not very long. the effects of this relatively-low dosage will be fading quickly perhaps in several hours. in the meantime it is helping to counteract your allergic response so it is doing its job side-effects and all. water will not flush it out of your system. time will do that for you. niacin has side-effects but an "allergic reaction" is rare since this is an essential b vitamin that we all need. i am guessing but your niacin reaction may not have been truly allergic in nature. read about niacin reactions on the internet. i have an allergic reaction. i don't know how i got it all over my body. should i see a dr?
i had an allergic reaction and was given an oral 30mg dose of prednisone. how long will it stay in my system? . the allergic reaction was from niacin. the prednisone has caused my heart to beat faster. how long will this last and will lots of water help flush it faster from my body?