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Type 2 diabetes mellitus

Medically reviewed by Last updated on Feb 13, 2024.

What is type 2 diabetes?

Harvard Health Publishing

Type 2 diabetes is a chronic disease. It is characterized by high levels of sugar in the blood. Type 2 diabetes is also called type 2 diabetes mellitus and adult-onset diabetes. That's because it used to start almost always in middle and late adulthood. However, more and more children and teens are developing this condition. Type 2 diabetes is much more common than type 1 diabetes, and is really a different disease. But it shares with type 1 diabetes high blood sugar levels and the complications of high blood sugar.

During digestion, food is broken down into basic components. Carbohydrates are broken down into simple sugars, primarily glucose. Glucose is a critically important source of energy for the body's cells. To provide energy to the cells, glucose needs to leave the blood and get inside the cells.

Insulin traveling in the blood signals the cells to take up glucose. Insulin is a hormone produced by the pancreas. The pancreas is an organ in the abdomen. When levels of glucose in the blood rise (for example, after a meal), the pancreas produces more insulin.

Type 2 diabetes occurs when your body's cells resist the normal effect of insulin, which is to drive glucose in the blood into the inside of the cells. This condition is called insulin resistance. As a result, glucose starts to build up in the blood.

In people with insulin resistance, the pancreas "sees" the blood glucose level rising. The pancreas responds by making extra insulin to maintain a normal blood sugar. Over time, the body's insulin resistance gets worse. In response the pancreas makes more and more insulin. Finally, the pancreas gets "exhausted." It cannot keep up with the demand for more and more insulin. It poops out.  As a result, blood glucose levels start to rise.

Type 2 diabetes runs in families. Obesity greatly increases the risk of diabetes.

Symptoms of type 2 diabetes

The symptoms of diabetes are related to high blood glucose levels. They include:

Extremely high blood sugar levels also can lead to a dangerous complication called hyperosmolar syndrome. This is a life-threatening form of dehydration. In some cases, hyperosmolar syndrome is the first sign that a person has type 2 diabetes. It causes confused thinking, weakness, nausea and even seizure and coma.

The treatment of type 2 diabetes also can produce symptoms. Too much glucose-lowering medicine, relative to dietary intake, can lead to the complication of low blood sugar (called hypoglycemia). Symptoms of hypoglycemia include:

You can correct hypoglycemia by eating or drinking something that has carbohydrates. This raises your blood sugar level.

Type 2 diabetes affects all parts of the body. It can cause serious, potentially life-threatening complications. These include:

Diagnosing type 2 diabetes

Diabetes is diagnosed by testing the blood for sugar levels. Blood is tested in the morning after you have fasted overnight.

Typically, the body keeps blood sugar levels between 70 and 100 milligrams per deciliter (mg/dL), even after fasting. If a blood sugar level after fasting is greater than 125 mg/dL, diabetes is diagnosed.

Your doctor will examine you to look for:

Laboratory tests are also used routinely to evaluate diabetes. These include:

Expected duration of type 2 diabetes

Diabetes is a lifelong illness. However, people with type 2 diabetes can sometimes restore their blood sugar levels to normal just by eating a healthy diet, regularly exercising, and losing weight.

Aging and episodic illness can cause the body's insulin resistance to increase. As a result, additional treatment typically is required over time.

Preventing type 2 diabetes

If a close relative — particularly, a parent or sibling — has type 2 diabetes, or if your blood glucose test shows "prediabetes" — defined as blood glucose levels between 100 and 125 mg/dL — you are at increased risk for developing type 2 diabetes. You can help to prevent type 2 diabetes by:

If you already have type 2 diabetes, you can still delay or prevent complications:

Treating type 2 diabetes

Diet and exercise

In most cases, type 2 diabetes treatment begins with weight reduction through diet and exercise. A healthy diet for a person with diabetes is:

For some people, type 2 diabetes can be controlled just with aggressive weight loss and exercise. Even if medications are required, diet and exercise remain important for controlling diabetes.


Metformin. Oral metformin remains the first go-to drug in the treatment of type 2 diabetes. It's available as a generic, so it's very affordable. Also, metformin does not cause hypoglycemia (low blood sugar) when used alone. The most common side effects are abdominal discomfort and irregular bowel habits.

