Insulins are divided into rapid-acting, very rapid-acting, intermediate-acting, and long-acting, based on the number of hours until their "peak" action. Peak action occurs when the concentration of insulin is greatest in the blood, and has its greatest glucose-lowering effect.
RAPID-ACTING (SHORT-ACTING) INSULINS: These include Humulin R (HR), and Novolin Toronto (also known as regular) insulins. Rapid-acting insulins start being absorbed in 30-60 minutes, and have their peak action within 2 to 3 hours of injection in most individuals. Their duration of action is approximately 6 to 8 hours but there is a great degree of variation. Short-acting insulins are often used before eating to control the large rise of blood glucose that often occurs after a meal. Ideally, short-acting insulin is taken approximately 30-45 minutes before the meal, as it takes up to 2 hours to see its main effect. However, most people take their insulin just before eating.
VERY RAPID-ACTING INSULIN: We are now in an age when the insulin molecule can be manipulated in order to give it more desirable properties. The first such "insulin analog" is Humalog (insulin Lispro), in which a chemical change has been made to the insulin molecule. This gives the insulin a very desirable property of extremely rapid absorption. Peak action of Lispro insulin is about 30 minutes after injection, and insulin levels fall off rapidly after 1 to 2 hours. This allows the insulin, if given before the meal, to control the post-meal rise of glucose much better, and to reduce the chance of hypoglycemia (because insulin levels drop faster). This may well be its greatest advantage. It can essentially substitute for any short-acting insulin.
INTERMEDIATE-ACTING INSULINS: These insulins, which include NPH and Lente, start being absorbed 3-4 hours after injection, and have their peak action after 7-9 hours. Again, there is considerable variation as the duration of action may be as much as 12-16 hours after injection. Used in the morning, its greatest action would be in the afternoon. One of the best uses is injection at bedtime to control the morning glucose of the next day.
LONG-ACTING INSULIN: This mainly includes the UltraLente insulin. Human UltraLente insulin has a peak effect occurring after 10-12 hours and its duration of action may be 16-18 hours, and very occasionally longer. It is not a popular insulin. It is mainly used in individuals in whom intermediate-acting insulins taken at bedtime act too quickly, resulting in hypoglycemic reactions during the night. By using a human UltraLente, one can slow down the peak action by an hour or two, often preventing hypoglycemia at night, but controlling the fasting sugar of the next day.
Better long-acting insulins (actually insulin analogs; again, insulins chemically altered to improve their absorption properties) will be available in a few years.