Nicotinic acid (niacin) by itself is an anti-vitamin. It interferes with the activity of nicotinamide (B3) in your metabolic pathways. Your liver has to aminate (attach ammonia) to it to make the amide. Until your liver does this task, you have the flush from a temporary acute shortage of nicotinamide.

B6 is good for circulation, if you are low on it -- accompany it with some magnesium.. If you are not low on it (eating enough veggies), it doesn't make much difference. Also, remember to get up and move around. And stay hydrated -- I always forget those things.

But it's best if the B complex vitamins are present in approximately correct ratios to one another -- B1, B2, B3, B6.. I don't know the optimal ratios from memory, but a large excess of one tends to cause symptoms of shortages in the others.