Insulin Aspart vs Lispro

When you eat, your body normally releases quick bursts of insulin to keep blood sugar from spiking. If you have diabetes, mealtime (bolus) insulin steps in to do that job. Two of the most commonly prescribed options are insulin aspart and insulin lispro. But what’s their difference?

In this guide, you’ll learn how each one works, how they’re given, and what research shows when they’re compared side by side.

What they are and why they act fast

Both insulin aspart and lispro are lab-made “rapid-acting analogs” of human insulin designed to absorb quickly after you inject them under the skin.

• • Insulin aspart – Available as NovoLog and the ultra-rapid version Fiasp, made by Novo Nordisk.

• • Insulin lispro – Available as Humalog and the ultra-rapid version Lyumjev, made by Eli Lilly.

Normally, injected regular insulin molecules tend to clump together into hexamers (clusters of six molecules). Your body can’t absorb these large clusters quickly, so they must break down into single molecules (monomers) first, which takes time.

In lispro and aspart, the structural changes reduce the tendency to form hexamers. Instead, they stay mostly as monomers, which can be absorbed much more quickly. As a result, both start working in approximately 10-20 minutes.

Insulin aspart starts working in about 18 minutes, with its full glucose-lowering effect peaking 1-3 hours. The “faster-acting” version, Fiasp, kicks in about 5 minutes earlier than regular aspart.

Standard lispro starts working in 23-27 minutes, but its newer version, Lyumjev, starts slightly faster at 15-18 minutes. Both reach peak effect around 1-2 hours.

Effectiveness in blood sugar control

Clinical trials and systematic reviews consistently show that insulin aspart and lispro are equally effective at lowering blood sugar. Major organizations like the American Diabetes Association (ADA) do not recommend one over the other.

In a systematic review, both insulin aspart and lispro lowered HbA1c by a similar amount in people with type 1 and type 2 diabetes. Another study reached the same conclusion for individuals with type 2 diabetes who required intensive therapy with an insulin pump and metformin.

In people with type 1 diabetes who had no remaining insulin production (C-peptide-negative), an earlier study demonstrated that they are equally effective in controlling both sugar and fat metabolism. This is especially important in type 1 diabetes, as lack of insulin can lead to dangerous ketone buildup (diabetic ketoacidosis).

Side effects and safety profile

The most common side effect of both insulins is low blood sugar (hypoglycemia). This can happen if your dose is higher than what your food or activity level requires. Symptoms include:

• • shakiness
  • sweating
  • dizziness
  • blurred vision
  • fast heartbeat
  • confusion

Hypoglycemia is dose-related, which means it can often be prevented with the right adjustments. If you experience frequent lows, your insulin dose or meal plan may need to be changed under medical supervision. Regular glucose checks are essential, especially during dose changes or if you’re fasting for a procedure.

You’ll also want to keep an eye out for other, less common issues, including:

• • low potassium levels or hypokalemia
  • redness, itching, or swelling at the injection site
  • allergic reactions
  • lipodystrophy (changes in fat under the skin where you inject repeatedly)

Delivery options and administration

Both insulin aspart and insulin lispro are available in subcutaneous injection (vials, cartridges, pens) and can be used intravenously under medical supervision. You can take them as a subcutaneous injection about 5 to 15 minutes before a meal, or right after you start eating.

Aspart is available in a standard concentration of 100 units/mL (U-100), but lispro is available in two concentrations, U-100 and U-200.

How insulin lispro is given

Lispro is a fast-acting mealtime insulin. You inject it right before or after eating, usually as part of a regimen that includes a long-acting insulin.

1. Humalog (Eli Lilly)

Forms available:

• • Vials (for syringes)
  • Cartridges (for reusable pens)
  • Prefilled pens

Strengths:

• • U-100 (100 units/mL)
  • U-200 (200 units/mL) in specially designed pens

Devices:

• • Humalog KwikPen: Disposable prefilled pen delivering 1-unit increments, up to 60 units per injection. Each pen contains 300 units.

• • Humalog Junior KwikPen: Prefilled disposable pen that delivers half-unit increments (0.5 units) up to 30 units. Each pen contains 300 units.

• • HumaPen Savvio: A reusable pen device designed to work with Humalog cartridges. You can dial insulin doses from 1 to 60 units in single-unit increments. It includes an Insufficient Remaining Dose (IRD) feature, which prevents you from selecting more units than remain in the cartridge, minimizing wasted doses and avoiding dosing errors.

2. Lyumjev (Eli Lilly’s faster-acting formulation of insulin lispro)

Forms available:

• • Vials
  • Cartridges
  • Prefilled pens

Strengths:

• • U-100 (100 units/mL)
  • U-200 (200 units/mL), only available in KwikPens

Devices:

• • Lyumjev KwikPen: Standard disposable prefilled insulin pen that delivers in 1-unit increments (like Humalog KwikPen). Also contains 300 units of insulin per pen.

