Hello,
I am a vet and would highly warn against the slow-kill route. See attached link from the Heartworm Society. For an active dog, it is definitely not best. There is actually an increased risk of thrombus with slow kill because worms are dying over an extended period of time. Obviously I have never met or performed an exam on your dog so I do not know all the details, but generally would advise against slow-kill.
https://www.heartwormsociety.org/ve...ks-out/507-the-ahs-protocol-vs-slow-kill
