Whether you decide to take niacin for lowering cholesterol or reducing anxiety, there is a good reason why most pharmacists and physicians specifically advise you to take it just after a meal - doing so helps reduce the sudden (15 to 20 minutes afterward) "flush" that some people experience. For the same reason, if you are looking at large daily doses, you want to break it up in small multiple doses throughout the day rather than taking it all in one go.
Although you do not need a doctor's prescription for niacin/Vitamin B3, if you are going to take large mega (eg above 2000 mgs/day) doses, it'd be prudent to consult your physician and get appropriate monitoring - especially for your liver & liver enzymes. Niacin in high doses can be toxic and there are a number of serious potential side-effects including liver damage, exacerbate an existing case of gout, birth defects, etc.