Injected insulin that functions as basal insulin is called “long-acting” and provides a relatively low level of insulin for a long period. Insulin that functions as bolus insulin is called “short-acting” or “rapid-acting” and provides a higher level of insulin that is used quickly.
Three types of long-acting insulin are commonly used: NPH (brand name Humulin N), insulin glargine (Lantus), and insulin detemir (Levemir). NPH insulin lasts 10–16 hours in the body. It may initially be taken as a single daily injection, but eventually it usually needs to be taken twice a day. The main advantage of NPH insulin is that it is inexpensive. Its main drawback is that the timing of its peak of action is unpredictable, which can lead to hypoglycemia (low blood glucose) if meals are not timed with injections properly. (An insulin’s “peak” is when it is most active in the body. It varies by type of insulin, and ideally injections are timed so that the insulin’s peak coincides with the rise in blood glucose that follows a meal.)
Insulin glargine is a long-acting insulin that can last up to 24 hours and has little peak in its action, which reduces the risk of hypoglycemia. Another advantage of insulin glargine is that it only requires one injection each day for the vast majority of people with Type 2 diabetes.
The newest long-acting insulin, insulin detemir, usually lasts 16–20 hours. In general, it has less of a peak than NPH but is not as “flat” as glargine. Insulin detemir tends to be the most predictable of the long-acting insulins. It has also consistently been shown to cause less weight gain than the other insulins (or even mild weight loss). Detemir and glargine cost about the same, but both are more expensive than NPH. They cannot be mixed with other insulins in the same syringe, while NPH can. All three basal insulins are available in prefilled pens that do not require syringes or vials. Doses of insulin can be dialed into the pen, and the pens can be carried conveniently in a coat pocket, purse, or knapsack.
The oldest type of short-acting insulin is Regular insulin (brand names Humulin R and Novolin R). It lasts about 6–8 hours and has its peak about 2 hours after injection. It does not start working (lowering blood glucose) until about 30–60 minutes after injection, so it can be difficult to coordinate the timing of injections with meals. For example, if you take an injection of Regular insulin right before you eat lunch, your lunch will likely raise your blood glucose level before your insulin starts working to lower it. You would need to inject the insulin 30–60 minutes before eating lunch to match the rise in blood glucose with the action of the insulin. In spite of this inconvenience, Regular insulin is still widely used because it is very inexpensive, and because many physicians have years of experience prescribing it.
Three other forms of short-acting insulin (considered “rapid-acting”) are insulin aspart (NovoLog), insulin lispro (Humalog), and insulin glulisine (Apidra). These insulins each have a slightly different chemical structure, but all last less than 5 hours and start to work within 15 minutes. They are all relatively expensive but are easier to coordinate with meals than Regular insulin. In general, these three rapid-acting insulins match up better with the body’s release of glucose into the blood after eating, resulting in a lower risk of hypoglycemia, but are no better than Regular insulin at lowering the HbA1c level. All short-acting insulins are available in easy-to-use insulin pens.
Long- and short-acting insulins are also available in premixed combinations such as 70% NPH and 30% Regular, also known as “70/30.” Although premixes may initially appear to be more convenient, they are difficult to tailor to individual needs due to the fixed proportions of the combinations. A person who needs, for example, more short-acting insulin but not more long-acting insulin is out of luck when using a premix. For more information on the different types of insulin, see “Insulin Action times.”