Insulin Basics

Everyone needs the hormone insulin to keep blood glucose within a normal range. In people without diabetes, the pancreas makes insulin. When you have Type 1 diabetes, your body doesn’t make enough insulin, which is why you must inject insulin. If you have Type 2 diabetes, you may not need additional insulin at first (you may use oral medication and/or non-insulin injectables along with lifestyle modifications). However, because of the progressive nature of the disorder, you will likely need insulin at some point. The table below describes insulin use in adults with diabetes.

Insulin Use in Adults


Most children and adolescents diagnosed with diabetes have Type 1 and use insulin. So if you have diabetes and use insulin, you are not alone. This article reviews important characteristics, focuses on different concentrations of injectable insulin, and provides insulin safety tips.

Insulin characteristics

You need to know three characteristics about insulin. These characteristics will help you predict how it will work once injected. For each type of insulin you use, you should know:

• the onset, or how long it takes insulin to get into your bloodstream and start lowering your blood glucose;

• the peak, or time when a particular insulin has the most impact on lowering your blood glucose; and

• the duration, or how long your insulin’s effect on blood glucose will last.

Regardless of the type of insulin, you should know what to expect after you inject it. The category of the insulin describes the onset of insulin action. For example, rapid-acting insulin starts working within 15 minutes after you administer it, reaches maximum effect in about one hour, and lasts for two to four hours.

Regular or short-acting insulin reaches the bloodstream within 30 minutes, has maximum effect between two and three hours, and lasts for three to six hours once injected.

Intermediate-acting insulin reaches the bloodstream two to four hours after being injected, reaches maximum effect between four and 12 hours, and lasts for 12 to 18 hours.

Long-acting insulin slowly enters the bloodstream over several hours and tends to lower glucose levels evenly over a 24- or 36-hour period. Keep in mind that insulins within each category may vary slightly. These details are based on years of research, and listed in the package insert.

Insulin concentration

In less than five years, several different concentrations of injectable insulin have become available. This is important for you to know so you get not only the correct type of insulin, but also the right concentration. If the concentration is incorrect, you may not receive the right dose.

U-100 insulin concentration

U-100 is by far the most commonly used concentration (strength of insulin. U-100 means that each milliliter [mL] of U-100 insulin contains 100 units. That means a standard 10 mL vial of U-100 insulin has 1000 units. Most insulin pens hold 3 mL and have 300 units of insulin. Also, be sure you know both the generic and trade names of your insulin to be sure you have the right product. U-100 insulin aspart injection (generic name) now approved with two different trade names, Novolog® and Fiasp®. While they are both rapid-acting insulin, they work a little differently — be aware if you are prescribed one of these medications.

U-200 insulin concentration

At least two insulins — a rapid-acting (insulin lispro [Humalog®]) and a long-acting (insulin degludec [Tresiba®]) — are available in both U-100 and U-200 concentrations. A word of caution: the generic and trade names are the same for both concentrations, which may cause confusion. The U-200 concentration has 200 units of insulin per mL, and therefore, twice as many (600) units of insulin in a 3 mL pen as the U-100 concentration (300 units). To avoid incorrect dosing, it is important to know the concentration of insulin you are prescribed.

U-300 insulin concentration

Currently, one insulin (insulin glargine, known as Toujeo®) is available in this concentration. This is different from other insulin glargine formulations because the medication has 300 units per mL (compared to the U-100 concentration with 100 units/mL or U-200 concentration with 200 units/mL). In other words, the U-300 concentration is three times as potent as the U-100 concentration. Once again, you should use the medication exactly as prescribed to ensure you are getting the correct dose.

U-500 insulin concentration

A version of regular insulin (Humulin® R) is available in U-100 and U-500 concentrations. The U-500 concentration delivers five times as much insulin in 1mL as the standard U-100 concentration. This insulin may be prescribed to control high blood glucose in children and adults with diabetes who need more than 200 units of insulin a day. While this insulin comes in a pen, vials of U-500 insulin are still available. If you take U-500 from a vial, be sure that you use a special U-500 insulin syringe; it has a green rather than an orange cap like the typical U-100 syringes. Make sure you have the right tools to dose this insulin correctly.

The main takeaway is that if you are prescribed insulin in a U-100 concentration and use U-200, U-300, or U-500 by mistake, you will get two, three, or five times, respectively, the amount of insulin you need. Too much insulin will dramatically increase your risk for hypoglycemia (low blood glucose). In contrast, if you need a stronger concentration of insulin such as U-200, U-300, or U-500 and inadvertently use the U-100 concentration, your risk for hyperglycemia (high blood glucose) will increase.

The bottom line is make sure you have the right type (know both the trade and the generic names) and the right concentration of insulin.

Thinking about insulin therapy can be overwhelming. Make sure you and your support system stay informed about the details of your insulin treatment so you can live a long, healthy, and safe life with diabetes.

Want to learn more about insulin? Read “What Does Insulin Do?” “Types of Insulin,” and “Insulin: What You Need to Know.”