What type of reaction can you get from niacin?

A flushing sensation that is mild not too discomforting. This usually occurs at higher doses (400mg) and not in 100mg tabs. "Flush-free" B3 (niacin) is a common product. I had the flushing syndrome with GABA as well, quite similar. Taking a B-50 or B-100 complex vitamin is a good way to balance niacin with all the other B vitamins.