I know dogs on Heartworm preventative is not always 100% effective even when given properly and according to directions and that dogs can still get heartworm while on the monthly dosage, which is why some preventatives or websites selling it have guarantees that cover part of the costs up to a certain limit as long as certain criteria have been met.
__________________  Jeanie and Tibbe One must do the best one can. You may get some marks for a very imperfect answer: you will certainly get none for leaving the question alone. C. S. Lewis |