Q. I'm confused about Novolog and Novolin insulins. What is the difference between them and can one be substituted for the other? - James Castille, San Francisco, CA

A. Understanding insulin can be confusing because there are so many types, some of the names sounds alike, and even have different variations of the same name. Types of insulin differ with respect to their source, how fast they begin to act, and how long their effect persists. Today most people use synthetic brands of insulin, such as Humulin and Novolin, which are identical to natural human insulin. Older insulin brands such as Iletin are extracted from the pancreases of cattle or pigs and differ slightly from human insulin, making them more likely than synthetic types to cause side effects such as skin rash and fat deposits under the skin.

With respect to speed of onset and duration of effect, there are prompt, short, intermediate, and long-acting insulins. Lispro (brand name Humalog or NovoLog) is a very-fast-acting insulin designed to take effect in less than 30 minutes and reach its peak sugar-lowering effect after 60 to 90 minutes. It is usually given 5-10 minutes before a meal and is effective for 3 to 5 hours.

Regular insulin (brand name Humulin R or Novolin R) is defined as short acting. It begins lowering blood sugar from 30 to 60 minutes after you inject it. The peak effect from an injection occurs after two to three hours and the effect lasts for a total of 5 to 7 hours.

NPH (brand name Humulin N or Novolin N) is an intermediate-acting insulin that starts working in about 1 to 2 hours and can last from 16 to 24 hours.

Insulin glargine (brand name Lantus) is a newer form of long-acting insulin. It starts to work within 1 to 2 hours and continues acting for about 24 hours. Lantus is different from other forms of insulin (sometimes referred to as the "poor man's pump") since it does not have a peak effect. Instead, it lowers blood sugar to a relatively constant level during the 24-hour period.

Many people use more than one type of insulin to control their blood sugar. Certain types of insulin should not be mixed together in the same syringe, so check with your doctor or pharmacist before doing so. Some people use premixed combinations of Regular and NPH. Examples include Humulin 70/30, Novolin 70/30, and Humulin 50/50. The first number is the percent NPH and the second indicates the percent regular insulin. They provide the same activity as if NPH and Regular insulin were injected separately but are easier to administer since only one dose needs to be measured.

Some people with Diabetes use a combination of fast-acting and long-acting insulin to help control blood sugar levels throughout the day. Your doctor will help decide which type(s) of insulin are best for you.