Quote Originally Posted by BobBong
http://en.wikipedia.org/wiki/Niacin

"Large doses of niacin can cause liver damage, peptic ulcers, and skin rashes. In the high doses used for controlling cholesterol levels (anything above 100mg/day), nicotinic acid can cause skin flushing and skin itching as well as headaches, lightheadedness and low blood pressure. Niacin may also aggravate peptic ulcer disease. Most stomach-related side effects usually go away over time. Taking niacin with food may prevent stomach discomfort. People with a history of liver disease or stomach ulcers should not take niacin supplements. High doses of vitamin B3 (75-mg or more) may cause liver damage. People who are pregnant, diabetic, or who suffer from asthma, liver disease, gallbladder disease, gout, glaucoma, or ulcers, should seek the advice of a medical professional prior to taking this vitamin. Nicotinic acid, but not nicotinamide in doses larger than 200 mg causes flushing by dilating the blood vessels, which can also cause the blood pressure to drop. High-dose nicotinic acid (approximately 3 grams daily) has caused impaired glucose tolerance in otherwise healthy individuals."
that pretty much sums it up