• • Lyumjev Junior KwikPen: The Junior version allows dosing in 0.5-unit increments.

• • Lyumjev Tempo Pen: Delivers the same insulin in the same capacity and increments as the KwikPen, but features smart technology. When paired with the Tempo Smart Button, a small device that attaches to the pen, it can:

• • • • • record each injection (time, dose)
        • sync data to the TempoSmart app on your smartphone
        • track insulin use, glucose readings, and patterns digitally

3. Admelog (Sanofi)

Admelog, made by Sanofi, is a follow-on version of Humalog (Eli Lilly’s insulin lispro). It was approved in 2017, before insulins were officially moved into the biologics category, which is why the FDA doesn’t technically call it a “biosimilar.”

Still, in practice, it works exactly like Humalog and is often placed on insurance formularies as the preferred, more affordable lispro option. Admelog comes only in the U-100 strength, available in 10 mL vials and SoloStar pens.

How insulin aspart is given

Insulin aspart also comes in a few different forms.

1. NovoLog (Novo Nordisk, also known as NovoRapid in some markets)

Forms available:

• • Vials (for syringes)
  • Cartridges (for reusable pens)
  • Prefilled pens

Strengths:

• • U-100 (100 units/mL)

Devices:

• • NovoLog FlexPen: Disposable prefilled pen delivering 1-unit increments. Each pen contains 300 units.

• • NovoPen Echo: A reusable pen device that uses PenFill cartridges. It allows 0.5-unit increments, which is especially helpful for children or anyone who needs smaller, fine-tuned doses. It also has a dose memory function, showing you the last dose you gave and how long ago you injected.

2. Fiasp (Novo Nordisk’s faster-acting formulation of insulin aspart)

Forms available:

• • Vials
  • Cartridges
  • Prefilled pens

Strengths:

• • U-100 (100 units/mL)

Devices:

The Fiasp FlexTouch pen is the only prefilled disposable pen available for Fiasp. Each pen is preloaded with 3 mL (300 units) of U-100 Fiasp. It delivers doses in 1-unit increments, up to 80 units in a single injection.

3. Merilog (Sanofi)

Approved in 2025, Merilog became the first FDA-approved biosimilar insulin aspart in the US, directly referencing NovoLog. Unlike Admelog, Merilog went through the biosimilar approval pathway, meaning it is officially recognized as equivalent to its originator.

It also comes in U-100 strength only, available in vials and prefilled pens similar to NovoLog’s FlexPen or FlexTouch devices.

Important:

• • Insulin pumps should only use rapid-acting U-100 insulins.
  • Do not mix insulins in a pump. Premixed or long-acting insulins can cause clogging, unpredictable absorption, and dangerous blood sugar swings.
  • U-200 insulins are not approved for pump use. They come only in pens and should never be used to fill a reservoir.

Always follow the insulin manufacturer’s and pump manufacturer’s instructions for safe use.

Pricing considerations

In terms of cost, insulin aspart and lispro are priced fairly close to each other. The main differences show up when comparing pens versus vials or looking at the faster-acting versions.

For NovoLog (insulin aspart), the list price is about $73 for a 10 mL vial and roughly $140 for a 5-pack of 3 mL FlexPens. In comparison, Humalog (insulin lispro) is priced at $67 per 10 mL vial and around $160 for a 5-pack of 3 mL KwikPens.

The faster-acting options cost significantly more. A Fiasp (faster-acting aspart) 10 mL vial lists for $290, while its 5-pack of pens is $558.83. For lispro, a Lyumjev 10 mL vial runs about $275, and the 5-pack of pens is $530.

Admelog (insulin lispro) and Merilog (insulin aspart) provide more affordable options. However, insulin use varies widely. Some people need as little as 5 units per meal, while others require over 100 units daily. Because of this variation, estimating an “average” monthly cost for insulin lispro or aspart is difficult.

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How clinicians help you choose

Here’s how they narrow it down with you:

Recap

Insulin aspart (NovoLog, Fiasp, Merilog) and insulin lispro (Humalog, Lyumjev, Admelog) work fast and are equally effective for blood sugar control. Both have the same main risk, which is hypoglycemia.

The real differences come down to insurance coverage, device options, and whether you need features like U-200 pens or half-unit dosing. Ultra-rapid types like Fiasp and Lyumjev act a little faster but cost much more. Affordable versions like Admelog and Merilog make treatment more accessible.

Neither insulin is better overall. The best choice depends on your coverage, dosing needs, and what works best for your daily routine.

Frequently Asked Questions