Ugh...you're in the belly of the beast for HW infection right now...you're right to worry. I would check with the vet about whether you maybe need to give a different form of prevention this month (maybe a topical). I honestly don't know, but it would worry me, as we're in the middle of the "bad season."
Trifexis is the only one I actively avoid because it's so unpredictable which dogs will react. It's a very, very small subset that react, but nobody's figured out why they do, so you can't say "dogs of this breed, or type, or age" are more likely. It seems like it's just random.
NOLA vets seem to love Trifexis -- so many adopters and people down there tell me it's all their vets prescribe. I can't figure out why. Maybe Elanco has a super-star clinic rep down there!
I would consider switching to a topical, since they may be better tolerated by tender-tummied dogs (Revolution, or especially Advantage Multi). In SE Louisiana, I think that Advantage Multi is probably the most reliable protection because there's no documented resistance to it -- Auburn Univ. found it to be the only product with no resistance in the ivermectin-resistant strain of heartworms! After 4 months, it provides steady state protection (not just a quick larvae kill and then being filtered out by the liver, as all the others do, but constant protection all month long)...and it protects against mites, which can be an issue here. The downside is that it stinks, badly, for a few days. If you have kids, you have to keep their faces out of the dog's mane for a while. I don't think the flea protection is as good as Trifexis/Comfortis, but it's been good enough for me -- and I want the best HW protection because of where we live. Our rescue's vet has her own dogs on it, and she shifted us to it exclusively for our foster dogs. I've used a lot of it the past few years because so many of our foster dogs are on it, and I've never seen a serious adverse reaction.
FWIW, I've heard some vets say off the record that they don't prescribe AM or other topicals because they think their clients are idiots and won't apply topical products correctly. They need to be applied to the skin -- separating the fur to expose the skin. These vets think that's too complicated for their clientele, who may just squirt it on top of the fur. You may have to tell your vet "I'm not an idiot and can read the insert on how to apply it correctly."
If your dog is otherwise healthy, you might also talk with your vet about the injection called Proheart6 that lasts for 6 months -- a lot of people love it for the convenience (you only have to worry about HW med twice a year, no chance of vomiting it up). However, it can only be given to adult, healthy dogs (not puppies, old dogs, or sick dogs).
If you want to stick with a pill, a lot of dogs seem to tolerate Interceptor pretty well, and it's cheap. In SE Louisiana, I personally choose to avoid Heartguard and its generic equivalents -- the emergence of ivermectin-resistant HW is real here, and it's no joke. I've known people here whose dogs came up positive who never missed a dose of HG (though the company will pay for treatment if it happens, and you buy it from a vet....not worth it to me) -- the resistant strain is still rare, but it's here.
ETA: You should probably report your dog's reaction to the FDA through their web portal. I believe they are monitoring Trifexis, or at least they were.
