NAD Q

Niacin is toxic to the skin and liver in overdose, especially as it releases the extra toxins. Studies in laboratory animals have demonstrated behavioral changes when large doses of niacin are given. Sullivan, WT (June 10, 1958). "Behavioral changes in rats and guinea pigs induced by the administration of indole 3-acetic acid and 6-aminonicotinamide".
Niacin in large doses is also known to increase the level of HDL (good) cholesterol in blood, and is often prescribed for patients with low HDL, and consequently high risk of heart attack.

Too much vitamin is harmful to the body.