I took it several years ago for the same reason. I had the same reaction - severely - everywhere - from head to toe - in the middle of the night. “never again” after that. One of the most unpleasant experiences I have ever gone through.
The key is starting small and building up. I used to have a BP of 152/96. My BP dropped significantly after a month. After a dozen years on Niacin it is still around 118/65. Did amazing things for cholesterol too.
Wife, sister, and quite a few co-workers take it with similar results. One says it helped his ED quite a bit. It was the main item for high blood pressure years ago, before the drug companies figured out how to patent a range of BP drugs. When my dad switched, he lost quite a bit of daily energy.