What to Know About Rapid-Acting Insulin

How It Works and What Kinds Exist

Rapid-acting insulin, also called fast-acting insulin, is absorbed into the bloodstream within minutes. It has a chemical structure that is quickly absorbed into the bloodstream to lower blood glucose (sugar) in response to food. As such, it is a form of bolus insulin.

Woman holding insulin syringe
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People with type 1 diabetes and some with type 2 diabetes may use rapid-acting insulin as part of their diabetes management plan. Sometimes people who have diabetes use more than one type of insulin to balance out blood sugar throughout the day.

Rapid-acting insulins became available in the 1990s. They are a synthetic form of insulin called analogue insulin. (Synthetic insulins are also called human insulins, to differentiate them from the earliest animal-sourced forms.) Analogue insulin has been refined in the lab to either take effect more quickly, in the case of rapid-acting insulin, or work more uniformly over time (for long-acting forms), as compared to the first synthetic insulins introduced in the early 1980s. 


Fast-acting insulin is a type of bolus insulin, which is the extra surge of insulin the body produces in response to the increase in blood glucose triggered by the ingestion of food or beverages. Therefore, rapid-acting insulin is usually taken with meals to help control post-prandial (after-eating) blood sugar.

It is different from basal insulin—the "background" insulin supplied by the pancreas on an ongoing basis. In people who do not have diabetes, the pancreas bases the amount of bolus insulin it produces on the carbohydrate load of the meal.

People with type 1 diabetes, an autoimmune condition, need to take insulin because their pancreas does not produce it. Insulin may be part of diabetes management for people with type 2 diabetes who are unable to control their blood glucose (sugar) with diet, exercise, and medications.

How Rapid-Acting Insulin is Administered

Rapid-acting insulin is often used in insulin pumps, in addition to basal insulin, to mimic how a healthy pancreas produces insulin after a meal. While the continuous flow of basal insulin is needed to manage normal fluctuations in blood sugar between meals and during sleep, additional units of rapid-acting insulin typically are taken at mealtimes to offset the effects of rising blood sugar from food.

Rapid-acting insulin can also be administered via a pen or a needle and syringe, or in a newer inhaled form. Research has found that patients are more likely to take their medication as prescribed when using the pen as compared to a syringe and needle. In addition, using a syringe and needle led to more inpatient hospital stays when compared to using the pen.

Types of Rapid-Acting Insulins

There are four formulations of rapid-acting insulin.

  • Lispro is marketed under the names Humalog and Admelog. Humalog is the oldest of the three rapid-acting insulins and has been commercially available since 1996.
  • Aspart is marketed under the brand names NovoLog and Fiasp.
  • Glulisine is best known by the brand name Apidra.
  • In 2015, an inhaled version of rapid-acting insulin became available in the United States. It is sold under the brand name Afrezza (insulin human insulation powder).
Onset, Peak, and Duration of 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

Before Taking

People with type 1 diabetes likely will be prescribed rapid-acting insulin shortly after being diagnosed with the condition.

If you have type 2 diabetes, your doctor may recommend starting an insulin regimen if you're unable to control your blood glucose with diet, exercise, and other medications or if you are experiencing symptoms. 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.

Precautions and Contraindications

Any time you make changes to your insulin regimen or diabetes medication (such as introducing a new brand or product), you should be especially vigilant in monitoring your blood glucose for the first few weeks for signs the changes need to be tweaked, keeping your doctor apprised as you do.

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

If you are sensitive to rapid-acting insulin or allergic to any of its ingredients, you should not take it. Because it is inhaled, people with impaired lung function, such as those with chronic obstructive pulmonary disease (COPD) or asthma, as well as those who smoke, may not be able to use Afrezza.


Dosage for any insulin is based on a person's weight, dietary needs, and other individual factors. Your doctor will suggest an initial regimen, but most people start by 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.

A benefit of fast-acting insulin is that it allows you to adjust insulin levels according to your mealtimes, exercise levels, and other activities. It will take some time and your doctor's guidance to figure out your optimal dose of rapid-acting insulin.

How Rapid-Acting Insulin Works

Rapid-acting insulins are quickly absorbed into the bloodstream. Your doctor may instruct you to inject your dose of rapid-acting insulin just before eating to manage the rise in blood sugar that occurs when food is eaten.

How to Take and Store

Rapid-acting insulin may come in a pre-filled pen or in a vial meant to be injected with a needle and syringe. Many people find the pen to be easier to use.

All three short-acting insulins are available in the pen form, while Afrezza is inhaled. A research study found no significant difference between Humalog (lispro) and NovoLog (aspart) in most outcomes. It also found people taking lispro had slightly more inpatient stays when compared to those taking aspart.

Storage directions for rapid-acting insulin vary based on form. Read the directions for your specific medication and reach out to your doctor if you have any questions. Make it a habit to always double-check your insulin dose before administering it.

As per the package instructions, 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 unused. Avoid light and heat exposure and keep all insulin medication stored 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) in the medication or if it becomes 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.


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 can affect how your body processes insulin and increase the risk of hypoglycemia or hyperglycemia. These include hormones (estrogen, androgen, and others), metformin, beta-blockers, and even decongestants containing pseudoephedrine. Tell your doctor about all the drugs and supplements you are taking, and be sure to keep up with the home-based monitoring and any other medical testing prescribed for you.

All forms of insulin lower blood potassium levels, so it's important to know that certain other medications and supplements may compound this potassium-lowering effect. These include 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.

Some food and nutritional supplements can 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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