Didn't speak of horse worm medication: I state HEART WORM.
If you are talking about Horse worm medication, there are a few out there to use but you need to know your % before you actually use it b/c you can make your dog(s) very ill if you give it incorrectly.
we use ivomec.. its about $40 @ a feed store but please consult a vet first on dosages because like they stated before if the wrong dosage is given it will be fatal and cannot be used while they are pups
Ivermectin is fairly safe to use in dogs and the toxic amount is a really large amount.
Here is the dosage for Ivermectin 1%
•The dosage of medication administered depends on the weight of the dog. If it's being used to treat heartworm, the dosage is much lower than if it's being used to treat parasitic infections. For use as a heartworm preventative, the dosage is .0015 to .003mg per pound, administered monthly. Many manufacturers of ivermectin for dogs offer a beef-flavored chew, to make it easier for dogs to swallow. To treat parasites, the dosage is .15mg per pound. For a dog weighing 30 lbs., for example, the dose would be 4.5mg. Depending on the severity of the infection, this may be repeated until the parasites are completely gone.