What to Know About Rapid-Acting Insulin

An Important Component of Diabetes Treatment

Rapid-acting insulin, also called fast-acting insulin, is a type of synthetic (man-made) insulin. It is an injectable medication prescribed for people with diabetes to help control glucose (blood sugar) levels.

It is absorbed into the bloodstream quickly—typically within minutes—to mimic the action of bolus insulin, the surge of insulin released by the pancreas in response to the ingestion of food. For this reason, it is injected just prior to meals and snacks.

Rapid-acting insulin is administered with a needle and syringe, a pre-filled pen, or an insulin pump. There is also a version of rapid-acting insulin that can be inhaled.

Woman holding insulin syringe
Tuomas Marttila / Getty Images


Rapid-acting insulin is prescribed for people with type 1 diabetes who must take insulin because their pancreas does not produce it.

A healthy pancreas releases bolus insulin when blood glucose increases after food or beverage consumption. Rapid-acting insulin mimics bolus insulin and is taken prior to meals and snacks.

Rapid-acting insulin is also used to bring overly high blood glucose to a normal level.

Some people with type 2 diabetes may require supplemental insulin if they're unable to control their blood glucose (sugar) with diet, exercise, and medications, although they typically need only basal insulin.

Basal insulin, bolus insulin's counterpart, is what is normally pumped out continuously by the pancreas to keep a steady level of insulin in the bloodstream at all times.

Types of Rapid-Acting Insulin

There are four formulations of rapid-acting insulin approved by the U.S. Food and Drug Administration (FDA) to treat diabetes, each available under different brand names and as generics.

They differ somewhat in how quickly they're absorbed by the body, when they start working, and how long they last.

  lispro glulisine aspart inhaled insulin
Brand Names Humalog, Admelog Apidra Novolog, Fiasp Afrezza
Onset 5-15 minutes 5-15 minutes 5-15 minutes 1 minute
Peak action 30-90 minutes 30-90 minutes 1-3 hours 12-15 minutes
Duration 3-5 hours 3-5 hours 3-5 hours 2.5-3 hours

Before Taking

Insulin is prescribed as soon as type 1 diabetes is diagnosed (usually during childhood, the teenage years, or young adulthood).

For someone with type 2 diabetes, however, insulin typically is not a first-line treatment. It will only be prescribed if other methods of managing the condition (diet, exercise, and/or medication) aren't effective at controlling blood sugar levels, or if a person is experiencing symptoms.

Precautions and Contraindications

Although supplemental insulin is necessary and life-preserving if you have type 1 diabetes, there are some circumstances in which you should take precautions when using it. Among them:

  • You are allergic to insulin or any ingredients in insulin products. This will have a bearing on which type or rapid-acting insulin your healthcare provider prescribes.
  • You develop hypoglycemia (low blood sugar).
  • You have liver or kidney impairment, in which case your healthcare provider will want to monitor your organ function regularly while you're on insulin.
  • You drink alcohol, which can cause blood glucose to decrease. Ask your practitioner if it is safe to drink if you are taking insulin.
  • You plan to travel across time zones, as this can affect your blood sugar levels.
  • You become sick, over-stressed, or change your diet—all of which may affect your blood glucose.

Afrezza, inhaled insulin, is not safe for people whose lungs are compromised. This includes people with chronic obstructive pulmonary disease (COPD) or asthma, as well as those who smoke.


According to the American Diabetes Association, the standard (and most common) strength of insulin is 100 units of insulin per milliliter of fluid (U-100). For people who are extremely resistant to insulin, supplemental insulin also is available in a U-500 strength.

Your healthcare provider will determine your insulin dosage based on your weight, dietary needs, how sensitive you are to insulin, and other individual factors. However, there are some general rules for calculating how much rapid-acting insulin to take under two particular circumstances:

  • With meals: The dosage for rapid-acting insulin taken with a meal typically is based on a ration of insulin to carbohydrates in that meal—most commonly, one unit of insulin per 12 to 15 grams of carbs.
  • To bring down overly high glucose levels: In general, one unit of rapid-acting insulin is needed to lower blood sugar by 50 mg/dl.

How to Take and Store

Insulin can be taken using a needle and syringe, a pen (that may be pre-filled or holds a cartridge of insulin that is inserted into it), or an insulin pump (a device that is worn on the body to release a steady dose of basal insulin but also can deliver individual bolus insulin as needed).

Your care provider will show you how to administer your insulin using your chosen method. You may find it easier to use a pen than a syringe and needle.

With either method, it's important to rotate the site of each injection. Never share needles or pens with someone else.

Read the instructions for storing your insulin and follow them closely. Most insulins should be stored in the refrigerator (but never the freezer) until ready to use.

Once a vial or pen is opened, it can be kept at room temperature. Insulin in a pump that is exposed to temperatures above 98.6 degrees should be thrown away. All insulin should be discarded if not used within 28 days.

Examine each vial of insulin before you use it. It should be clear and colorless. If it's not or you see anything floating in it, do not use it.

Keep insulin out of sight and reach of children and pets. Learn what the rules are for disposing of used needles and syringes in your town.

Side Effects

Rapid-acting insulin is safe for most people to use. There are a few common side effects, however. Chief among the potentially serious side effects that can occur is extreme fluctuation in blood glucose levels.

While taking insulin, make sure you know the signs of both hyperglycemia (high blood glucose) and hypoglycemia. Discuss with your healthcare provider what you should do in either case.


Most people gain weight as a side effect of insulin therapy. Constipation is not uncommon.

Skin reactions at injection sites, such as itching, rash, and swelling, can occur. It also possible for fat to build up under the skin, causing it to feel thick, or, conversely, for fat to breakdown, leaving depressions in the skin.

Afrezza may cause coughing and sore throat.


The risk of heart failure increases if you’re combining insulin therapy with drugs called thiazolidinediones. Your healthcare provider will monitor your heart function closely when you have diabetes, particularly if you’re combining these two drugs.

Be aware of warning signs of heart trouble, including 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 (for example, estrogen), metformin, beta-blockers, sufonylurias, GLP-1, SGLT-2, and pseudoephedrine (a decongestant).

Tell your healthcare provider about all the drugs and supplements you are taking, and keep up with 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 this can be exacerbated by certain other medications and supplements, including diuretics, albuterol (used in asthma inhalers), and pseudoephedrine. 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 eating garlic is unlikely to affect blood sugar levels (up to 50 milligrams per day), there is some evidence that higher doses (up to 1.5 grams) may improve blood glucose control. Let your healthcare provider know if you choose to take this or any other supplement.

11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Gary Gilles
Gary Gilles is a licensed clinical professional counselor (LCPC) who has written about type 1 diabetes and served as a diabetes counselor. He began writing about diabetes after his son's diagnosis as an infant.