Amy Campbell is a registered dietitian and Certified Diabetes Educator who has been working in the field of diabetes for many years. She is the author of several books about diabetes, including 16 Myths of a Diabetic Diet and Staying Healthy with Diabetes: Nutrition and Meal Planning. In addition, Amy is a lecturer and frequent contributor to several diabetes-related websites.

If you have diabetes, it’s highly likely that at one time or another, you’ll have high blood sugars, also known as hyperglycemia.

What is considered high blood sugar?

The American Diabetes Association recommends the following blood sugar (glucose) goals for people with diabetes:

  • Fasting and before meals: 70 to 130 mg/dl
  • Two hours after meals: Less than 180 mg/dl

Keep in mind that your own blood sugar goals may be different, so make sure to talk with your doctor about them. In general, blood sugar levels that are above these goals are considered high.

How do you know if your blood sugar is high?

The best way to know if your blood sugars are up (or down) is to check them with your blood glucose meter. It’s important to check your blood sugars as prescribed by your doctor or diabetes educator because you may not have any symptoms of highs. However, high blood sugar symptoms include:

  • Feeling thirsty
  • Feeling hungry
  • Having to urinate more often than usual
  • Blurry vision
  • Feeling tired
  • Weight loss
  • Dry or itchy skin
  • Cuts or sores that are slow to heal

What causes high blood sugars?

There are a lot of reasons for high blood sugars. These include:

  • Eating too much food, especially foods that contain carbohydrates
  • Too little physical activity
  • Missed or not enough diabetes medicine
  • Spoiled insulin, such as insulin that’s expired or left out in the heat
  • Illness, injury, or surgery
  • Emotional stress, such as a death in the family, divorce, or loss of a job
  • Certain medicines, including corticosteroids (prednisone)

What are the risks of high blood sugars?

If your blood sugar levels are high and stay high over a long period of time, your risk of developing diabetes-related complications goes up. High blood sugars can lead to:

  • Heart disease
  • Kidney disease
  • Eye disease (diabetic retinopathy)
  • Nerve damage
  • Teeth and gum problems

In the short term, two serious conditions can occur from very high blood sugars:

Diabetic ketoacidosis (DKA). This occurs when the body doesn’t have enough insulin. As a result, blood sugar levels rise very high. The body also starts to burn fat for energy, forming substances called “ketones.” Left untreated, DKA can lead to coma and can be life-threatening. People with type 1 diabetes are more likely to get DKA than people who have type 2 diabetes.

Diabetic hyperglycemic hyperosmolar syndrome (HHS). HHS is another very serious condition that can occur in people who have type 2 diabetes. In this condition, blood sugar levels are very high but there are no ketones. Illness, infection, dehydration, impaired kidney function, and not following your diabetes treatment plan can raise the risk of HHS. It’s also more likely to occur in older people and in people with issues such as dementia.

When to take action

It can be scary to think of what could happen if your blood sugars get too high. However, try not to let fear get in the way of taking charge of your blood sugars. The good news is that people can, and do, live long, healthy lives with diabetes. So, use this information as a chance to review your blood sugar and A1c goals with your diabetes care team; also, look at your blood sugar readings and see how often you’re within your target range. Target blood sugar goals exist for a reason: to lower the risk of complications. If, for example, your fasting blood sugars are frequently running above 130, or your after-dinner readings are at 200, it’s time to take action. Here’s how:

  1. Drink plenty of water or sugar-free beverages so you don’t get dehydrated.
  2. For at least three days, check your blood sugar before meals and before bed, and occasionally two hours after meals. Record your results in your log book, your meter, or in a smartphone app.
  3. Look for patterns. For example, are you running high before lunch most days?
  4. Think about the possible cause: too many carbs? Forgetting to take your medicine? Illness?
  5. Call your doctor or diabetes educator to discuss necessary changes in your treatment plan.

When to call your doctor right away

If you have type 1 diabetes or type 2 diabetes and are on multiple daily injections of insulin, call your doctor right away if your blood sugar is 250 mg/dl or higher for two checks in a row and you have any of these symptoms:

  • Feel sick
  • Have a fever
  • Are nauseous or vomiting
  • Feel weak or sleepy
  • Have stomach or chest pains
  • Are breathing quickly
  • Are unsure what to do

You will need to increase your insulin dose to prevent DKA. You’ll also need to keep checking your blood sugars every three to four hours and check for ketones. If you have DKA, you’ll need to be taken to the hospital for treatment.

If you have type 2 diabetes and take pills or insulin once daily, call your doctor right away if your blood sugar is higher than 300 mg/dl. You may need to start on insulin or increase your dose to prevent HHS.

More about high blood sugar:
3 Reasons Your Morning Blood Sugar Might Be High
4 Foods to Eat when Your Blood Sugar Is Running High
Why Do Fatty Foods Raise Blood Sugars?