The treatment for a positive dog does contain arsenic. The preventative meds do not. The most common preventative is ivermectin which is a very safe drug even at high doses.
Thanks for telling me that, I'm very glad to know it. As I said I hate giving all these things to my dog!
I first saw it as an injectable worming medicine in horses. It was so darn effective that we had some cases of colic due to the sudden load of dead worms. We began worming first with paste and THEN with the ivermectin. But they stopped the injectable form from general use because of problems with infection at the site, now you get it in a paste like everything else. It's still very effective, though not as dramatic as the injection.