Regular Insulin: What It Does, Benefits, and Risks
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What Is Regular Insulin?

What Is Regular Insulin?
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Published on April 30, 2025
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Regular (R) insulin is a short-acting, injectable insulin used for the management of diabetes. It is typically taken before meals to prevent glucose from rising after a person eats.

 Regular insulin is an older formulation, and it is generally considered to be more challenging to use than newer rapid-acting insulins on the market. Nevertheless, some people prefer to use it, often because it’s less expensive or because it may be available without a prescription.

Regular insulin cannot be substituted for another rapid-acting insulin without making adjustments. It should be taken under a doctor’s care to make sure you’re using it properly.

How Does Regular Insulin Work?

Regular insulin, also known as regular human insulin, RHI, or (R) insulin, is a man-made, injectable insulin that is used to lower blood sugar levels in people with diabetes.

Regular insulin is short-acting, which means it works relatively quickly to lower blood sugar levels, though not as quickly as newer forms of rapid-acting insulin. It’s typically taken 30 minutes before eating and reaches peak effectiveness roughly one to two hours after injection. The effects of regular insulin can last up to six hours.

“Regular insulin is typically used before meals to manage increases in blood glucose that occur after eating,” says Joshua J. Neumiller, PharmD, CDCES, the president of healthcare and education at the American Diabetes Association. “Regular insulin can also be used to correct high blood glucose levels.”

According to Tom Wadsworth, PharmD, the dean and an associate professor at L.S. Skaggs College of Pharmacy at Idaho State University in Pocatello, regular insulin is considered human insulin because it has the exact same chemical structure as the insulin made by the pancreas.

“This distinguishes it from insulin preparations derived from animal sources … [and] from ‘insulin analogs,’ which are human insulins that have been modified to alter their absorption and action profiles,” says Dr. Wadsworth.

Two commercially available regular insulin preparations include Humulin R, manufactured by Lilly,

 and Novolin R, manufactured by Novo Nordisk.

 Wadsworth says both brands work the same way and are generally considered interchangeable.
Premixed insulins, which contain both short-acting and intermediate-acting insulin in the same vial or pen, also may contain regular insulin. Novolin 70/30 and Humulin 70/30 both have 30 percent regular insulin. Using premixed insulin options can sometimes help people decrease the number of injections they need to administer.

Regular insulin is the only type of insulin that also comes in concentrations five times higher than standard doses, so it’s occasionally used by people who are very insulin resistant. This form of insulin is also used in hospitals when insulin needs to be given intravenously and absorption from subcutaneous fat isn’t an issue.

Regular Insulin vs. Rapid-Acting Insulin

Regular insulin, which can be used in an insulin pen, a syringe for injections, or in some cases an insulin pump, starts working relatively quickly. But there are mealtime insulin products on the market that kick in even sooner.

 Newer, rapid-acting insulins like Humalog and NovoLog take only about 15 minutes to reach the bloodstream and approach maximum effectiveness in 30 to 90 minutes. Rapid-acting insulin’s effects last around four hours, less time than regular insulin, which could make them easier to use before exercise or bedtime.

In short, you need to be especially careful with your timing when you take regular insulin. To be most effective, regular insulin must be used about 30 minutes before you start eating, which can make it more difficult to live spontaneously or to navigate situations where you are unsure what food will be available, such as at parties or restaurants. You’ll want to plan ahead even for small snacks between meals.

“Rapid-acting insulins work faster and give more flexibility,” says Wadsworth. “With regular insulin, if you eat too soon or too late, your blood sugar can go dangerously high or low.”

What Are the Benefits of Regular Insulin?

While it’s an older type of insulin, some people prefer regular to newer options. It’s widely available, even without a prescription, but it’s rarely used for daily diabetes management these days.

Regular insulin might be a better choice for those who have type 2 diabetes than those with type 1. One review suggests that those with type 2 can achieve similar glucose control using either rapid or regular insulin. For people with type 1, who tend to have more volatile blood sugar levels, regular insulin may lead to worse results than newer alternatives.

Some people with diabetes use regular insulin because it costs less than rapid-acting insulin. Newer forms of insulin can be very expensive, especially for people without insurance. But Neumiller says the cost gaps are narrowing. In the United States, some people who are uninsured may now be able to get insulin for free, and Medicare and multiple states have a capped out-of-pocket cost for most insulin, which is typically $35 for a 30-day supply.

Some people with diabetes use regular insulin because it costs less than rapid-acting insulin. This medication is available in pharmacies without a prescription, but you do need to ask the pharmacist for it, and you should take it under the care of a diabetes healthcare provider.

