I go on and off brewing my own kombucha at home. I don’t do it currently because it’s a pain to keep the culture alive by constantly brewing it. I end up with much more than I can drink. I like to use green or white tea and brew the same way I make most green and white teas – with maybe 50 C water, brewed for four minutes (1 tsp tea per cup of water.) Then I add brown sugar, let cool, and place the fungus in with the tea into a glass container. I cover it with paper towel and rubber bands, and let ferment for about 2 weeks. As it brews it gets less sweet, more vinegary. The result is a lightly effervescent, slightly alcoholic drink that is living (probiotic,) and tastes like a slightly sweet, slightly sour brew that has a tang to it. It is mainly a digestive tonic for me. 

I think these dosages are well tolerated usually but I understand it can be hard on the liver, so you may need to be monitored through blood tests. Therapeutic doses of niacin are much higher than recommended daily allowances.