The Highs and Lows of Blood Glucose: What Everyone With Diabetes Should Know

Reviewed by Robert Ehrman, MD

Checking your blood glucose can be frustrating—if your numbers are too low, too high, or you are not sure what the numbers mean, your meter can start to feel like your worst enemy.

high blood glucose

How high is too high?

Generally a blood glucose reading above 250 mg/dL is too high, especially if you have more than one reading at this level.

Symptoms of high blood glucose

For many people, the symptoms feel the same as they did before they were diagnosed. Others have no symptoms at that level.

Some common symptoms are:

  • Feeling tired and sleepy
  • Feeling groggy and not able to think clearly
  • Dry mouth, thirst and dry, itchy skin
  • Urinating more often.
  • Blurred vision
  • Headache
  • Nausea and vomiting

What to do?

It is easy to get dehydrated when your blood glucose is too high. Drinking extra water can help to flush the extra glucose out of your body. Exercise may help to lower your glucose level but can be hard or unsafe to do when you are feeling tired and groggy. If you take a rapid-acting form of insulin, ask your healthcare professional about a correction dose of insulin that you can take at those times. If your blood glucose stays at that level for more than a day, call your healthcare provider and ask for advice. If your family or others notice you are confused, you need to go to the emergency room or call 911.

low blood glucose

How low is too low?

Generally a blood glucose level of less than 70 mg/dL is too low. That is the point at which many people begin to have symptoms.

SYMPTOMS OF low blood glucose

Although there are common symptoms, not everyone feels the same way.

Some early symptoms are:

  • Feeling weak and hungry
  • Sweating or feeling cold and clammy
  • Shaking, especially feeling like you are “shaking on the inside”
  • Dizziness
  • Having nightmares or being very restless while sleeping.

What to do?

Although it is tempting to eat everything in sight, the treatment that works best is to drink or eat a fast-acting source of sugar.

Here is a general plan of action:

• If your blood glucose is between 50 and 70 mg/dL, 15 to 20 grams of carbohydrates is usually enough to raise it. That is the amount of carbohydrates in the following:

✓ 3–4 ounces (½ cup) of juice

✓ 3–4 ounces regular soda

✓ 3–4 glucose tablets

✓ 3–5 pieces of hard candy

  • If your blood glucose is less than 50 mg/dL, take twice that amount.
  • It will take time for the treatment to work, so wait 15 minutes and check your blood glucose again. If it is still too low, then take the same amount of carbohydrates again. If you don’t treat it and just rest, your blood glucose may continue to drop until you pass out.

What if you can’t check your blood glucose?

Then the rule of thumb is to treat yourself as if your blood glucose is too low. If you feel better, then you were probably too low. Another clue is to think about your symptoms or what might be causing them. Low blood glucose usually comes on quickly and may be related to missing a meal, eating less or later than usual, being more active or drinking alcohol. High blood glucose usually comes on more gradually and may be related to being less active or eating more than usual. Stress or an infection or illness, such as a cold or the flu, can also raise your blood glucose level.


Blood glucose monitoring can sometimes feel like your worst enemy. It can be frustrating when it is hard to figure out what is causing your readings to fluctuate or when they do not reflect your hard work. It can feel like you studied hard, did the best you could and then failed the test. But remember, blood glucose levels are not who you are or a judgment of your efforts or character. Blood glucose readings are just numbers. They give you information about your body and your diabetes to figure out how to manage it so you reach your targets and goals. By knowing what to do when your blood glucose is too high or too low and using the information to take action, monitoring can become your friend instead of your enemy.

“Rule of 15” for treatment of low blood glucose

Check your blood glucose.

Take 15 grams of carbs.

Wait 15 minutes.

Check your blood glucose again.

Repeat if needed.

Be informed.

If low blood glucose is not treated, it’s possible to become confused or even pass out.

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