Diabetics may need more insulin when they take niacin, though not all diabetics react this way. High blood pressure patients who use Reserpine type medications may experience nausea and considerable drop in blood pressure, when using high dosages of niacin. This might not be dangerous, but could be very inconvenient. I know of no studies about this. Liver function tests might show false indications for a person on niacin.
Smoking also disturbs glucose tolerance tests.
Your skin turns pink, as though you had a sunburn. This flush might start almost immediately, or hours after you take the pill, and at the beginning it might last for a few hours. If you drink a glass of cold milk after taking the tablets it reduces the flush. Or take niacin with the solid part of your meal. Regulate your schedule so that their is little flushing in public.