What to Know About Rapid-Acting Insulin

How It Works and What Kinds Exist

Woman holding insulin syringe
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Rapid-acting insulin, which is absorbed into the bloodstream within minutes, has many potential benefits for people with diabetes.

A rapid-acting insulin is one whose chemical structure has been changed so that the insulin is more quickly absorbed into the bloodstream. This change to the chemical structure of the insulin does not affect its ability to lower blood sugar, but it does affect the rate at which the insulin is absorbed. 


Rapid-acting insulin is usually taken with meals to help control what’s called post-prandial (after eating) blood sugar.

All people with type 1 diabetes and some people with type 2 diabetes need to take insulin in order to process glucose from food.

In type 1 diabetes, this is because the pancreas no longer makes insulin. In type 2 diabetes, the pancreas makes insulin, but the body no longer responds as well to it, sometimes making injections of insulin necessary.

People with type 1 diabetes and some people with type 2 diabetes need to take both basal and bolus insulinBasal insulin is the background insulin that is normally supplied by the pancreas and is present 24 hours a day, whether or not you eat. Bolus insulin refers to the extra amounts of insulin the pancreas would naturally make in response to glucose taken in through food. The amount of bolus insulin produced depends on the size of the meal. Rapid-acting insulin is a type of bolus insulin.

Rapid-Acting Insulin in Pumps

Rapid-acting insulin is also used in insulin pumps to provide a continuous low level of insulin. This continuous flow of basal insulin is the ongoing insulin that is needed to manage normal fluctuations in blood sugar between meals and during sleep. Additional units of rapid-acting insulin are given at mealtimes to offset the effects of rising glucose from food.

Types of Rapid-Acting Insulins

There are currently four rapid, or fast-acting, insulins. The first is generically referred to as lispro but is marketed under the name Humalog and manufactured by Eli Lilly. Humalog is the oldest of the three rapid-acting insulins and has been commercially available since 1966.

A second rapid-acting insulin was approved by the U.S. Food and Drug Administration in 2000. This insulin carries the generic name aspart but is marketed under the brand name NovoLog and is produced by Novo Nordisk.

Yet a third rapid-acting insulin was added in 2006. It is called insulin glulisine and is best known by its brand name Apidra. It is marketed by Sanofi-Aventis.

In 2015, another type of rapid-acting insulin became available in the United States. It's an inhaled insulin by the brand name Afrezza.

Before Taking

People with type 1 diabetes will likely start taking a rapid-acting insulin shortly after being diagnosed.

People with type 2 diabetes may find they’re able to maintain their blood sugar within optimal levels through lifestyle changes (such as diet and exercise) or a first-line medication such as metformin. If not, their doctor is likely to suggest taking some form of insulin. Depending on their glycemic control goals, most people with type 2 diabetes will be able to manage the disease with basal insulin and will not need to use rapid-acting insulin at mealtimes.

According to American Diabetes Association guidelines, people with type 2 diabetes may want to begin taking insulin if they’re experiencing rapid weight loss, symptoms of hyperglycemia, or have very high A1C (greater than 10%) or blood glucose (greater than 300 mg/dL) levels. Your doctor may also recommend starting some kind of insulin regimen if your A1C levels are more than 1.5% over your recommended target.

Precautions and Contraindications

Any time you make changes to your insulin regimen or medication (such as introducing a new brand or product), you should be especially vigilant in monitoring changes in blood glucose levels with the help of your doctor, especially in the first few weeks.

In addition, rapid-acting insulins should not be used when you’re experiencing hypoglycemia. As with all insulins, your doctor may suggest regular liver and kidney monitoring if you have liver problems or kidney impairment and take rapid-acting insulins.

Allergic reactions have been reported with insulin. If you are sensitive to rapid-acting insulin or allergic to any of its ingredients, you should not take it. People with impaired lung function, such as in COPD and asthma, and current smokers may not be able to use Afrezza.


Your dosage for any insulin is based on weight, dietary needs, and other individual factors. Your doctor will suggest an initial regimen, but most people generally start with taking short-acting insulin two to three times a day, before meals or as needed. Basal (long-acting) insulin will make up the remainder of your insulin needs for the day.

The benefits of short-acting insulins are that they allow you to adjust insulin levels according to your mealtimes, exercise levels, and other activities. It may take time, but you’ll work with your doctor to find the optimal dose for your daily life.

