rash, hives, itching, dizziness, fainting, difficulty breathing; tightness
in the chest; swelling
of the mouth/ face/ lips/ tongue, changes
in vision, decrease
in urine or dark-colored urine; fast or irregular heartbeat, nausea, vomiting, loss of appetite, muscle weakness, tenderness, or yellowing of the skin or eyes.
(http://www.drugs.com/sfx/niacin-side-effects.html
http://www.medicinenet.com/niacin-oral/article.