Your body is known to hypersensitive to
Niacin. Flushing, hypotension is known adverse effect of
niacin in some individuals.
The spectrum of these reactions can vary widely; reactions may present as typical allergic symptoms (eg, itching, urticaria, rash, edema) after a single dose, or may manifest as severe idiosyncratic dermatologic reaction.
As for marks are concerned, Hydroquinone cream against prescription plus sun protection can lighten existing lesions.