High levels of blood glucose in general are more likely to cause long term problems than short ones. However, if someone has type one diabetes then they can go into DKA (diabetic ketoacidosis) which can lead to a coma. People with type 2 diabetes don't generally produce ketones so it is not as much of an issue for them. Low levels are actually caused by the insulin that people with insulin dependent diabetes need to stay alive! My son spent a few days of his holiday in a Bulgarian hospital a few years ago because of a hypo that caused a seizure. It is a constant balancing act between the amount of insulin, the carbohydrate intake and the exercise that the person takes. People who have many years of experience of looking after their own diabetes will still often struggle to maintain good control, because there are so many variables and the body doesn't always behave as expected! The basics are that insulin will bring blood glucose levels down and glucose will raise them.