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, also called fast-acting insulin, is absorbed into the bloodstream within minutes and is often used in the treatment of diabetes. This form of insulin has a chemical structure that is quickly absorbed into the bloodstream and has the same ability to lower blood glucose (sugar) as other forms of insulin. 

Sometimes people who have diabetes use more than one type of insulin to balance out blood sugar throughout the day.


Most 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 enough insulin. In type 2 diabetes, the pancreas typically makes an adequate supply of insulin, but the body does not respond well to it, and insulin injections can sometimes help with glucose metabolism.

People with type 1 diabetes and some people with type 2 diabetes need to take both basal and bolus insulin. Basal 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 insulin the pancreas would naturally make in response to glucose taken in through food.

In a person who doesn't have diabetes, the amount of bolus insulin produced depends on the carbohydrate load of a meal. Rapid-acting insulin is a type of bolus insulin.

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

Rapid-Acting Insulin in Pumps

Rapid-acting insulin is often 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 typically taken at mealtimes to offset the effects of rising glucose from food.

Types of Rapid-Acting Insulins

There are currently 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 1966.
  • Aspart is marketed under the brand names NovoLog and Fiasp.
  • Glulisine and is best known by its brand name Apidra
  • 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 be prescribed rapid-acting insulin shortly after being diagnosed with the condition.

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 you have type 2 diabetes, your doctor may recommend starting an insulin regimen if your weight, blood glucose, or A1C levels are not in the target range 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 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.

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

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.

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 it.

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. This same study showed that patients were more likely to take their medication as prescribed when using the pen when compared to the vial. Lispro led to slightly more inpatient stays when compared to aspart, and using a vial led to more inpatient hospital stays when compared to using the pen.

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 you haven’t used them. 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) 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.


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 any other medical testing prescribed for you.

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.

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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