I've been taking niacin (B 3 flush kind) for decades. I've taken it mainly for migraine headaches...not after the migraine begins but as a preventative; can take 250 mg four or five a week. I also take the herb feverfew as an herbal dilator.
To the people who got a bad flush at 500 - too much...won't hurt you though. Just chop the tabs in half and take AFTER eating...less intense flush and more a mild one if taken after eating. I think more effective also.