Uncontrolled diabetes can be extremely dangerous, resulting in loss of vision, damage to the nerves, blood vessels, kidney and increased risk of heart attack. Diabetes cannot be cured but it can be controlled either by diet alone, oral medication or by administering insulin. Some complications of diabetes are hard to control and can even lead to premature death.

Adverse effects

High levels of glucose in the blood causes diabetic neuropathy. It can cause damage to the blood vessels supplying the nerves and the nerves itself causing gradual loss of sensation primarily in hands and feet which may extend up the limbs.
It causes damage to the blood vessels in the light-sensitive retina at the back of the eye, which affects both the eyes and if left untreated can result in loss of vision and blindness. People with diabetes should have their eye examined yearly.
Damage to the blood vessels in the kidneys and destroys the kidneys function of removing wastes and excess water from the body. Symptoms include vomiting, breathlessness, drowsiness etc.
It can cause hypoglycemia (a disorder in which the blood sugar falls to abnormally low levels) by the gradual build up of glucose lowering drugs in the blood or excessive dose of insulin in relation to food intake. New born baby of a diabetic mother can also develop hypoglycemia.
Bacteria thrives on glucose rich blood and urine, people with uncontrolled diabetes are more prone to urinary tract infection.
It can cause chemical imbalance in the blood causing nausea, vomiting, abdominal pain, confusion etc. It effects type I diabetes patients.