Flushing

Flushing, vasodilation, and itching are the primary indicators.

Niacin and Coumadin (Warfarin) are compatible, but their use together should be monitored by your doctor. Niacin has minor anticoagulant effects and its use, even without Coumadin, requires doctor supervision if used to affect blood lipids. Niacin can have liver toxicity if taken in high amounts. This is especially true of extended-release or sustained-release niacin. Immediate-release niacin (regular niacin, without any delaying aspect) has the least liver problems associated with it.

From Provet Healthecare Information website: Over-upplementation can result in toxicity with doses of 133-145mg nicotinic acid/kg body weight per day causing haemorrhagic diarrhoea, fits and death in dogs.