I’m very comfortable with the safety of heartworm preventatives. I give the preventative once a month and treat for at least 3 months past the last possible exposure. I do not use an extended duration of 45 days because in the original studies using heartgard dogs became heartworm positive when dosed at 45 days.
I extend out for at least three months beyond exposure because dogs given only one dose ( moxidectin was effective with one dose) after exposure became heartworm positive but when dosed for 3 consecutive months did not. I do not like heartworms and as I’m in a heartworm endemic area will not risk it.
