A woman needs more niacinamide while breastfeeding.

http://www.drugs.com/cons/niacinamide.html

Of much greater concern is large doses of fat soluable vitamins while nursing. Eating raw garlic while nursing might give the milk a strange flavor, and the infant might consume less as a result. Therefore the amount of raw garlic eaten should be greatly limited.

Niacinamide is very nontoxic. If you are concerned though, then supplementation could be limited to just 20 mg a day while breastfeeding.