One of our ducks arrived 2 months old from a breeder with harsh living conditions and in bad health. We did not realize at the time - starting out with zero practical experience, our previous (first) purchase was from a lady who really loved her ducks and took good care of them, so we assumed this was what everybody did.
When she came in (together with 2 more from the same breeder) our 5 ducks, the original gangsters, decided that she was the one of the three newbies that they absolutely didn't want around. They all ran directly at her, she got pecked, stomped, chased, it was bad.
She had an eye infection which was cured by antibiotic fed by a syringe. But more importantly, she could not walk more than 15 ft without flopping down.
We kept her under a special regimen for a month and a half:
- Over the day, she'd be mostly closed in a (very) large dog crate with frequently refreshed water (infused with a vitamin/mineral mix with lots of niacin) and cooked peas (niacin) sprinkled over by deactivated yeast (again a great source of B-complex vitamins). Cooked peas is actually what she preferred to all regular duck food (grains, pellets); the other stuff would just remain mostly uneaten. Sometimes during the day we would let her out just to catch some sun and for us to see how it's going.
- Over the night, she'd be in a smaller (but still quite large) dog crate inside the duck coop (the super large one would not fit

so that the flock would not consider her a stranger. Again, water with the booster and the peas with yeast.
After 3 weeks or so the walking got better; the distance she could go was slowly but surely increasing.
After 4 weeks or so we declared her well enough and socialized enough (and us tired enough of moving crates around) that she could stay in the duck house without a crate - would not get bullied. This worked fine.
After 6 weeks we declared her well enough to live in the same way as all the other ducks, no special treatment anymore.
I mean, when the duck gets hard to catch, obviously she got much better at running
This point was in mid-June. Since then she's been doing well. One of her feet is still rotated inwards and we believe that's how it's going to stay. She walks somewhat like a clown (more so than the others, anyway) but can walk at speed and without rest stops. She's also a full member of the flock, no social problems.
So yes, niacin, niacin, niacin. It's a great help. But don't necessarily expect a speedy fix or a complete one. Maybe, but maybe not entirely.
The only special behavior that remained is that she really, really loves to be in the pool (we have 2 kiddie pools) and stays in there most of the day. We're guessing because it takes the weight off her feet and makes her comfortable. All ducks love water but this one is really deeply in love with it
Big thanks to folks in the Quack Shack part of the forum who supported me, a newbie, in this duck recovery adventure!