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Unmanaged blood sugar levels can affect a person’s daily quality of life. That’s why choosing the right type of insulin to manage post-meal glucose is essential. Two commonly prescribed rapid-acting insulins are Fiasp and Humalog. Both help control blood sugar spikes after meals to prevent diabetes-related complications.

While both share the same purpose of managing glucose levels, they’re distinct in their onset, duration, side effects, and even cost. Patients with diabetes often compare Fiasp vs. Humalog when looking for the most effective insulin option. Learn how these two compare to find the best medication that suits your needs.

Key Takeaways

  • Fiasp and Humalog are both rapid-acting insulins approved by the FDA for treating diabetes in adults and children.
  • Fiasp contains insulin aspart, while Humalog contains insulin lispro.
  • Fiasp includes niacinamide and arginine as inactive ingredients (excipients), which help accelerate insulin absorption compared to Humalog.
  • According to studies, Fiasp begins working within approximately 2.5 minutes after injection, while Humalog typically starts working around 15 minutes after administration.
  • Humalog peaks between 30 and 90 minutes, whereas Fiasp reaches its peak effect at approximately 63 minutes.

Is Fiasp the Same as Humalog?

No. Fiasp and Humalog are not the same, although they belong to a class of rapid-acting insulin and are both approved to treat diabetes in adults and children.

The active ingredient in Fiasp is insulin aspart — a rapid-acting insulin released quickly into the bloodstream after injection. It’s manufactured by  Novo Nordisk and was approved by the FDA in 2000 to control blood sugar in adults and children with diabetes.

Humalog is also a rapid-acting insulin that was FDA-cleared in 1996 to treat Type 1 diabetes in adults and children at least three years old and Type 2 diabetes in adults. It’s manufactured by Eli Lilly and is often prescribed with an intermediate- or long-acting insulin.

They’re formulated differently and have slightly distinct effects on the body.

Fiasp Insulin on Desk Counter

Are Fiasp and Humalog Interchangeable?

Fiasp and Humalog aren’t considered interchangeable. One reason is they have different formulations — Fiasp contains insulin aspart while Humalog contains insulin lispro. They also vary in their inactive ingredients.

Fiasp’s Inactive Ingredient

    • Niacinamide
    • Arginine
    • Glycerin
    • Metacresol
    • Dibasic sodium phosphate
    • Phenol
    • Zinc
    • Water

Humalog’s Inactive Ingredient

    • Hydrochloric acid
    • Metacresol
    • Dibasic sodium phosphate
    • Glycerin
    • Trace amounts of phenol
    • Sodium hydroxide
    • Zinc oxide (Zinc ion)
    • Water

Some users may also find the side effects of Fiasp more tolerable than Humalog or vice versa, which is why they shouldn’t be swapped unless your doctor recommends it.

Fiasp vs. Humalog: How Do They Differ? 

Explore how Fiasp differs from Humalog.

1. Mechanism of Action

Both Fiasp and Humalog work by mimicking the body’s natural insulin response after meals. They carry glucose from the bloodstream into cells for energy or storage.

One advantage of Fiasp is it’s formulated with niacinamide and arginine. These excipients accelerate the absorption of sugar into the cells, allowing the drug to start working faster than traditional insulin aspart.

 

2. Fiasp vs. Humalog Dosing

Both insulins are prescription-only medications and come in various strengths and dosages.
Fiasp Humalog
100 units/mL (U-100) available as:
  • 10 mL multiple-dose vial
  • 3 mL single-patient-use Fiasp FlexTouch® pen
  • 3 mL single-patient-use PenFill® cartridges for use in a PenFill cartridge device
  • 1.6 mL single-patient-use PumpCart® cartridges for use in a compatible pump
 100 units/mL (U-100) is available as:
  • 10 mL multiple-dose vial
  • 3 mL multiple-dose vial
  • 3 mL single-patient-use KwikPen® prefilled pen
  • 3 mL single-patient-use Tempo Pen prefilled pen
  • 3 mL single-patient-use Junior KwikPen® prefilled pen
  • 3 mL single-patient-use cartridges
200 units/mL (U-200)
  • 3 mL single-patient-use KwikPen® prefilled pen
Humalog Mix 75/25
  • 10 mL multiple-dose vial
  • 3 mL single-patient-use KwikPen® prefilled pen
Humalog Mix 50/50
  • 10 mL multiple-dose vial
  • 3 mL single-patient-use KwikPen® prefilled pen

How to Administer Fiasp 

Fiasp is injected into the thigh, upper arm, or abdomen at the beginning of a meal or within 20 minutes after starting a meal. There’s no fixed dosage guideline as the amount of insulin aspart to take depends on several factors, such as your weight.

For patients with Type 1 diabetes, a doctor may initially prescribe 0.4 to 1 unit/kg of body weight daily. This dose is often taken with an intermediate- or long-acting insulin. Type 2 diabetes may start with 0.3 to 0.5 units/kg of body weight per day.

Generally, doctors prescribe a lower dose at the beginning of the treatment and adjust it until the right dosage is reached.

How to Administer Humalog

Humalog is administered subcutaneously in the buttocks, thigh, upper arm, and abdominal wall within 15 minutes before a meal or immediately right after it.

Like Fiasp, the dosage can vary and often starts with the lowest dose, increasing until blood sugar control is achieved.

The dose for patients with Type 1 diabetes typically ranges from 0.4 to 1.0 units/kg per day. Patients with Type 2 diabetes are often more insulin resistant, requiring a higher daily dose of about 1 unit/kg.

Flextouch vs. KwikPen Devices

Fiasp is available in the Flextouch pen, while Humalog and its generics use the KwikPen. Some people prefer one over the other based on injection feel, dialing ease, and grip.

