when I was getting treatment for parasites drugs were rotated and life cycle of parasites was taken into consideration as some of the meds only worked for a certain stage of parasite as well as some were all body meds while others were only gut intestinal tract meds. Never used the product DE but did use other holistic products during off days of meds. They do develop resistance - I have read how pigs and other farm animals develop drug resistance or the parasites within them develop resistance to certain meds- its a huge industry keeping the herds clean - my cousin actually develops pharma meds for animals just for this purpose