When is blood sugar considered to be too high or too low?

Slight fluctuations in blood sugar levels are completely normal, and also happen every day in people who don’t have diabetes, in response to the food they eat. Between around 60 and 140 milligrams of sugar per deciliter of blood (mg/dl) is considered to be healthy. This is equivalent to a blood sugar concentration of between 3.3 and 7.8 mmol/l. “Millimoles per liter” (mmol/l) is the unit that blood sugar is measured in. It describes the amount of a certain substance per liter.

If someone has readings over 7.8 mmol/l (140 mg/dl), they are considered to have hyperglycemia. These high blood sugar levels mainly occur if there isn't enough insulin or the insulin doesn't work properly. Without the effect of insulin, the organs can't make good use of the sugar in the blood, so the sugar builds up. If type 1 diabetes is left untreated, blood sugar levels can increase to over 27.8 mmol/l (500 mg/dl). Such high levels tend to be uncommon in type 2 diabetes.

Blood sugar levels below 3.3 mmol/l (60 mg/dl) are considered to be too low. But, as you can see in the illustration below, there are no clear-cut borders between normal blood sugar levels and too high or too low blood sugar levels.

Blood sugar: Normal range between hyperglycemia and hypoglycemia

Signs of hyperglycemia

Hyperglycemia doesn't always have immediately noticeable effects. Someone might have it for years without having any physical symptoms. But very high blood sugar can cause the following symptoms:

• Extreme thirst
• Frequent urination
• Tiredness
• Listlessness
• Nausea
• Dizziness

If someone has extremely high blood sugar levels, they may feel confused and drowsy or even lose consciousness (diabetic coma).

What can you do if your blood sugar levels are too high?

If you have the above symptoms over a long period of time or if they keep occurring, it's important to see a doctor. They could be a sign of diabetes.

If these symptoms occur in people who have already been diagnosed with diabetes, they could be a sign that the treatment is no longer effective enough. Then medication can be prescribed or adjusted to make your blood sugar levels go back down. You may stay at the hospital while your blood sugar levels are being stabilized. If you are feeling drowsy, confused or lose consciousness, the emergency services should be called (112 in Germany and many other countries, 911 in the U.S.).

Signs of hypoglycemia

Low blood sugar is most common in people who use insulin or take certain tablets to reduce high blood sugar. This is because things like unexpected physical activity, eating too little food or drinking too much alcohol can mean that you need less insulin than you thought, causing your blood sugar to drop too low.

Signs that your blood sugar is too low may include:

• Racing pulse
• Cold sweats
• Pale face
• Headache
• Feeling incredibly hungry
• Shivering, feeling weak in the knees
• Feeling restless, nervous or anxious
• Difficulty concentrating, confusion

The severity of these symptoms depends on the blood sugar levels and can vary from person to person. The symptoms don’t occur all at once. If you aren’t sure whether your blood sugar is too low, you can measure it to make sure. 

Your blood sugar levels may also drop at night, making you feel tired and weak in the morning.

Mild hypoglycemia doesn't usually have any serious health effects. But severe hypoglycemia can lead to a loss of consciousness and become life-threatening.

What can you do if your blood sugar levels are too low?

It is important to react quickly enough and eat or drink something, like dextrose sugar or a sugary drink (no "diet" or "zero" soft drinks with artificial sweeteners!).

If someone has severe hypoglycemia they may feel drowsy and confused, and might even become unconscious. People who have type 1 diabetes often carry a pre-filled syringe on them in case that happens, containing the hormone glucagon. Glucagon makes the liver release sugar into the bloodstream. Someone else can then inject the hormone if necessary. If this is not possible, it is important to call the emergency services immediately (in Germany and many other countries: 112, in the U.S.: 911) and ask for medical help.

If your blood sugar levels keep on dropping too low, you should see your doctor. It could then be a good idea to change your lifestyle or medication.

Sources

• Bundesärztekammer (BÄK), Kassenärztliche Bundesvereinigung (KBV), Arbeitsgemeinschaft der Wissenschaftlichen Medizinischen Fachgesellschaften (AWMF). Nationale Versorgungsleitlinie: Therapie des Typ-2-Diabetes (S3). AWMF-Registernr.: nvl-001g. December 2014.

• IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

  Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. We do not offer individual consultations.

  Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.