According to new guidelines from the American Diabetes Association (ADA), doctors should consider adding either a GLP-1 receptor agonist or SGLT-2 inhibitor if blood sugar levels are still too high despite metformin or a person has an increased risk of cardiovascular or kidney disease. The ADA recognizes that the high cost of these newer drugs can be an obstacle to their use.

Glucagon-like peptide 1 (GLP-1) receptor agonists. These drugs mimic the effects of a substance the gut produces naturally to spur the pancreas to make more insulin. These drugs also slow down the emptying of the stomach, which may make people feel full longer. Examples include liraglutide (Victoza), semaglutide (Ozempic) and dulaglutide (Trulicity).

Sodium-glucose co-transporter 2 (SGLT2) inhibitors. These drugs work by blocking the kidney's ability to reabsorb sugar. They act as diuretics and produce other effects on the kidney, which are likely what makes them effective in preventing heart disease and lowering blood pressure. Examples include empagliflozin (Jardiance), canagliflozin (Invokana), and dapagliflozin (Farxiga).

Both have side effects. When a SGLT2 blocks the kidney's ability to reabsorb sugar, it increases sugar levels in the urine, exposing your kidneys and bladder higher levels of glucose. This can lead to infections. These medications are associated with a fourfold increase in vaginal yeast infections and a more modest increase in urinary tract infections. GLP-1 receptor agonists slow down digestion, which can help with weight loss but can also produce unpleasant symptoms, including nausea, vomiting, and diarrhea, in 15% to 45% of people who take them.

GLP-1 receptor agonists and SGLT-2 inhibitors have additional health benefits beyond helping with blood sugar control:

Weight loss. Both can help a person with diabetes shed pounds. But the GLP-1 receptor agonists are more effective for weight reduction compared with the SGLT-2 inhibitors.

Heart disease. Drugs from both classes lower the risk of heart attack and stroke in people with diabetes. For people with heart failure and diabetes, SGLT-2 inhibitors have been shown to decrease hospitalizations and improve survival. Because of these benefits, doctors are beginning to use SGLT-2 inhibitors even in nondiabetic heart failure patients.

Chronic kidney disease. SGLT-2 inhibitors can slow down the progression of diabetic kidney disease and may help prevent kidney failure related to other conditions.


Because type 2 diabetes develops when the pancreas cannot make enough insulin to overcome insulin resistance. In advanced type 2 diabetes, or for people who want to tightly control glucose levels, insulin may be needed more than once per day and in higher doses.

Treatment plans that include both very long-acting insulin and very short-acting insulin are frequently the most successful for controlling blood sugar. Very short-acting insulin is used with meals, to help control the spike in blood sugar levels that occur with a meal. If a person does not eat on a regular schedule, very short-acting insulin can be particularly helpful.

The most worrisome side effect of insulin is low blood sugar (hypoglycemia). The usual symptoms are sudden weakness, sweating, feeling unwell and not thinking clearly. However, people with low standing diabetes may lose the ability to feel these symptoms even with a dangerously low blood sugar level.

Other medicines

In addition to medicines that help control the level of blood sugar, people with type 2 diabetes often take other medicines that reduce the risk or to slow the onset of the complications of diabetes. These include medications that:

When to call a professional

If you have diabetes, see your doctor regularly.

People with high blood sugar levels have a higher risk of dehydration. Contact your doctor immediately if you develop vomiting or diarrhea and are not able to drink enough fluids.

Monitor your blood sugar as advised by your health care team. Report any significant deviations in blood sugar levels.


Your treatment plan is likely to require adjustment over time. Insulin resistance increases with age. And the insulin-producing cells in the pancreas may wear out as the pancreas tries to keep up with the body's extra insulin needs.

After the first few years, the majority of people with type 2 diabetes require more than one medicine to keep their blood sugar controlled.

The prognosis in people with type 2 diabetes varies. It depends on how well an individual modifies his or her risk of complications. Heart attack, stroke, and kidney disease can result in premature death. Disability due to blindness, amputation, heart disease, stroke, and nerve damage may occur. Some people with type 2 diabetes become dependent on dialysis treatments or require a kidney transplant because of kidney failure.

Additional info

American Diabetes Association

American Dietetic Association

National Institute of Diabetes & Digestive & Kidney Disorders

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.