dont use niacin. dont know if it clears the urine or not but from what i have read too much is not good for you. read around. i'm sure you'll find what you're looking for.

ps. go to yahoo answers. there is this chick there her user name is trish m. she works at a lab for drug testing and she knows her shit. ask her whatever you want. more often than not she'll get you an answer. good luck!