Is it safe to take niacin 50mg or 100mg orally everyday as a supplement for the body?

Safe, yes. Issue is. why? Vitamin (except Vit D3) deficiency=rare in 1st world countries. Vitamins=simple molecules present in nearly all foods; humans have not maintained every gene/enzyme to make them from other molecules, generally missing only a single step. Yet they are cheap to gather & big business sales. However niacin, B3, at >1,000 mg/day, improves lipoproteins for some.