It's a histamine reaction, short duration allergy pill will solve your issue.
maybe do some research first:
another option is to use inositol hexanicotinate instead, I prefer this since I can go much higer in doses.
One caveat is that high doses of Niacin can play havoc with your methylation cycle:
Guard Against Homocysteine
Some concern has been raised by a recent report, which found that niacin raises homocysteine levels. But in fact, this finding represents yet further proof of niacinís benefits for heart health, and an opportunity to make them even stronger. That is, since persons taking niacin alone, even with this Hcy-raising effect, still end up with lower rates of both heart attack and death than those not receiving the supplement, it is reasonable to speculate that taking niacin with homocysteine-lowering nutrients may result in even greater reductions in risk.