We don't have that in Australia unfortunately, and if I order anything international it would take too long. I already have them on heaps of vitamins, including B's and yeast, which is the weird part. I will call the vet in the morning and confirm if what they have given is enough, or if they perhaps have an intramuscular option. The feed I was using was specifically for ducklings too, which makes it all the more frustrating! I have to wonder if congenital deficiency plays a significant role as well, as both batches of eggs from the same breeder have all had significant problems, and the vet said the problem was "embryonic".Go get the liquid b complex TSC usually carries it. It will say injectable but we use it orally 1 ml per duckling over a tasty treat so you know they are getting full dose. I hope it’s not too late for full recovery but I know the liquid b complex has helped so many over come niacin deficiency.
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
In your experience, how long do the ducklings take to respond to the treatment?