 Because it is available inexpensively and without a prescription, some people who require insulin for diabetes consider regular insulin a fallback option in case of emergencies. Though it is more difficult to use than the newer rapid-acting analogs, regular insulin could be a useful option for those without health insurance or without access to their normal supplies.

What Are the Possible Side Effects and Risks of Regular Insulin?

As with any type of insulin, side effects are possible. The most important side effect of insulin use is low blood sugar (hypoglycemia). People who use insulin need to be constantly vigilant to prevent low blood sugar events. It is possible that regular insulin could create an enhanced risk of hypoglycemia, because it is more challenging to time its administration well before meals. It also had a longer-than-desired duration of action, often longer than it took for the meal to be absorbed.

The timing of doses for regular insulin is very specific, so it may be wise to eat on a regular schedule. If you don't eat on a regular schedule or if your meal is unexpectedly delayed, it can easily lead to low blood sugar, says Wadsworth.

Other possible side effects include:

  • Low potassium, which can lead to abnormal heart rhythms
  • Allergic reactions
  • Redness, itching, or other adverse effects at the injection site
  • Swelling in limbs
Injecting medicine repeatedly in the same place on the body can cause skin irritation or a condition known as lipohypertrophy, which is when a lump of fatty tissue develops underneath the injection site. This can interfere with your body’s ability to absorb insulin, so it’s a good idea to rotate injection or pump sites.

There is also some evidence that suggests that human insulins, like regular insulin, are more likely to cause weight gain than newer analog insulins.

The Takeaway

  • Regular insulin is a synthetic, short-acting insulin used before meals to manage blood sugar in people with diabetes. It’s an older formulation than other insulin medications, but it’s safe and effective when taken properly.
  • Regular insulin starts working about 30 minutes after it’s injected, peaks in about two to four hours, and lasts up to six hours. It’s crucial to time the dose properly and match it to carbohydrate intake to avoid blood sugar swings.
  • Compared with newer rapid-acting insulin, regular insulin takes longer to work and may be more challenging to use, but it can be more affordable and can be purchased at a pharmacy without a prescription.

Resources We Trust

EDITORIAL SOURCES
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy. We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.
Resources
  1. Regular Insulin Injection (Short-Acting Insulin). Cleveland Clinic. July 14, 2022.
  2. Herman WH et al. 100 Years of Insulin: Why Is Insulin So Expensive and What Can Be Done to Control Its Cost? Endocrinology and Metabolism Clinics of North America. October 14, 2022.
  3. Types of Insulin. Centers for Disease Control and Prevention. May 15, 2024.
  4. Humulin R U-500. Lilly.
  5. Products Diabetes. Novo Nordisk.
  6. Rafi E et al. Navigating Insulin Options for Diabetes Management. The Journal of Clinical Endocrinology and Metabolism. February 25, 2025.
  7. Health & Wellness Insulin Basics for Diabetes. American Diabetes Association.
  8. Eledrisi MS et al. Comparison of Insulin Analogs and Human Insulins: A Narrative Review. Journal of Diabetes and Endocrine Practice. August 10, 2023.
  9. Leading the Fight for Insulin Affordability. American Diabetes Association.
  10. Munguia C et al. Regular Insulin. StatPearls. July 3, 2023.
  11. Lipohypertrophy. Cleveland Clinic. April 29, 2022.
  12. McFarlane SI. Insulin Therapy and Type 2 Diabetes: Management of Weight Gain. The Journal of Clinical Hypertension. October 6, 2009.
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Elise M. Brett, MD

Medical Reviewer
Castle Connolly Top Doctor
Elise M Brett, MD, is a board-certified adult endocrinologist. She received a bachelor's degree from the University of Michigan and her MD degree from the Icahn School of Medicine at Mount Sinai. She completed her residency training in internal medicine and fellowship in endocrinology and metabolism at The Mount Sinai Hospital. She has been in private practice in Manhattan since 1999.

Dr. Brett practices general endocrinology and diabetes and has additional certification in neck ultrasound and fine-needle aspiration biopsy, which she performs regularly in the office. She is voluntary faculty and associate clinical professor at the Icahn School of Medicine at Mount Sinai. She is a former member of the board of directors of the American Association of Clinical Endocrinology. She has lectured nationally and published book chapters and peer reviewed articles on various topics, including thyroid cancer, neck ultrasound, parathyroid disease, obesity, diabetes, and nutrition support.
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Jessica Freeborn

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Jessica Freeborn has worked as a health and wellness freelance writer since 2021. She is passionate about encouraging people to take control of their health and stay informed about the latest medical advancements. She has two nursing degrees and has used her healthcare experience to enhance her writing and research.

As someone with type 1 diabetes, she understands the complexities surrounding diabetes management and wants to provide people with accurate information and dispel misconceptions about diabetes treatment.

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