elmhar

Generally speaking, herbs are higher on the list of possible suspects than pure vitamins.

That said, supplements contain fillers, binders, coatings & dyes, all of which have potential for causing reaction.

In my family, vitamins w/red dye coating are notorious for causing itchy rash. As each new generation grows up, we see another wave of those who can't tolerate the dye. So, sometimes these intolerances are genetic.

How potent was your niacin? Plain niacin can cause histamine to be dumped (if given in large enough dose) which can cause an itchy "flush" rash -- but a niacin rash will usually go away in a few hours if not sooner, IME. There are some niacin compounds that reduce the likelihood of rash.

An allergy rash, like a dye allergy rash, can take much longer to go away.

Best wishes.