How long will it take Niacin to flush out your body of methamphetamines? How will the test show up if someone took 1000 mg of niacin 7 hours before?

Usually meth is detected in very negligible amount in blood after 48 hours. It usually excreted  through kidney within 24 to 48 hours. And there is no specific antidote that flush meth from your body. But maintaining hydration and taking as much as fluid is the key to excrete as much meth as possible early.

Meth metabolism half life is around 10 hours and excretion time up to 48 hours for abusers. It can be prolonged in chronic abuse of meth. How long have you been using it?

So wait for 2 to 3 days to become meth free in your blood concentration.