this increases when our blood sugar levels are too high.[7] Bacteria love to feed off this glucose, which can cause a build-up of plaque and eventually lead to gum disease. Such conditions can also cause infections that can take a long time to heal.

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What should my blood sugar levels be?

Your doctor or healthcare team will explain to you what a healthy blood sugar level should be but as a general rule, healthy blood sugar levels are:[8]

  • At home with a self-testing kit - a normal range is  4 to 7mmol/l before eating and under 8.5 to 9mmol/l 2 hours after a meal
  • If your HbA1c level is tested every few months - a healthy range is below 48mmol/mol (or 6.5% on an older measurement scale)

If you don’t have type 2 diabetes but experience any of the symptoms listed above, then you should visit your doctor—they will likely measure your blood sugar levels. If you have type 2 diabetes and self-monitor your blood sugar levels, test your blood sugar levels as soon as you can and take the necessary steps to lower them to a healthier level.

What to do if your blood sugar levels are too high

If your blood sugar levels are too high then you should follow the plan suggested to you by your doctor or healthcare team. In most cases, you can effectively lower your blood sugar levels by:

  • Staying hydrated: drinking plenty of sugar-free fluids throughout the day will keep you hydrated and blood sugar levels down. 
  • Adjusting your diet: reducing your intake of processed foods, choosing healthier carbohydrates, and eating plenty of fruits, vegetables, and healthy fats can all help maintain lower blood sugar levels.
  • Moving more: upping your exercise is an easy way to reduce your blood sugar levels. Try and incorporate 30 minutes of exercise that leaves you feeling out of breath into your daily routine.
  • Taking medication (if prescribed): if you take medication to manage your blood sugar levels, make sure you take this as directed by your doctor.

If you don’t already monitor your blood sugar levels yourself but are interested in starting, speak to your doctor first to discuss your options.

How to prevent hyperglycaemia

Keeping your blood sugar at healthy levels will help reduce the risk of hyperglycaemia.[9] You can reduce your blood sugar levels by:

  • Avoiding foods high in sugar and carbohydrates that will cause your blood sugar to spike
  • Getting plenty of exercise and making sure you live an active lifestyle
  • Taking your medication correctly and as advised by your doctor
  • Looking after yourself when you’re ill, stressed, or rundown
  • Monitoring your blood sugar levels (if you can) to learn what a healthy blood sugar level is for you

Hyperglycaemia and type 2 diabetes

Managing your blood sugar levels when you have type 2 diabetes isn’t always easy and sometimes despite your best efforts, your blood sugar levels will just be too high. But by following the advice of your doctor and making sure you’re aware of the signs of hyperglycaemia you can act quickly to safely bring your levels down.


[1] Polydipsia. Diabetes.co.uk. Retrieved 1 April 2022. Accessible here.

[2] Tiredness and Diabetes. Diabetes.co.uk. Retrieved 1 April 2022. Accessible here

[3] Wang, W., Lo, A.C.Y. (2018). Diabetic Retinopathy: Pathophysiology and Treatments. Int J Mol Sci 19(6):1816. Accessible here.

[4] Casqueiro, J.,  Casqueiro, J., Alves, C. (2012). Infections in patients with diabetes mellitus: A review of pathogenesis. Indian J Endocrinol Metab 16(Suppl1): S27–S36. Accessible here.

[5]  Rosen, J., Yosipovitch, G. (2018). Skin Manifestations of Diabetes Mellitus. Endotext [Internet]. Accessible here.

[6] Diabetic Ketoacidosis. NHS. Retrieved 1 April 2022. Accessible here

[7] Gupta, S., Nayak, M., T., Sunitha, J.D., et al. (2017). Correlation of salivary glucose level with blood glucose level in diabetes mellitus. J Oral Maxillofac Pathol 21(3): 334–339. Accessible here.

[8] Checking your blood sugar levels. Diabetes UK. Retrieved 1 April 2022. Accessible here

[9] Forouhi, N.G., Misra, A., Mohan, V., et al. (2018). Dietary and nutritional approaches for prevention and management of type 2 diabetes. BMJ 361:k2234. Accessible here.

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