3. Onset of Action

According to a study, Fiasp works within 2.5 minutes after injection, making it one of the fastest insulin options.

Meanwhile, Humalog typically begins working within 15 minutes of injection, which is also considered fast.

The faster onset of action of Fiasp makes it suitable for people who eat spontaneously, are sensitive to sugar fluctuations, or often forget to inject insulin before meals.

4. Peak Effect

Fiasp insulin peaks at 63 minutes, though some patients report a faster peak.

Humalog peaks between 30-90 minutes, depending on each person’s response. Healthy individuals who took 0.1 to 0.4 units/kg of insulin lispro saw a peak in serum levels 30 to 90 minutes after dosing.

Between the two, Fiasp may be slightly better at controlling post-meal spikes, while Humalog may be ideal for those looking for a steadier glucose-lowering effect.

4. Duration of Action

Fiasp lasts between 3-7 hours, depending on the dose. Below is the duration of action for each dose:

    • 0.1 unit/kg: ~5 hours
    • 0.2 unit/kg: ~6 hours
    • 0.4 unit/kg: ~7 hours

Meanwhile, the effects of Humalog often last 2-4 hours, but this can also vary from person to person.

Some factors that may affect insulin’s duration of action include:

    • Injection site
    • Body temperature
    • Blood flow to the injection site

5. Side Effects

Rapid-acting insulins like Fiasp and Humalog share similar symptoms, which include:

    • Hypoglycemia
    • Allergic reactions
    • Hypersensitivity
    • Injection/Infusion site reactions
    • Lipodystrophy
    • Weight gain

In rare cases, these drugs may cause serious side effects that include:

    • High blood sugar levels
    • Low blood sugar levels
    • Low potassium levels
    • Hypersensitivity reactions
    • Fluid retention and heart failure

If you experience severe or unusual symptoms, seek medical help immediately.

6. Costs

A 15 mL of Fiasp 100 units/mL costs $606.57, while a 3 mL of Humalog 100 units/mL costs up to $97.97 in local pharmacies. Retail prices vary based on the pharmacy, location, and insurance coverage.

People without insurance who pay out of pocket can save on their prescriptions by purchasing their insulins from Canadian online prescription delivery platforms, like Buy Canadian Insulin.

A 100 units/mL vial of Fiasp costs only $58, while Fiasp FlexTouch 100 units/mL pens start from $124.

Meanwhile, a 100 units/mL vial of Humalog costs $52, with Humalog KwikPen prices starting at $102.

Fiasp is more expensive than Humalog, but its rapid onset makes it worth it for some people.

Both medications are often used long-term, which can be expensive. Buying from Canadian online pharmacies can lower your monthly medication expenses, even if you’re uninsured.

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Switching from Fiasp to Humalog

Your doctor may recommend switching from Fiasp to Humalog or vice versa for reasons that may include:

    • Cost: Humalog is generally cheaper than Fiasp
    • Insurance formulary changes: Your insurance may cover one insulin but not the other
    • Issues with side effects and availability: If you can’t tolerate the side effects of Fiasp, your doctor may suggest Humalog or other brands of rapid-acting insulin.

Some alternatives to Fiasp and Humalog include:

    • NovoLog: contains insulin aspart
    • Lyumjev: contains insulin lispro
    • Apidra: contains insulin glulisine
    • Lispro (generic Humalog): contains insulin lispro

Fiasp is often compared with NovoLog as both are rapid-acting and contain insulin aspart. So, is Fiasp the same as NovoLog? Fiasp is a newer version of NovoLog but with an added niacinamide for faster insulin absorption.

Humalog is also often assumed to be interchangeable with NovoLog — but it’s not. NovoLog contains insulin aspart, while Humalog has insulin lispro.

Fiasp vs. Humalog: Which Is Better?

Although both Fiasp and Humalog are rapid-acting insulins, they contain different active ingredients. Fiasp has insulin aspart, while Humalog contains insulin lispro. They both lower blood sugar levels, but they’re not interchangeable.

Your choice of insulin depends on factors like how fast you need the insulin to take effect, side effect tolerance, and cost or insurance coverage.

You can switch from Fiasp or Humalog, but only under your doctor’s supervision. They can help monitor the side effects, assess patterns of glucose levels, and suggest lifestyle changes to mitigate the side effects.

Frequently Asked Questions

Although Fiasp and Humalog regulate blood sugar levels, they contain different formulations, affecting their onset of action and absorption time. Fiasp is formulated with insulin aspart and works faster than Humalog, which contains insulin lispro.

Fiasp may increase the risk of side effects, such as low blood sugar, injection site reactions, and allergic reactions. Some users also report gaining weight and having low potassium levels. If you experience symptoms associated with these conditions, let your doctor know immediately.

Fiasp is a rapid-acting insulin option proven to be effective for many patients. It may be the best insulin for some diabetes patients who only experience minor side effects when using it. Insulin therapy is not one-size-fits-all, so the best choice boils down to what suits your preferences, needs, and health goals.

Yes, several types of rapid-acting insulin may work better for certain people than Humalog. Some examples include NovoLog, Lyumjev, Admelog, and NovoRapid.

Yes. One advantage of Fiasp is that it’s formulated with niacinamide and arginine, which results in faster absorption.

Both Fiasp and NovoLog are rapid-acting insulin, but the former is typically considered better because of the added niacinamide and arginine in its formulation. These two substances allow the body to absorb the drug quickly, benefiting people sensitive to post-meal sugar fluctuations.

Original Publish Date: 12/15/2020 – Updated 5/20/2025