Hyperglycemia, also known as high blood sugar, is a condition where there is too much glucose (sugar) in the blood. It is a common problem for people with diabetes, but it can also affect people who are seriously ill, such as those who have recently had a stroke or heart attack, or have a severe infection. High blood sugar can occur for many reasons, including eating too much, being sick, not taking enough glucose-lowering medication, or not using enough insulin or other diabetes medication. Symptoms of hyperglycemia develop slowly over several days or weeks and include high blood glucose, high levels of glucose in the urine, frequent urination, increased thirst, and blurred vision. If left untreated, high blood sugar can cause damage to nerves, blood vessels, and organs, and can lead to serious complications such as ketoacidosis or hyperglycemic hyperosmolar nonketotic syndrome. People with diabetes must use medication, such as oral diabetes medications or synthetic insulin, and/or lifestyle changes to help keep their blood sugar levels in range. Checking blood glucose levels often and treating high blood glucose early can help avoid problems associated with hyperglycemia.