It's funny you mention an itchy nose. Niacin can cause a 'flush', which can be anything from redness to itchiness. I got a really bad niacin flush years ago and it felt like someone dropped ants all over me. It lasted about 20 minutes.
Today I took 2 zantrex before I left for work. My hands got a little itchy. I did notice, though, that at one point I couldn't breath very deeply or I would feel like I might choke. It only lasted a few minutes, and it was only if I inhaled very deep.