Per the studies done the only heart worm preventative found to be 100% effective was Advantage Multi – it was a small study though so my guess is that if it were larger Advantage Multi would have some failures as well (just a hunch). Most vets are observing equal numbers of failures with Interceptor/Sentinel, Heartguard and Revolution. The failures are mostly seen in large breed, outdoor dogs.
http://veterinarynews.dvm360.com/dvm/Veterinary+news/Leading-parasitologist-reveals-heartworm-preventiv/ArticleStandard/Article/detail/703785
http://www.yourpetsbestfriend.com/your_pets_best_friend/2008/04/heartworm-preve.html
