Okie drake, I am having a hard time following your line of thought on this statement. IF someone is using 1% Ivermectin it is a "treatment" for the intestinal parasites but not heartworms. At that dose Ivermectin will kill the migrating L4 heartwom larvae and intestinal parasites but even at that dose to the best of my knowledge it will not kill adult heartworms. I have read some materials that indicated it may "weaken" adult heartworms but still does not kill them. Which by the way may be the "key" to some of the resistance issues but we can discuss that later. Commercially prepared monthly heartworm products work exactly the same way except that they use another product (Pyrantel) to "kill" the intestinal parasites and Ivermectin at a much lower dose to "kill" the migrating heartworm larvae. I think efficacy against a broader spectrum of parasites compared to some of the commercially prepared products may also play a role in the decision making of some individuals as well. But sure cost is ALWAYS an issue. No way are you gonna convince me that using a higher dose is more likely to cause resistance than using a lower dose. However I do think it is possible that the use of all heartworm prevention products in dogs that are "positive" for heartworms may indeed be the "key" to some of the resistance issues. You are quite welcome, I am enjoying the discussion.