The Difference Between Fast-Acting and Slow-Acting Insulin

Posted on September 07, 2021 at 12PM

Just like any drug, insulin has its activity profile that is measured in three numbers which include onset, peak, and duration. These are also known as the characteristics of insulin and they help to know the differences between fast-acting insulin and slow-acting insulin.

Fast-acting insulin is used due to its fast peak. The insulin acts within a short time to reduce the level of sugar in the blood. Slow-acting insulin, on the other hand, is preferred for its long duration. Its effect in reducing the level of sugar in the blood lasts for several hours.

If you have insulin problems and it needs to be taken frequently, then you will need to go for slow-acting insulin.

Characteristics of Insulin

The three characteristics of insulin include the following:

  • Onset: This refers to the length of time it takes insulin to reach the bloodstream and reduce the blood sugar level.
  • Peak time: This is the time at which insulin has its maximum strength in terms of reducing the blood sugar level.
  • Duration: This refers to how long it takes the insulin to continue lowering blood glucose.

Fast-Acting Insulin

Fast-acting insulin refers to a synthetic (or man-made) form of insulin. This type of insulin is an injectable medication that is prescribed for those that have diabetes to control the level of glucose (or sugar) in their blood.

Fast-acting insulin is absorbed into the bloodstream within a short period, usually within minutes, to act like bolus insulin (i.e. the surge of insulin that the pancreas releases when food is ingested). As a result, this form of insulin is injected just before taking meals or snacks.

Fast-acting insulin can be administered with a needle and syringe, an insulin pump, or a pre-filled pen. There is also a form of fast-acting insulin that the patient can inhale.


Fast-acting insulin is prescribed for those suffering from type 1 diabetes. These people need to take insulin since their pancreas cannot produce it.

A healthy pancreas secretes bolus insulin whenever the level of glucose in the blood increases after ingesting food or beverage. Fast-acting insulin acts like bolus insulin and it is usually administered before taking snacks and meals.

Also, fast-acting insulin is used to reduce high blood glucose to a normal level.

Some patients with type 2 diabetes may need supplemental insulin if they cannot control their blood sugar with exercise, diet, and medications. Although such people need only basal insulin. Basal insulin is bolus insulin’s counterpart and it is the insulin that is usually secreted continuously by the pancreas to keep insulin at a normal level in the bloodstream every time.


According to the American Diabetes Association, insulin has a standard U-100 strength. Those that are highly resistant to insulin may require supplemental insulin with a U-500 strength.

Your physician will determine the dosage that you need depending on your weight, dietary needs, sensitivity to insulin, and other factors. However, some general rules are used to calculate the amount of fast-acting insulin an individual needs to take. These rules are determined by the following circumstances:

Taken with meals

If fast-acting insulin needs to be taken with a meal, its dosage usually depends on a ratio of insulin to carbohydrates that are in the meal; usually one unit of insulin per about 12 to 15 grams of carbohydrates.

To lower extremely high glucose levels

Generally, one unit of fast-acting insulin is required to lower the level of sugar in the blood by 50 mg/dl.

How long does it take fast-acting insulin to start working?

How long it takes for fast-acting insulin to begin working depends on the specific products, but can start working within 5 minutes. It could also take as long as 30 minutes depending on the specific type of fast-acting insulin used.

The following are the various fast-acting insulin products and the time it takes for each of them to start working:

  • Humalog: The onset of action time is between 10 and 20 minutes.
  • Novolog:  Its onset of action time is between 10 and 20 minutes.
  • Apidra: It takes between 10 and 20 minutes to start working.
  • Afrezza:  It starts working within 3 to 7 minutes.

Slow-Acting Insulin

Slow-acting insulin helps to create a healthy baseline blood sugar level. In other words, when food gets into the body, the blood sugar increases from a lower point, thereby making it easier to manage.

Most people use an electronic insulin pump to administer slow-acting insulin thereby eliminating the need for manual injections. However, those that don’t have access to pumps or decide not to use a pump due to their lifestyles may go for injections.

The three types of slow-acting insulin that are on the market include detemir, glargine, and degludec.


Slow-acting insulin is usually injected once a day to keep the level of blood sugar steady. The insulin is injected by using a needle or pen device. Ensure that you inject your slow-acting insulin at the same time daily to avoid stacking the doses. Stacking refers to the process of taking your doses very close together, thereby making the activity of the insulin overlap.


Each of the slow-acting insulin has its required dose. These dosages differ based on the type of diabetes the patent has and if there’s a history of insulin use.

When someone starts using new insulin, the doctor usually recommends that they start with a smaller portion of the target dose so that the body can have time to adapt to the additional insulin. Once the body has adapted to the extra dose, the doctor will then gradually increase the dosage to give a full dose.

An insulin regimen may be adjusted if there’s a change to the person’s diet, levels of physical activity, or if the person falls ill. The insulin dosage may also be changed by any factor that can affect the blood sugar and negative reactions to the hormone (for instance, weight).

A trial and error process is usually involved when trying to find the ideal dosage of slow-acting insulin. Thus, it requires carefully monitoring the levels of blood sugar.

How long does slow-acting insulin last?

The duration differs and it is determined by the specific type of slow-acting insulin used. However, it should last between 22 and 24 hours. Highlighted below are the types of slow-acting insulin and the duration it takes for each product to last:

  • Lantus: It lasts between 22 and 24 hours.
  • Basaglar: It lasts between 22 and 24 hours.
  • Levemir: Its action lasts for about 24 hours.


So, how is fast-acting insulin different from slow-acting insulin?

Fast-acting insulin becomes effective and wears off within a shorter period than slow-acting insulin. Fast-acting insulin is usually used about 30 minutes before taking a meal so that it can have enough time to work. This type of insulin is clear and does not settle out when the vial sits for some time.

According to the American Diabetes Association, there are three main differences between fast-acting insulin and slow-acting insulin:


Fast-acting insulin lowers the levels of sugar in the blood within 15 to 30 minutes while it takes several hours after injection for the effect of slow-acting insulin to be felt.

Peak time

Fast-acting insulin has a peak time of about 1 to 3 hours after it is injected. Slow-acting insulin, on the other hand, does not have a peak time. It helps to regulate the sugar levels in the blood at a fairly stable rate.


Fast-acting insulin can control the level of sugar in the blood for a few hours. Slow-acting insulin, on the other hand, can remain effective for about 24 hours or more (depending on the specific type of slow-acting insulin product injected).

Frequently Asked Questions About Fast and Slow-Acting Insulin

Which insulin products last longer than slow-acting insulin?

Toujeo and Tresiba are referred to as ultra-slow-acting insulin products because they last longer than slow-acting insulin. Toujeo was approved in February 2015 by the FDA. It starts working within 6 hours and lasts for 36 hours without any peak.

Tresiba was approved in September 2015 by the FDA. It starts working within 1 hour and lasts for at least 42 hours. Just like Toujeo, it also has no peak.

Do I have to inject all insulin?

No. Not all insulins need to be injected. Afrezza is a fast-acting insulin that is inhaled via the mouth.

Do I need to be administered with more than one insulin?

Your doctor is in the best position to determine if you need more than one insulin.

Some type 2 patients may be required to use slow-acting insulin to put their blood sugar level under control, while others may need both meal-time and slow-acting insulin to effectively control their blood sugar level.

Patients using an insulin pump will need to use fast-acting insulin. The pump can give a slow and steady dosage of insulin to take you through the day the same way slow-acting insulin would do. It is, however, advisable to have slow-acting insulin as a backup in case the pump fails.