This does have Niacin Supplement, I can't find the ingredients for just the original Chick Feed though. Unless this is the same feed with different labeling.
Amprolium is a Thiamine Blocker, it won't hurt ducklings.
The Medicated feed in the older days used either Cyanide, or Arsenic as a Coccidiastat, & it was more deadly to ducklings then chicks. Chickens tolerate the poison better.