High blood sugars (hyperglycemia) can come on slowly or quickly depending on the type of diabetes you have and the cause of that particular high.

In fact, your blood sugars may have been sitting at a higher-than-normal level for so long that you don’t really even feel the symptoms of it at all.

Compared to low blood sugars (hypoglycemia), high blood sugars are much more difficult to spot or even notice. They also take longer to correct, and while they carry less immediate danger compared to lows, they have the potential to cause much more long-term damage throughout your entire body. 

Let’s take a closer look at the causes and symptoms of high blood sugar levels.

Symptoms of high blood sugar

What is high blood sugar?

A healthy human body thrives when blood sugar levels are generally between 70 to 130 mg/dL throughout the day. While everyone — including non-diabetics — will likely experience blood sugar levels above this range because of big meals, this is the general goal range.

With low blood sugars, there’s one number for everyone that is considered “low” — 70 mg/dL — because dipping below this becomes dangerous for everyone. But high blood sugars are more complicated because being slightly above the goal ranges listed below doesn’t equate to immediate danger and may actually be safer for some people based on other health conditions and their age.

According to the American Diabetes Asociation and the Mayo Clinic, optimal blood glucose levels are: 

Blood sugar goals for a non-diabetic:

  • Between 80 and 120 mg/dL for people age 59 and younger with no additional medical conditions
  • Below 100 mg/dL when fasted (before eating)
  • Between 100 and 140 mg/dL for people age 60 and older if they have additional health conditions like heart disease, kidney disease, etc. 

Blood sugar goals for a person with type 1 or type 2 diabetes:

  • Between 80 and 130 mg/dL “fasted” (before meals)
  • Less than 180 mg/dL within two hours after meals

High blood sugar levels become dangerous when:

  • You’re high for a consistent period of time, even when you wake up in the morning
  • You’re running above 180 mg/dL on a regular basis 
  • You’re running above 250 mg/dL on a regular basis — very dangerous

If you’ve already been diagnosed with any type of diabetes, then seeing a 120 mg/dL on your glucose meter before breakfast isn’t necessarily cause for alarm, but it does mean you’re running higher than your likely goal range.

If you haven’t been diagnosed with any type of diabetes and your fasting blood sugar level is over 120 mg/dL, this is a clear indicator of diabetes and should be discussed in detail with your healthcare team.

Causes of hyperglycemia

  • Type 1 diabetes
  • Pre-diabetes
  • Type 2 diabetes
  • Gestational diabetes
  • Stress
  • Illness
  • Injury
  • Surgery
  • Weight-gain
  • Obesity
  • Insulin resistance
  • Lack of regular exercise
  • Diet high in processed, fatty, sugary foods and beverages
  • Exceptionally large meals (Thanksgiving, Christmas, Easter, etc.)
  • Severe dehydration from repeated vomiting/diarrhea/stomach virus

Symptoms of high blood sugar

Depending on the cause of your high blood sugars, the symptoms could develop quickly or gradually. 

People with undiagnosed type 1 diabetes will develop nearly all of the symptoms of high blood sugar very quickly, over the course of a few weeks, because their blood sugar levels are rising rapidly while insulin production is declining rapidly.

People with prediabetes or type 2 diabetes may not notice these symptoms for months or years because gradually increasing insulin resistance and beta-cell dysfunction can take a long time to become severe enough to produce noticeable symptoms.

Earlier symptoms:

Contact your healthcare team if you are experiencing several of these symptoms, and ask to have your blood sugar levels tested.

Later symptoms:

  • Sour fruit smell on your breath
  • Lethargy — very heavy feeling in your limbs
  • Yeast infections (in men or women)
  • Dry mouth
  • Weight loss (type 1) or weight gain (type 2)
  • Shortness of breath
  • Weakness
  • Confusion / difficulty concentrating
  • Nausea

Contact your healthcare team if you are experiencing several of these symptoms, and ask to have your blood sugar levels tested. If you’ve already been diagnosed with diabetes, ask your healthcare team to help you fine-tune your medication dosages to help you achieve healthier blood sugar levels.

Emergency symptoms:

Visit your local emergency room immediately if you are experiencing any of these severe symptoms.

Tips for managing high blood sugars if you have diabetes

High blood sugars in a person with any type of diabetes are expected but if you are consistently running higher than your goal range, this means your body is getting too little of something or too much of something else.


In all people with type 1 diabetes and some with type 2 diabetes, insulin is the only way you will be able to maintain healthy blood sugar levels. Without insulin via pump, pen, or syringe, a person with type 1 diabetes will die within a few days.

But taking insulin is complicated. Just 1 unit too much or too little can lead to unwanted fluctuations in your blood sugar. Learning how to adjust, balance and dose your insulin around food, activity, stress, hormones, weight gain or loss, and daily life is a neverending process. 

Work with your healthcare to make sure your insulin doses are meeting your body’s current needs.

Diabetes medications

Non-insulin diabetes medications taken via pill or injection can help a person with diabetes tremendously. Many of these medications work by increasing the amount of insulin you produce if you have type 2 diabetes.

These medications can also decrease the amount of sugar your liver produces, slow down the digestion of your meals to prevent post-meal spikes, and help excess glucose from your diet pass through your urine so it never enters your bloodstream.

But your dosages of these medications need to be adjusted from time to time. If you’re noticing symptoms of high or low blood sugars, talk to your healthcare team immediately to get things fine-tuned.


When your blood sugar is high, you need to drink plenty of water along with anything else you do to help reduce it. Water is going to help flush out excess glucose and ketones. Even mild dehydration as a person with diabetes will only make your blood sugar levels higher because your blood becomes more concentrated.

Nutrition changes

Reducing the amount of processed, packaged, sugary foods in your diet will help tremendously no matter what type of diabetes you have. 

You don’t have to eat a perfect diet, but if you can strive for 80 to 90 percent of whole, real food that leaves you with plenty of room each day for a treat to prevent feelings of deprivation! Look at the bigger picture of your diet rather than striving for perfection. It doesn’t have to be all or nothing.


Yes, you can often lower your blood sugar level by exercising. Regular exercise will help your body need less insulin because the exercise will burn some of the glucose in your bloodstream.

However, if your blood sugar is above 240 mg/dL, check your urine for ketones using ketone strips or a blood ketone meter. If you have ketones, do not exercise. Exercising with severe high blood sugars can increase ketones and put you at risk of DKA.


The closer you are to a healthy body weight, the more sensitive your body will be to insulin. This means your body will simply struggle less to achieve healthy blood sugar levels and need less assistance from larger quantities of insulin or other medications. Even losing 5 or 10 pounds can make a difference in your sensitivity to insulin!

Read “How to Lose Weight with Diabetes” to learn more about healthy weight loss.

Managing stomach viruses, the flu, and repeated vomiting 

If you are a person with type 1 diabetes and you get a stomach virus that causes you to repeatedly vomit — which means you are unable to keep food or water down — you need to visit an emergency room immediately. If you have a glucagon kit, consider using it to prevent severe low blood sugar until you are being cared for in an emergency room.

Long-term effects of high blood sugar levels

Left untreated, high blood sugars will wreak havoc on your entire body, because glucose will build up in your bloodstream and essentially rot the vital nerve endings throughout your entire body. 

High blood sugars are serious and can severely impact your health in the short term and long term. Talk to your healthcare team immediately if you believe your blood sugars are consistently running higher than your goal range.