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Complications of Diabetes Mellitus


Erika F. Brutsaert

, MD, New York Medical College

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People with diabetes mellitus have many serious long-term complications that affect many areas of the body, particularly the blood vessels, nerves, eyes, and kidneys.

(See also Diabetes Mellitus.)

There are two types of diabetes mellitus:

  • Type 1, in which the body's immune system attacks the insulin-producing cells of the pancreas, and more than 90% of them are permanently destroyed

  • Type 2, in which the body develops resistance to the effects of insulin

In both types, the amount of sugar (glucose) in the blood is elevated.

People with either type 1 or type 2 diabetes are likely to have complications as a result of the elevated glucose level. However, because type 2 diabetes may be present for some time before it is diagnosed, complications in type 2 diabetes may be more serious or more advanced when they are discovered.

People with diabetes mellitus may experience many serious, long-term complications. Some of these complications begin within months of the onset of diabetes, although most tend to develop after a few years. Most of the complications gradually worsen. In people with diabetes, strictly controlling the level of glucose in the blood makes these complications less likely to develop or worsen.


Most complications of diabetes are the result of problems with blood vessels. Glucose levels that remain high over a long time cause both the small and large blood vessels to narrow. The narrowing reduces blood flow to many parts of the body, leading to problems. There are several causes of blood vessel narrowing:


  • Poor control of blood glucose levels causes the levels of fatty substances in the blood to rise, resulting in atherosclerosis and decreased blood flow in the larger blood vessels.

Types of Diabetes Complications

Blood vessel complications in diabetes

Atherosclerosis leads to heart attacks and strokes. Atherosclerosis is between 2 and 4 times more common and tends to occur at a younger age in people with diabetes than in people who do not have diabetes.

Over time, narrowing of blood vessels can harm the heart, brain, legs, eyes, kidneys, nerves, and skin, resulting in angina, heart failure, strokes, leg cramps during walking (claudication), poor vision, chronic kidney disease, damage to nerves (neuropathy), and skin breakdown.

Infectious problems in diabetes

People with diabetes often develop bacterial and fungal infections typically of the skin and mouth. When the levels of glucose in the blood are high, white blood cells cannot effectively fight infections. Any infection that develops tends to be more severe and takes longer to resolve in people with diabetes. Sometimes, an infection is the first sign of diabetes.

One such infection is a yeast infection called candidiasis. Candida yeast is a normal resident of the mouth, digestive tract, and vagina that usually causes no harm. In people with diabetes, however, Candida can overgrow on mucous membranes and moist areas of the skin causing rashes in those areas.

People with diabetes are also particularly likely to have ulcers and infections of the feet and legs because of poor circulation to the skin. Too often, these wounds heal slowly or not at all. When wounds do not heal, they typically become infected and this can result in gangrene (tissue death) and bone infection (osteomyelitis). Amputation of the foot or part of the leg may be needed.

Eye problems in diabetes

Damage to the blood vessels of the eye can cause loss of vision (diabetic retinopathy). Laser surgery can seal the leaking blood vessels of the eye and prevent permanent damage to the retina. Sometimes, other forms of surgery or injectable drugs may be used. Therefore, people with diabetes should have yearly eye examinations to check for early signs of damage.

Liver damage in diabetes

It is common for people with diabetes to also have fatty liver disease, in which abnormal fat deposits collect in the liver. Fatty liver disease can sometimes progress to more serious liver disease including cirrhosis. Doctors diagnose liver problems if liver blood tests are abnormal and confirm the diagnosis with a liver biopsy. Losing weight, good control of blood sugar levels, and treatment of high cholesterol can be helpful.

Kidney damage in diabetes

The kidneys can malfunction, resulting in chronic kidney disease that may require dialysis or kidney transplantation. Doctors usually check the urine of people with diabetes for abnormally high levels of protein (albumin), which is an early sign of kidney damage. At the earliest sign of kidney complications, people are often given drugs that slow the progression of kidney damage, for example, sodium-glucose co-transporter-2 (SGLT2) inhibitors (drugs that increase glucose secretion in the urine), angiotensin-converting enzyme (ACE) inhibitors, or angiotensin II receptor blockers (ARBs).

Nerve damage in diabetes


Foot problems in diabetes


  • Damage to the nerves (neuropathy) affects sensation to the feet, so that pain is not felt. Irritation and other forms of injury may go unnoticed. An injury may wear through the skin before any pain is felt.

  • Changes in sensation alter the way people with diabetes carry weight on their feet, concentrating weight in certain areas so that calluses form. Calluses (and dry skin) increase the risk of skin breakdown.

  • Diabetes can cause poor circulation in the feet, making ulcers more likely to form when the skin is damaged and making the ulcers slower to heal.