How Rapid-Acting Insulin Works

All rapid-acting insulins are quickly absorbed into the bloodstream. This quick absorption allows a person to inject a rapid-acting insulin just before eating to “cover” the rise in blood sugar that occurs when food is eaten.

Onset, Peak, and Duration in Fast-Acting Insulins
  Novolog (aspart) Apidra (glulisine) Humalog (lispro) Afrezza
Onset 5-15 minutes 5-15 minutes 5-15 minutes 1 minute
Peak action 1-3 hours 30-90 minutes 30-90 minutes 12-15 minutes
Duration 3-5 hours 3-5 hours 3-5 hours 2.5-3 hours
Difference in onset, peak, and duration among rapid-acting insulins

How to Take and Store

Rapid-acting insulin may come in a pen or a vial meant for injection beneath the skin. Your medical providers will show you how to administer a dose.

Storage directions for rapid-acting insulin vary somewhat based on form, so be sure to read the directions for your specific medication, and check with your doctor if you have any questions. Make it a habit to always double-check your insulin dose before administering.

Considering ask for the pen form, which all three short-acting insulins offer (by contrast, Afrezza is inhaled). A recent study found no significant difference between Humalog (lispro) and NovoLog (aspart), as well as pen and vial, in most outcomes. However, lispro led to slightly more inpatient stays as compared to aspart, as did use of a vial. The pen also made it easier to adhere to a regimen.

Pens should be kept refrigerated until ready to use. After opening, they should be stored at room temperature. They should be used or discarded within 28 days, even if you haven’t used them.

Generally, avoid light and heat exposure and keep all insulin medication stored at temps below 86 degrees (Apidra can reach up to 98.6 degrees). When unopened, store in the fridge. It’s okay to keep opened containers at room temperature, but they should be discarded after 28 days. They can also be refrigerated, but never freeze any form of insulin. If you notice anything floating (particulate matter) or the medication has become cloudy or discolored, discard it. Never share insulin with someone else.

Side Effects

People who have diabetes must carefully balance blood sugar and insulin needs with meals, exercise, and even periods of not feeling well to avoid the most common side effect of insulin, which is low blood sugar (hypoglycemia). When taking any insulin product, you’ll also want to carefully monitor for hyperglycemia. While hypoglycemia is more common, both are dangerous and should be avoided. If you experience either, see your doctor to have your dose adjusted.


Weight gain may be a side effect of insulin therapy. Skin reactions at injection sites, such as itching, rash, and swelling, are also not uncommon. Afrezza, which is inhaled, may cause coughing and sore throat.


Heart failure is an uncommon possible side effect of all insulins. The risk increases if you’re combining insulin therapy with drugs called thiazolidinediones. Your doctor will monitor your heart function closely when you have diabetes, particularly if you’re combining these two drugs. Pay attention to warning signs of heart trouble. They include shortness of breath, swelling of feet or ankles, or sudden weight gain. If you experience these or other severe symptoms, such as a fast heartbeat, drowsiness, dizziness or confusion, seek emergency medical help. 

Warnings and Interactions

Many medications—from hormones (estrogen, androgen, and others) to metformin, beta-blockers, and even over-the-counter decongestants such as pseudoephedrine—can affect how your body processes insulin and increase the risk of hypoglycemia or hyperglycemia. Tell your doctor about all the drugs and supplements you are taking, and be sure to keep up with the home-based monitoring and other medical testing they prescribe.

All forms of insulin lower blood potassium levels. Watch for supplements or other medications that may compound this effect, such as diuretics, albuterol (used in asthma inhalers), and pseudoephedrine (found in Sudafed). Symptoms of low potassium (hypokalemia) include muscle weakness that begins in the legs and moves upward, nausea or decreased appetite, and heart arrhythmias.

Note that two common drugs used to treat high blood pressure are known to raise potassium levels. ACE inhibitors may improve insulin sensitivity and allow you to lower your dose of insulin. Angiotensin II receptor blockers, or ARBs, may have the same effect.

Of course, food and supplements can also affect blood sugar and glycemic control. For example, while dietary use of garlic is unlikely to affect blood sugar levels (up to 50 milligrams per day), there is some evidence garlic at higher doses (up to 1.5 grams) may improve blood glucose control. Let your doctor know if you choose to take this or any other supplement.

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