I used to use Interceptor for heartworms and bought Program online for flea prevention. My vet said "there's this great new combo pill that will save you money" and we switched to Trifexis. That was a couple of years ago, and now I am changing to Sentinal. I did not realize Sentinal had been removed from the market for a while, but it is the same flea med as Program (in fact, Sentinal is marketed as Program Plus outside of the US) and the same heartworm med as Interceptor ( and as trifexis, but it is the Spinosad, not the milbemycin that seems to be the problem ).
Even Snopes cannot rule out Trifexis as the cause of deaths, although they won't say it is, either.
snopes.com: Trifexis Kills Dogs?