I think it's how far gone your auto immune issues are, and ability to flush toxins that are produced by the die off. I ended up in a very bad herx when I was ttrying to kill off the lyme parasite babesia,, because I've had the Lyme so long, my mast cells have been building up over the years, when I did a cleanse and used a bio film remover, all it took was a few high histamine foods eaten after two weeks of the cleanse, my immune system went bonkers and I went into anaphylaxsis. Vomiting pain completely covered in hives, there was no skin, just hives.. The mistake was I should have eaten very very simple mono diet, anti inflammation , etc, while I was doing it. And drank tons of water, which I did not, I am really bad about drinking water.