They use arsenic to kill adult heartworms. Vets recommend close monitoring of your pet during treatment with blood tests. Since the treatment contains arsenic I would recommend keeping dogs separate from other dogs especially if they like to eat stools, or vomit. Prevention is always the best way to go because heart worms can be devastating to the heart and lungs of your pets. From what I heard the treatment is very hard on your pet and you have my sympathy. I hope your dog makes a full recovery.