Report Which Heartworm Preventative Is Safe For My Border Collie? in Health and Genetics Posted June 19, 2013 One of my border collie/jrt mixes seems to have a sensitivity to heartworm meds. He has become quite ill with at least 3 different ones. I did have some Sentinel leftover from when I was hoarding it and thankfully he is ok with it. When I run out I will have to decide what to try next. His great-grandfather almost died from heartworm meds (they believe he does have a ivermectin sensitivity). He no longer gets HW meds but he is also in a low risk area. VA is a higher risk area but will do what I have to do... it was also suggested splitting the dose into 2 different doses (am and pm). With any meds there is a possibility of a reaction. You just have to decide how to handle it and you may have to go through some trial and error to find a solution. Trip's reactions: fever, diarrhea, lethargy, etc... so yes he has a bad reaction but I will continue the trial and error when it is time. I will just keep an eye on him when he is given the meds.