All insulin is not alike. You may already have heard of long-acting and short-acting insulin, but there are several other varieties too. Each has a specific role to play in helping regulate blood glucose (blood sugar) levels.

Different insulins or combinations of insulin suit different people's unique needs, and many people who use insulin require two or more types to achieve their blood sugar goals.

Your doctor determines which insulin to prescribe. But by learning more about how the different types of insulin work, you will be better able to monitor your response to your insulin. Then you can work together with your doctor to adjust your treatment plan to achieve and maintain the best possible control of your diabetes.

What is insulin?

Insulin is a hormone made in the pancreas and used every day in the human body. Whenever you eat, the food breaks down into sugar that moves into your bloodstream. Insulin is required to move that sugar from your blood into cells throughout your body so they can use it for energy.

Think of insulin as the key that opens cell doors to let blood sugar in.

Without insulin, all that sugar has nowhere to go, so it sits in your blood until it can be excreted, creating the high glucose levels that are the bane of people with diabetes.

Traditionally, insulin was only used to treat people with type 1 diabetes, whose bodies no longer make enough insulin. But insulin has also proven to be an excellent therapy for a growing number of people with type 2 diabetes, whose bodies make some insulin but struggle to use it effectively. Insulin is also sometimes used to treat other lesser-known forms of diabetes.

All popular types of prescription insulin must be injected into your bloodstream. If you swallowed insulin in a pill, the acid in your stomach would digest it, making it useless.

Early injections used insulin from cows and pigs. But modern medicine has developed laboratory-made insulin that is exactly the same as the insulin made in the human pancreas. Today this biosynthetic insulin is used almost exclusively.

The FDA has also approved Afrezza, an inhaled rapid-acting insulin that may provide an alternative to injections of rapid-acting insulin for some people.

Different insulins for different needs

Medical advances have created different types of lab-made insulin, called insulin analogs. They vary in how they act within the body in three important ways:

• Onset, which is how soon the insulin starts to lower your blood sugar after you take it
• Peak, the time when the insulin achieves its greatest effect on lowering blood sugar
• Duration, how long the insulin keeps working before it wears off

Individuals respond differently to the same medicine, and insulin is no exception. Your onset, peak, and duration times may vary from the averages mentioned below. That’s another good reason to work closely with your doctor to find the best insulin treatment plan for you.

Here are the most common types of insulin available today:

Rapid-acting insulin has the fastest onset, averaging 15 minutes. It typically peaks in 30 to 90 minutes and lasts one to five hours. Rapid-acting insulin covers insulin needs when meals are eaten at the same time as the injection is administered. Most people who use rapid-acting insulin also use a longer-acting insulin. Brands include NovoLog (generic: aspart), Apidra (glulisine), and Humalog (lispro).

The rapid-acting inhaled insulin Afrezza achieves peak levels in about 15 minutes and may last up to three hours. It is not recommended for smokers, people with chronic lung disease such as asthma, or to treat a condition called diabetic ketoacidosis.

Short-acting insulin kicks in a little slower and lasts a little longer: the typical onset is 30 to 60 minutes, peak is two to four hours, and duration is five to eight hours. It is injected 30 to 60 minutes before a meal. Brands include Humulin R and Novolin R, also generically called “regular (R).”

Intermediate-acting insulin lasts even longer. Onset averages one to three hours, peak comes in four to 12 hours, and duration is 12 to 18 hours. This insulin often covers an individual’s needs for about half a day or overnight. In many cases, users are also prescribed a rapid- or short-acting insulin as well. Intermediate-acting insulin is sold as Humulin N, Novolin N, and is generically called NPH (N).

Long-acting insulin, as you would expect, lasts longest of all—up to 26 hours—so the typical once-daily injection covers the body’s continuous need for insulin for about one entire day. Onset is generally one to two hours. These insulins are designed to have minimal peaks, delivering insulin at a steady rate. Name brands include Levemir (insulin detemir), Lantus (insulin glargine), and Toujeo (insulin glargine). The newest long-acting insulin is Tresiba (insulin degludec). Tresiba has been shown to last longer than 24 hours with regular use, which may help people who occasionally forget to inject it at the same time each day. (That doesn't mean you can skip a day, though.) People who take long-acting insulin may also be prescribed rapid- or short-acting insulin.

Pre-mixed insulins offer a convenient way to get a combination of two insulins in a single bottle or insulin pen. Each pre-mixed insulin contains intermediate-acting insulin as well as either short- or rapid-acting insulin. The amount of each type of insulin is identified by the numbers following the name brand. For example, Humulin 70/30 contains 70 percent intermediate-acting and 30 percent short-acting insulin. Several different pre-mixed insulins are available under the Humulin, Novolin, Humalog, and Novolog names. They are generally taken two or three times daily, before meals. Their onsets are usually five to 15 minutes, peaks vary from 30 minutes to five hours, and durations are 10 to 24 hours.

What about side effects?

The list of possible side effects for insulin is shorter than it is for many other medicines. Insulin side effects may affect some users but not others. Let your doctor know if you think you are having any side effects so your insulin treatment can be adjusted if necessary.

Two possible side effects are common to all forms of insulin.

First, because insulin acts to reduce the sugar in your bloodstream, taking too much insulin may cause hypoglycemia or abnormally low blood sugar. That’s a dangerous condition requiring immediate attention. An insulin overdose isn’t always the cause of hypoglycemia, however. It can result from skipped meals, strenuous exercise, a change of diabetes pills, or anything else that alters the delicate balance between the amount of insulin you take and the way your body uses it.

Second, some people who start using insulin notice that they gain weight. That could happen because the new insulin supply helps the body better use sugar in the bloodstream instead of excreting much of it. All those sugar calories can quickly add up. What’s more, the high volume of urine that the body had been producing to get rid of blood sugar may leave some chronically dehydrated. When insulin allows that excessive urination to stop and a healthy level of hydration is restored, the weight of the additional water shows up on your bathroom scale. Don’t be alarmed and don’t cut back on liquids; look elsewhere in your diet if you need to shed a few pounds.

Among other potential side effects, insulin may also produce flu-like symptoms in a small percentage of people. And some may experience undesirably low levels of potassium in their blood. Your doctor can confirm whether any other unusual symptoms may be side effects of your insulin.

Serious allergic reactions to insulin are rare but can be life-threatening. Seek medical care if you develop possible insulin allergy symptoms including rash, itching, swelling, headache, dizziness, trouble breathing, nausea, or vomiting.

It’s worth it

Like so many important things in life, taking insulin isn’t always easy—but the benefits are worth it. Together with exercise, a healthy diet, and other diabetes medicines you may take, insulin is a powerful cornerstone of diabetes care. It can help you achieve and maintain your blood sugar goals, reduce your risk of diabetes complications, and feel your very best for years to come.

To learn more about this topic:
Keep Your Insulin Safe
Why Insulin Is Not Just a Last Resort
Pump Iron, Not Insulin: How Workouts Curb Diabetes