I had another session where I needed 400 mg of niacin. I had eaten before, maybe 1 1/2 before. So I took it under the tongue, 200 right away.
I felt it absorbing, got the jaw even a little prickly, but ot enough to say ot was a flush. So I took 100 more, waited a while and then anoher 100mg.
Still no flush. All in all, about an hour passed from taking the first niacin to when I realized I wasn't going to flush. But going on a hunch, I decided to"do it" anyways-
With the accelerated heart beat and energy expended during.... I began to feel the flush in my face a bit, and my wife said I was somewhat red.
Today NO POIS!
Puting all the pieces together, here's what I figure is happening.
The flush is a good indicator that you've had enough, "under certain conditions". But if you take a lot of niacin or are more sensitive because its early in the day or you haven't eaten, then the flush comes faster. If you have eaten or are tired or whatever, the flush delays or doesn't even come, and so it might not indicate very well if you are ready or not.
I have found, that if for some reason I don't get the flush, that 400 mg is always enough to prevent POIS, and that you must wait for at least an hour from when you first ingest the niacin.
Sometimes you can't take 400mg. so there, the flush is the measure (or even the restriction). If you get the flush, wait that hour from first taking the niacin though, that seems to be the important part.