Fiasp Insulin: Why It's Different & How It Works

fiasp insulin - fiasp insulin review - what is fiasp insulin - rapid acting insulin - fast acting insulin

Updated: 8/16/22

Read time: 9 minutes

  • Two ingredients make Fiasp different than Novolog: niacinamide, a form of vitamin B3, increases how quickly the insulin is absorbed and L-arginine, an amino acid, increases the insulin’s stability.
  • Fiasp’s label allows for flexibility in dosing by recommending bolusing at the start of a meal or up to 20 minutes after your first bite.
  • Considerations should be given to Fiasp’s onset, peak, and duration as well as a meal’s composition and your unique insulin response when determining bolus timing.

  • Fiasp insulin is one of the fastest acting insulins on the market. Unlike other mealtime insulins that recommend pre-bolusing before a meal, Fiasp suggests that it can be taken up to 20 minutes after you start eating.

    Manufactured by Novo Nordisk, Fiasp is a type of rapid-acting insulin that advertises flexibility. It can be taken “at first bite” or “during meal." For those people living with diabetes who frequently forget to pre-bolus, the flexibility of Fiasp’s dosing recommendations could be a game changer, but like all diabetes medications, FIasp isn’t one-size-fits-all.

    The One Drop Beginner's Guide to Insulin

    How is Fiasp Different?

    Fiasp insulin is essentially Novolog insulin with two additional ingredients added: niacinamide (a form of vitamin B3) and L-arginine (an amino acid).

    The vitamin B3 speeds up how fast your body absorbs your Fiasp dose, according to the official Fiasp websiteThis means it appears in your bloodstream in about 2.5 minutes, two times faster than regular Novolog insulin. 

    Another ingredient, an amino acid called L-arginine is also added. L-arginine is used “to stabilize the formulation."

    Using Fiasp in Insulin Pumps vs. Injections

    Fiasp can be used in an insulin pump in place of other rapid-acting insulins. But you should make this transition with careful guidance from your diabetes healthcare team. Differences in onset, peak, and duration of insulin action may change the timing and amounts of insulin you require.

    Some studies show that using Fiasp rather than traditional Novolog in an insulin pump can reduce hypoglycemia and improve overall time in range while other studies show no significant differences.

    Comparing Fiasp to Today's Rapid-Acting Insulin Options

    Take a look at all six of the rapid-acting insulin options currently available to see how Fiasp compares.





    Fiasp (aspart)

    16-20 minutes

    90-130 minutes

    5-7 hours

    Novolog (aspart)

    21-25 minutes

    60-90 minutes

    3 to 5 hours

    Humalog (lispro)

    10-20 minutes

    30-90 minutes

    3 to 5 hours

    Apidra (glulisine)

    10-20 minutes

    30-90 minutes

    2 to 4 hours

    Lyumjev (lispro-aabc)

    15-17 minutes

    120-175 minutes

    5 to 7 hours

    Afrezza (inhaled)

    12 minutes

    35-45 minutes

    1.5 to 3 hours

    One thing that may stand out is the 16-20 minute onset time of Fiasp. Although Fiasp appears in the bloodstream after only 2.5 minutes, studies show that its first measurable effect often doesn’t occur until about 16-20 minutes post injection.

    So if Fiasp appears in the blood in 2.5 minutes, has measurable effects in 16-20 minutes, peaks in 90-130 minutes, and continues to lower blood sugar for 5-7 hours, when is the ideal time to take it?

    The answer: It depends.

    Did You Forget to Pre-Bolus?

    Fiasp could be especially beneficial for you if you forgot to pre-bolus.

    In clinical trials, Fiasp demonstrated modest improvements in A1C (an estimated -0.15% difference) vs. individuals treated with Novolog when insulin was dosed 0 to 2 minutes before a meal.

    When Fiasp dosing occurred 20 minutes after the start of a meal, A1C results were nearly identical (+0.04% estimated difference in post-meal Fiasp users) to those dosing Novolog immediately before a meal.  

    Another trial showed that individuals using long-acting insulin (insulin degludec) who either used Fiasp immediately before eating, used Novolog immediately before eating, or used Fiasp 20 minutes after the start of a meal showed that Fiasp was not inferior to Novolog.

    The flexibility provided by Fiasp could help you stay on track with meds and provide reassurance that a dose 20 minutes after a meal has started is better than a skipped dose.

    How to Bolus for Ice Cream

    Side Effects of Fiasp

    Besides hypoglycemia—which is a potential side-effect of any insulin—common  adverse reactions occurring in more than 5% of adult users with type 1 diabetes included:

    • Nasopharyngitis
    • Upper respiratory tract infection
    • Nausea
    • Diarrhea
    • Back pain
    • Skin problems (like eczema), rash, itching

    In Fiasp users with type 2 diabetes, the most common side effect was urinary tract infection that occurred in 5.9% of individuals using Fiasp plus insulin glargine.

    Remember, while this list might sound concerning, there’s actually a similar looking list for nearly every type of insulin. Most people will not experience these side effects, but they're still important to know.

    The Cost of Fiasp

    Fiasp was first sold in 2017 at the same price of Novolog, but out-of-pocket costs for Fiasp could be higher compared to Novolog or Humalog.

