Fast-Acting Insulin 101: 7 Things You Need to Know

Last modified on February 23rd, 2023

fast-acting insulin pen lying on light wooden background7.4 million people in the United States are taking insulin, and that number is projected to continue climbing (1). 

Whether you use insulin or know someone who does, such as a family member, co-worker, student, or friend, it’s important to know why it’s needed and how to use it. 

There are different types of insulin: fast-acting, ultra-fast acting, short-acting, intermediate, long-acting, and ultra-long-acting.

In this post, you will learn 7 things you need to know about fast-acting insulin.

***This article is for general informational and educational purposes only and is not intended to treat, cure, or diagnose any disease. The information in this article is not a substitute for medical care or advice provided by your healthcare team. Please consult your doctor or dietitian for specific, personalized treatment.***

1.What is Insulin?

When you eat food, your body turns carbohydrates (carbs) like pizza or fruit into sugar (glucose) during digestion to be used for energy. Your body cannot use sugar without insulin, a hormone that comes from an organ called the pancreas.

How Insulin Works in the Body

Insulin acts like a key and “unlocks” the doors of the cells in your body so that sugar can get inside and be used for energy. Your body has billions and billions of cells. They’re found in your liver, heart, lungs, skin; they make you, you. Every single cell in your body needs sugar to work.

infographic explaining how insulin works in the body and why insulin is needed

If you don’t have diabetes, the pancreas sends out what’s called a bolus of insulin whenever food is eaten. A bolus of insulin is a large amount of insulin to help your body use that sugar from food. If you have diabetes and take insulin, then fast-acting insulin is your bolus insulin. 

To remember the term bolus, think of a BOWL of spaghetti. A BOWL-US (bolus) of insulin is needed to cover the food you eat. 

In someone without diabetes, the pancreas also releases very tiny amounts of insulin all day and night to keep the blood sugar constantly stable when not eating. This is called basal insulin or long-acting insulin.  

Depending on the type of diabetes, your pancreas either doesn’t make insulin, OR your body doesn’t use it like it should. Because the pancreas releases insulin at different times, different types of insulin are needed when your pancreas doesn’t work. 

Injecting insulin is not always a perfect science. You literally need to replace the action of an organ in your body! With that being said, there are side effects when injecting insulin. 

2.Side Effects

There are two main side effects to consider when taking insulin: hypoglycemia caused by too much insulin and diabetic ketoacidosis (DKA) caused by too little insulin.


Hypoglycemia is when there is not enough sugar in your blood. The clinical definition of hypoglycemia is a blood sugar less than or equal to 70, but always speak with your healthcare team about what number is too low for you (2). 

Many things can cause hypoglycemia, but a common reason is taking too much insulin without eating enough carbs or considering how much movement/exercise will be done.


If there isn’t enough sugar in your blood, you may feel weak, dizzy, sweaty, shaky, look pale, feel mad easily, have trouble thinking or speaking, have clammy skin or numbness in your hands and feet, or pass out. 

Treatment for Hypoglycemia 

When there isn’t enough sugar in your blood, you want to eat or drink foods that will raise your blood sugar fast such as:

  • Honey
  • Glucose tablets
  • Grapes, pineapple, or watermelon
  • Juice or regular soda
  • Fruit snacks
  • Fruity candy like Skittles or Sweet Tarts
  • Raisins
  • Banana

Once your blood sugar is above 70, eat a meal. If it’s not yet mealtime, have a snack with a mixture of carbs and protein. This ensures your blood sugar won’t drop again right away since your body is digesting a balanced snack such as:

  • Graham crackers with cream cheese
  • Granola bar and a cheese stick
  • Greek yogurt and fruit
  • Crackers and cheese
  • ½ peanut butter or lunch meat sandwich

Diabetic Ketoacidosis (DKA)

Diabetic Ketoacidosis (DKA) is a life-threatening condition that happens when there is not enough insulin in your body. Remember, insulin is needed so you can use sugar for energy. If your body can’t use sugar for energy, it uses fat instead. 

When fat is used for energy, chemicals called ketones are made. If enough ketones build up, they make your blood too acidic. This isn’t good by the way as acidic blood puts your life at risk. 

DKA Symptoms

First, it’s important to note that DKA may be starting to develop if you have high blood sugar levels (>250), extreme thirst, frequent bathroom trips, and high levels of ketones in the urine. Call your doctor if you notice these symptoms and cannot bring your blood sugar down.

If you have the above symptoms and begin to experience any of the issues listed below, you are likely in DKA and need to go to the nearest hospital Emergency Room right away:

  • Vomiting 
  • Stomach pain
  • Trouble breathing
  • Fruity smelling breath
  • Extremely tired or weak
  • Confusion 

How to Treat DKA

Once you’re in full blown DKA, the only treatment to help is at the hospital where they will hook you up to an IV for fluids, electrolytes, and insulin. 

While DKA and hypoglycemia can and likely will happen to anyone with diabetes, one way to reduce the frequency of these side effects is knowing the names of insulin to ensure you are using that type of insulin correctly.

