I have just tried Fiasp with my TSlim2 pump. It did not work at all, It did not lower my glucose.
The first session ended with occlusion alarms the second day, tested the tube and the canula, there were no occlusions. I never have problems with scar tissue either.
I have changed back to using Novolog.
How many times did you try it?
two cartridges
I have heard from other people it doesn’t work for them. Mostly I have heard it stops though after you have been using it a little while, 2 weeks, 2 months??? I am one that I have it in a pen and the first time I tried it, it was great. I am on Humalog through a pump and just wanted it for the off and on highs. But I decided to try it for a full day. I kept giving myself more and more insulin and was wondering what was going on and then it dawned on me maybe I am having trouble with the fiasp itself. It turns out if I use it once a day it’s okay, but if I try to use it a second time for the day it takes a lot longer to work. I swear I even notice a difference if I use it 2 days in a row.
Same here. After 1-2 months it virtually stopped working in my Omnipod pump. I switched back to Humalog, and absorption returned to normal.
I think Marie is right, I’ve also noticed if I don’t use it continuously it seems to work for short periods, though it was never any faster than Humalog.
Do you have a sensitivity to Vitamin B3 (niacin)? Fiasp is just Novolog mixed with that. Since you are OK on straight Novolog, beyond from the obvious comment, “Stick with it”, it might be worth checking out the possibility of some reaction to the extra ingredient.
For me the extra maybe makes the expected difference, but all we are talking about is a 5 minute, 10 minute absolute max, acceleration to the start of adsorption of the insulin. This might make a difference to me because I only know how much I am eating when I start eating. However in many cases I need a 20 minute warning to manage the post-prandial highs. In some sense it is a waste of effort; an extra 10 minutes really isn’t enough.
I use it because it doesn’t cost me any more and, apart from the odd stinging which isn’t an issue for me, it has no downside. I don’t care about the stinging. If Novolog cost me less (impossible for me) I’d use it instead.
Okay that’s interesting, @jbowler I have never noticed a stinging with it. But when it is working it does seem to work about 30 minutes faster than Humalog for me.
Interesting. For us, we have been using it for about a year in pens, and about 4 months in pumps. It still works well without trouble, and is definitely a lot faster on average than Novolog, although there are times, maybe 30% of the time, when it is just as fast as Novolog.
Weirdly, there are numerous occasions where it is quasi-instantaneous in a pump. At first, I was putting it down to coincidence, but it was so often the case that I asked around and found out from another parent of T1 teen that it was true for them also.
Wow, I never experienced that. I’m glad it is working well for Kaelin though. 
I’ve used Fiasp extensively and for me it has an onset nearly 4x’s faster than humalog via MDI. I’ve occasionally felt an injection sting, it’s curious but certainly not a deal breaker.
From time to time I’ve also noticed it seems to lose its efficacy, especially its faster onset. I concluded I likely develop Fiasp specific insulin antibodies, and I give it a rest for a couple of months.
Which reminds me nows a good time to retry it, as I stopped 8 weeks ago tomorrow for a rest and reset.
That’s interesting. For me, it isn’t just NOT faster than Humalog, but not even an insulin reaction! Like water! I’ve not heard of developing insulin antibodies. That’s scary. I still have several vials of Fiasp so I could try it again. The problem here with switching among insulins is that a prescription is needed! For me, that means paying for a Dr prescription since they now charge for them unless it coincides with an appt. And insurance only allows one fast acting insulin prescription!
One of the few benefits of living in the great white north 
I used Fiasp for a week or so early last year. I did act faster than my Humalog, but I developed a rash and a big red spot where I had injected into my arm. I went back to Humalog. Several days later the rash finally disappeared.
Richard, I am curious because you don’t seem allergic to much. Was it direct injections or a pump?
@Michel, I used a pump at first and had itching and no rash. I stopped the pump usage and used Humalog again. The next time had a high above 170 I injected Fiasp. There was a half inch red spot around the injection ad a lot of rash around that. I have not used any more Fiasp and I have never used Novolog.