I wondered the same thing as I took herbs many years ago and while I did kill some, I got worse in general, and then ultimately nothing was working anymore. I was scared to try again.
but I did, for about the last 6 months, this time it's going ok, albeit very slow, but I'm slowly improving...what I did different this time...started with low dose and built up as per tolerance, also taking kidney cleansing herbs, intestinal herbs (though I did these before too) and infrared sauna. If I get a bad herx reaction, I don't take any anti-parasitic herbs till it subsides, when I've pushed it before it didn't help, now I wait till I feel better and then continue. and at anytime if I don't start feeling better, I will stop them completely, I don't want to go down the bad path again, but so far so good.