3.Insulin Names

Like most drugs, insulin has generic and brand names. Don’t worry, they both work the exact same way, the generic is just usually cheaper. 

Below is a list of brand and generic names for fast-acting insulin which starts working in 5-15 minutes. Generic names are in parentheses.

  • Humalog (lispro)
  • Novolog (aspart)
  • Apidra (glulisine)

Ultra fast-acting insulin works much quicker (in 1 to 2.5 minutes) than fast-acting insulin and is listed below:

  • Fiasp (aspart)
  • Lyumjev (lispro-aabc) 

Short-acting insulin starts to work in 30-60 minutes and covers meals:

  • Regular

Next, we have intermediate insulin listed below which starts to work in 2-4 hours and is typically taken morning and night. 

  • NPH

Finally, long-acting or basal insulin starts working in 1 to 3 hours and is meant to cover blood sugars between meals and overnight. Long-acting and ultra-long-acting insulins are listed below with ultra-long insulin being in bold:

  • Levemir (Detemir)
  • Lantus, Basaglar, or Semglee (Glargine)
  • Tresiba (Degludec)
  • Toujeo (Glargine)

4.Correct Usage of Fast-acting Insulin

Now that you know insulin names, you will next learn when to use a fast-acting insulin such as Humalog. 


Remember, fast-acting insulin helps your body use sugar from food. Aim to take fast-acting insulin when food is on your plate, or 5-15 minutes BEFORE taking that first bite. This way, the insulin will be working as soon as your body starts turning carbs into sugar.

Because ultra-fast-acting insulin starts to work much quicker than fast-acting, you can take it at the start of a meal or even 20 minutes after a meal.


NEVER take fast-acting insulin if you’re NOT eating, or at bedtime. If you take this insulin and don’t eat, you will have a low blood sugar. If you skip a meal, then you skip an injection. Bottom line. 


Very young children may need to inject fast-acting insulin right AFTER eating. The reason for this is because appetites and the amount kiddos eat may change rapidly. One minute they load up their plates with mac and cheese, the next, they take two bites and say they’re full. 

If your blood glucose is very high, you may need to take insulin even though you’re not eating to bring it back down into normal range. This is called a correction dose. For this, you would contact your doctor’s office for guidance.

Infographic comparing when to use Bolus insulin, such as Humalog, and Basal insulin, such as Lantus.


5.Where to Inject

It is best practice to inject into fatty tissue to help your body absorb the insulin. You can inject into your tummy, love handle area/hips, upper thighs, buttocks, or back of your arm. 

Do not inject straight into muscle. For example, if your thighs are very muscular, try using your tummy or back of arms. Rotate your injection sites (meaning don’t inject into the same spot repeatedly) to avoid excessive scarring or bruising. 

For example, if you inject into your left lower tummy at breakfast, switch to your right lower tummy at lunch. If you do develop hard lumps or scars from injections, take a break from injecting into that site for a few weeks.

6.Insulin Storage

Keep any unopened boxes of insulin in the refrigerator. 

Opened vials or pens can stay out at room temperature for about 30 days. The insulin will typically expire after 30 days, but it is best to always consult the manufacturer instructions that come in each insulin box for specifics. 

Never leave insulin in extreme temperatures such as a hot car in the summer. This same guideline also applies to insulin that is inside an insulin pump.

7.Insulin Pump Usage

When using an insulin pump, fast-acting or ultra-fast-acting insulin ONLY goes in the pump. An insulin pump tries to copy a pancreas’s insulin release. Insulin pumps have bolus and basal settings. 

Remember, basal insulin keeps your blood sugar steady between meals and overnight. Basal insulin settings in a pump give you very tiny amounts of fast-acting insulin 24/7. This is similar to what a long-acting insulin would do in someone taking insulin injections.

Bolus insulin is what covers the food you eat. Therefore, you must tell the pump how many carbs you’re eating so it can give you the right amount of insulin. 

Basal and bolus settings can be adjusted in an insulin pump to give you more or less insulin. 


To conclude, fast-acting insulin is a life-saving medication used by people living with diabetes to help their bodies use sugar for energy.

It’s important to know the different insulin types, and it is best to always double check with your pharmacist that you have the correct insulin at pick-up.

Again, because if fast-acting insulin is used, it must be taken 5-15 minutes before eating and never if a meal is skipped. 

Regardless of the insulin type, injection sites may become scarred or bruised if you constantly inject into the same area, so remember to always rotate your sites.

As a reminder, you can take an opened insulin vial or pen with you on the go, just don’t leave it in any extreme temperatures, and store any unopened insulin in the fridge. 

Lastly, if you’re considering getting an insulin pump, know that you will only be using fast-acting insulin as the pump tries to copy a pancreas’s insulin production. Check out this article for everything you need to know about diabetes insulin pumps or click on one of the pump brands at the end of this article. 

After reading this post, I hope you go forward with confidence in taking your insulin. Thanks for reading!

Interested in Pump Therapy?

Learn more about how fast-acting insulin works with insulin pumps, by clicking on an insulin pump brand below. After doing a little research on the insulin pumps, contact your healthcare provider or Certified Diabetes Care and Education Specialist. 


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