This is from here :
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
Possible overdose
If you search "Can I overdose my duckling on niacin?" you'll likely end up finding several answers claiming, that you can't since niacin is water-soluble and isn't stored in the body like other vitamins, but just passes through without being stored into any tissue.
The problem with this claim is humans, and dogs can have serious side effects when given too much niacin, so why can't birds?
I decided to research this a bit more, and found this
link which state's,
"Limited research indicated that nicotinic acid and nicotinamide are toxic at dietary intakes greater than about 350 mg per kg (160 mg per lb) of body weight per day (NRC, 1987). Clinical signs for niacin toxicosis in chicks include reduced egg production, growth retardation, short legs and coarse, dense feathering. High dietary levels of niacin (0.75% to 2.0%) fed to broilers were detrimental to dimensions and mechanical properties of bone (Johnson et al., 1992; 1995; Leeson and Summers, 2001). There was no change in the mineral content of the tibia, but bone strength decreased with increased susceptibility to fracture."
That said, it appears niacin can actually have reverse effects when birds are given too much, and can cause a weakened overall bone strength, as well as increased susceptibility to fracture.
