"Flush" is an old word that seems to have many meanings. In this case, a side effect of taking niacin. which can be purchased "over the counter" can be "flushing" or a sudden feeling of great heat in the body, like a fever and sometimes, with a reddening of the face. Like a "hot flash". It usually doesn't last long but is somewhat unpleasant. Some niacin manufacturers have figured out a way to make it "flush-less". I don't know how they do that, but it's labeled that way.
"Flush" is an old word that seems to have many meanings. In this case, a side effect of taking niacin. which can be purchased "over the counter" can be "flushing" or a sudden feeling of great heat in the body, like a fever and sometimes, with a reddening of the face. Like a "hot flash". It usually doesn't last long but is somewhat unpleasant. Some niacin manufacturers have figured out a way to make it "flush-less". I don't know how they do that, but it's labeled that way.