    Out-of-pocket costs for Fiasp may be about $570 for a box of 5 u100 pens. You can look-up your insurance coverage for Fiasp at NovoCare. You can also see if you qualify for NovoCare’s Patient Assistance Program based on your household income. This program is designed for those living below the poverty level.

    If Fiasp isn’t on your insurance’s list of “covered medications," you may need a letter from your doctor stating your unique need for this insulin in order to get prior authorization from your insurance plan.

    What People Are Saying About Fiasp

    We looked around the web and asked on Twitter to see what people had to say about their experience with Fiasp: 

    “I’ve used it extensively in pens, now in an Omnipod pump. I’ve found that its rapid onset of action has diminished slightly over time, but it’s still worth it over Novorapid for me. It’s rounded off my post meal spikes significantly and allows correction boluses to work quicker.” - @CountCarbula 

    “I switched to it in the summer from Humalog -- I love how fast it is, but you do have to watch out those lows a bit more.” - @WelfordP

    “Yes, been using it for a while. Yes, I had to reduce the dosage a bit. Used Novolog previously. I don’t know what I’m going to eat always so this is perfect for me 2 minutes before eating or up to 20 minutes after eating.” - @DianeBerinato

    “Used it for a while last summer but found after a while it became inconsistent -- a lot of unexplained highs. Went back to Humalog happily.” - @maireadyo

    “My diabetes educator gave me a sample pen in July. I didn’t see a difference between it and my Humalog. Stayed with Humalog.” - @christifunky

    “It was SUPER fast -- even a little too fast? I did reduce my insulin dosage a little. There was no burn for me. Felt like my usual Humalog. It was only a one-pen trial so it was a brief experiment, but I may have chosen to stay with my Humalog even if I'd had a choice!” - @KatherineRubida

    “I found no difference with [Novolog], but it did sting during a bolus so went back to [Novolog].” - @james_thurlow

    “Fiasp is hell. If you're okay with your current insulin, don't change. And if you do, do it when you have time to check and adjust a lot. I get high blood sugars I’ve never had before. I use about 30% more than Humalog and was still high. And I'm hungrier, and put on weight as a result.” - @Moumone1

    “It worked beautifully for about 3 weeks: flat CGM profile and no post-meal spikes. Then it stopped working. I'm now on an almost double TDD. Waiting for my clinic to change back to Humalog.” - @markgdeakin

    “I bought pens the last time I was in Canada. I use it for corrections only. It’s much faster than Apidra (which previously was fastest for me) and eons faster than Humalog and out of my system in about 3/4 the time. Since I only bought one box to try, I am only using it for corrections. I might when I get close to the 28 days so I don’t waste it.” - CSBD49

    “I use Fiasp in my pump. There's a few ups and downs with it but on the whole, it's fine. Anything over 40 carbs then you need to split the bolus. I bolus just as I sit down to eat. The biggest mistake is people saying it's [Novolog] with added ingredients and then [using] it as they did [Novolog]. Its profile is different so treat it as a completely new insulin. As most people have found, injecting / bolusing the whole [dose] up front tends to cause a [low blood sugar] if there's a carb-load in that meal. Again, different foods need different splits. For me, an ordinary meal split would be 80/20 over two hours. To start with, I found Fiasp very frustrating and had been tempted on many occasions to just bin it, but now I'm used to it, and know what you can and cannot expect from it. I'm quite happy using it. It's absolutely vital that your basal is correct when using Fiasp, getting that spot on is the key factor.” - PumpkinUK

    “I am on my 2nd sample Fiasp pen after making the switch from Humalog pens. I have found the insulin dosage to be the same between Fiasp and Humalog, but Fiasp kicks in a little quicker and tails off quicker. Humalog worked well for a long time until a couple of months ago when it wouldn’t kick in for hours and dropped me rapidly around 4 hours. My Express Scripts insurance formulary only covers Humalog, but my endo is going to try to get it approved.” - ChiGuy

    “I only know, using for 10 days, that Fiasp, for me, has less ‘oomph’ than Novolog, and I think mixing (even in different shots) will be beneficial. The ratio now depends on initial glucose level and what I’m eating and inexperience. I also haven’t figured out how to do corrections well with Fiasp. Note: my first vial of Fiasp went cloudy and I stopped using it. I called, Novo Nordisk and they rapidly sent a return mailer and wanted it back, which I did. They also arranged for my pharmacy to provide a new vial at no charge.” - MonkMaster

    “I have been using my Fiasp pen to bolus before dinner for about a week, and it seems to reduce the post-meal high somewhat, and sometimes causes my blood sugar to go low three hours after eating. Everything is still preliminary, so I will continue testing until my pen runs out.” - Don1942

    Whether you're newly diagnosed with diabetes or you've been living with the condition for years, getting accurate and reliable information on diabetes from an expert is crucial. Consider trying a 14-day free trial of One Drop Premium and connecting with a clinical health coach. Every One Drop coach is a certified diabetes care and education specialist with years of experience. 

    This article has been clinically reviewed by Alexa Stelzer, RDN, LD, CDCES, clinical health coach at One Drop.

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