Because diabetes can affect the body’s ability to fight infections, a foot ulcer, once it forms, easily becomes infected. Because of neuropathy, people may not feel discomfort due to the infection until it becomes serious and difficult to treat, leading to gangrene. People with diabetes are more than 30 times more likely to require amputation of a foot or leg than are people without diabetes.

Foot care is critical (see Foot Care). The feet should be protected from injury, and the skin should be kept moist with a good moisturizer. Shoes should fit properly and not cause areas of irritation. Shoes should have appropriate cushioning to spread out the pressure caused by standing. Going barefoot is ill advised. Regular care from a podiatrist (a doctor specializing in foot care), such as having toenails cut and calluses removed, may also be helpful. Also, sensation and blood flow to the feet should be regularly evaluated by doctors.


Complications of Diabetes

Tissue or Organ Affected



Blood vessels

Fatty material (atherosclerotic plaque) builds up and blocks large or medium-sized arteries in the heart, brain, legs, and penis.

The walls of small blood vessels are damaged so that the vessels do not transfer oxygen to tissues normally, and the vessels may leak.

Poor circulation causes wounds to heal poorly and can lead to heart attacks, strokes, gangrene of the feet and hands, erectile dysfunction, and infections.


The small blood vessels of the retina are damaged, leading to formation of new fragile blood vessels that tend to bleed.

Vision decreases, and ultimately, blindness occurs.


Blood vessels in the kidneys thicken.

Protein leaks into urine.

Blood is not filtered normally.

The kidneys malfunction, and ultimately, chronic kidney disease occurs.


Nerves are damaged because glucose is not used normally and because the blood supply is inadequate.

Legs suddenly or gradually weaken.

People have reduced sensation, tingling, and pain in their hands and feet.

The nerves that control internal body processes such as blood pressure and digestion are damaged.

Swings in blood pressure occur (especially when the person stands).

Swallowing becomes difficult.

Digestive function is altered, and sometimes nausea or bouts of diarrhea occur.


Blood flow to the skin is reduced, and sensation is decreased, resulting in repeated injury.

Sores and deep infections (diabetic ulcers) develop.

Healing is poor.


White blood cell function is impaired.

People become more susceptible to infections, especially of the urinary tract and skin.

Connective tissue

Glucose is not used normally, causing tissues to thicken or contract.

Monitoring and Preventing Diabetes Complications

At the time of diagnosis and then at least yearly, people with type 2 diabetes are monitored for the presence of diabetes complications, such as kidney, eye, and nerve damage. Doctors begin screening tests in people with type 1 diabetes 5 years after diagnosis. Typical screening tests include the following:

  • Foot examination to test sensation and look for signs of poor circulation (ulcers, hair loss)

  • Eye examination (done by an eye specialist)

  • Urine and blood tests of kidney function

  • Blood tests for cholesterol levels

  • Sometimes an electrocardiogram

Worsening of complications can be prevented or delayed by strict blood glucose control or by early drug treatment. Risk factors for heart problems, such as increased blood pressure and high cholesterol levels, are evaluated at each doctor visit and are treated with drugs if necessary.

Another common problem in people with diabetes is gum disease (gingivitis), and regular visits to the dentist for cleaning and preventive care are important.

Did You Know...

  • People who can strictly control their blood glucose levels may be able to minimize or delay diabetes complications.

Prevention of hypoglycemia

One of the challenges of trying to strictly control the levels of glucose in the blood is that low blood glucose levels (hypoglycemia) may occur with some commonly used antihyperglycemic drugs (such as insulin or sulfonylureas). Recognizing the presence of low blood glucose is important because treatment of hypoglycemia is an emergency. Symptoms may include hunger pangs, racing heartbeat, shakiness, sweating, and inability to think clearly.

If hypoglycemia is very severe, sugar must get into the body quickly to prevent permanent harm and relieve symptoms. Most of the time, people can eat sugar. Almost any form of sugar will do, although glucose works more quickly than table sugar (typical table sugar is sucrose). Many people with diabetes carry glucose tablets or glucose gel packs. Other options are to drink a glass of milk (which contains lactose, a type of sugar), sugar water, or fruit juice or to eat a piece of cake, some fruit, or another sweet food. In more serious situations, it may be necessary for emergency medical practitioners to inject glucose into a vein.

Another treatment for hypoglycemia involves the use of glucagon. Glucagon can be injected into a muscle or inhaled as a nasal powder and causes the liver to release large amounts of glucose within minutes. Small transportable kits containing a syringe or autoinjector pen filled with glucagon are available for people who frequently have episodes of low blood glucose to use in emergency situations when sugar cannot be ingested by mouth.

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