Perhaps you (and many others here and everywhere) are thinking like I used to, that if it's herbal and "natural" it's good and, therefore, automatically safe but even some good things must be used properly. I've read some cautionary statements about wormwood (for instance) because of it's possible content of thujone (which some say is very low in today's extracts...), a chemical that can produce some negative side effects if consumed frequently or in high amounts and/or for long periods of time, so I think it's better to be cautious and even if I had a serious case of parasites, just in case I would not use it twice in a row. In any case, as some have mentioned before, it's best to rotate between different types of herbs so the parasites don't become resistant to any particular one.