If you have diabetes, your blood glucose, or blood sugar, levels are too high. Glucose comes from the foods you eat. A hormone called insulin helps the glucose get into your cells to give them energy. With type 1 diabetes, your body does not make insulin. With type 2 diabetes, your body does not make or use insulin well. Without enough insulin, the glucose stays in your blood.

Over time, having too much glucose in your blood can cause complications, including:

• Eye disease, due to changes in fluid levels, swelling in the tissues, and damage to the blood vessels in the eyes
• Foot problems, caused by damage to the nerves and reduced blood flow to your feet
• Gum disease and other dental problems, because a high amount of blood sugar in your saliva helps harmful bacteria grow in your mouth. The bacteria combine with food to form a soft, sticky film called plaque. Plaque also comes from eating foods that contain sugars or starches. Some types of plaque cause gum disease and bad breath. Other types cause tooth decay and cavities.
• Heart disease and stroke, caused by damage to your blood vessels and the nerves that control your heart and blood vessels
• Kidney disease, due to damage to the blood vessels in your kidneys. Many people with diabetes develop high blood pressure. That can also damage your kidneys.
• Nerve problems (diabetic neuropathy), caused by damage to the nerves and the small blood vessels that nourish your nerves with oxygen and nutrients
• Sexual and bladder problems, caused by damage to the nerves and reduced blood flow in the genitals and bladder
• Skin conditions, some of which are caused by changes in the small blood vessels and reduced circulation. People with diabetes are also more likely to have infections, including skin infections.

If you have diabetes, you need to watch out for blood sugar levels that are very high (hyperglycemia) or very low (hypoglycemia). These can happen quickly and can become dangerous. Some of the causes include having another illness or infection and certain medicines. They can also happen if you don't get the right amount of diabetes medicines. To try to prevent these problems, make sure to take your diabetes medicines correctly, follow your diabetic diet, and check your blood sugar regularly.

NIH: National Institute of Diabetes and Digestive and Kidney Diseases