Here is the Safeheart (milbemycin oxime) FDA drug summary for those interested. You can print this out and take it to your vet to show that you can safely give Interceptor (milbemycin oxime) at the lowest dose (2-10 pounds) for dogs up to 50 pounds to effectively protect against heartworm. Dogs over 50 pounds can get the Interceptor 11-25 pound dose. This lower dose will not protect against intestinal worms but at least you have the option to decide if you wanna drug your dog for something so benign and easily treatable as intestinal worms.
Unfortunately Novartis is no longer producing stand alone Interceptor. They now offer Sentinel which includes both milbemycin oxime for heartworm protection and lufenuron for flea protection. Your dog likely will not get the same flea protection if you dose down to the Safehart level of milbemycin oxime for heartworms. But again, at least you have the option to make the choice.
NADA 140-915 Safeheart? - supplemental approval (June 4, 1998)