my vet only recommends interceptor or sentinel for heartworm preventatives for my yorkies...i believe they have been documented as having the least amount of negative reactions and a low failure rate (meaning getting heartworm while being on the preventative) and i like the fact they have doses metered for 2-10 lbs (unlike other heartworm which is up to 25lbs)...i have a problem giving my kids - all who weigh less than 4lbs - the same amount of medicine that a 25lb dog would get
any drug given to our babies should be well thought out...but heartworm is prevalent in many parts of the US and i personally would rather be safe than sorry...i am sure you will get more responses from more of the experts on here
ooh and my vet wanted me to get my youngest (bree) on interceptor the minute she reached 2lbs