Yes, any animal that uses millions of eggs as a survival stragety, which most parasites do, will have some offspring that are resistant to whatever you throw at them Luckily, there are so many herbs, you will never run out, and if you have bigger worms, then use digestive enzymes, and get them that way.
(Zymex 2 is a good example, I like bromelain, it agrees with me and does the same thing.)
Julie