I have a retired black lab that is 12 yr. old. I have used Revalution made by Phizer all her life with no problems. Revalution not only prevents heart worms but kills all other parasite dog and cats can get such as fleas, ticks, all stomach worm, etc. It is very easy to apply just pop the little capsule and squeeze it between their shoulder blade once a month. It is a little pricey but not when you consider it will be the only treatment for all parasites. I get it from my local vet @ about 7.50 per dose. Jim