Can Niacin Really Clean Out Your System Of Methenphetamins Even

Can niacin really clean out your system of methenphetamins even if you've used just recently? A family member of mine is going for a urinalisys today right now and he just used about two hours prior. He said that would flush his system. Is that true? I know there are lores about stuff like that. I myself don't use but should I believe this family member that he'll be ok??? I'm disabled and he takes care of me. I'm not pleased with his choices. But he's all I've got. Will you ease my mind even if it's a no?
Thanks.