There are clear indications, through the first hand experiences of dog caretakers/guardians, that the heartworm treatment for dogs, Trifexis, is dangerous and possibly lethal.
Here is one extremely pointed and poignant comment:
“I gave Trifexis, I lost my dog.”
OR
“I am sick that I was unknowingly poisoning my baby girl, sick that this med is still being prescribed, and concerned that some (most?) vets aren’t doing their due diligence and researching what they recommend. It unfortunately only takes a quick Google search to see what’s happening!”

A notable aspect of the dangerous nature of this insecticide drug for dogs is that its damaging effects may only be apparent after one year. Beware.
Now having convinced myself that Trifexis is a dangerous treatment for dogs despite being widely dispensed by veterinarians, what is the connection with cats?
Well, a simple bit of research tells me that the insecticides used in Trifexis are also used in drugs for cats.
Trifexis contains: Spinosad and milbemycin oxide. They are respectively a crop insecticide and a heartworm larvae pesticide.
Milbemycin is similar to another pesticide called ivermectin, a dewormer.
So we have these three parasite killers: spinosad, milbemycin and invermectin.
All three are used in certain cat parasite treatments. So if a cat is being given two different treatments say for fleas and worms it seems possible that the cat will have two of these drugs inside him at the same time and when that happens the toxicity could be dangerous and damaging because Trifexis contains two of them and it kills dogs according to thinking dog owners. It is the combination of spinosad and invermectin which can kill.
A cat flea treatment, Comfortis, contains spinosad. Ivermectin is used in cat treatments for heartworm, ear mites and other parasites. Milbemycin oxide is contained in other anti-parasite products for cats and dogs such as Interceptor. There are others.
The labelling on Comfortis states: For Animal Treatment Only. What they are saying is that we can place animals in danger but not humans Humans have similar biology to cats and dogs. The manufacturers admit that this product can hurt people but then sell it for use on cats and dogs.
It does not take much imagination to see the possibility of cat receiving a combination of these insecticides in one or more treatment to then become very ill. Sometimes cats suffer from mysterious illnesses associated with the nervous system. They may go undiagnosed especially if the illness occurs a long time after the drug was administered. These insecticides kill parasites by attack their nervous system. They can also attack the nervous system of larger creatures. For example, one side effect of Trifexis is ataxia (lack of coordination in dogs and cats).
Really, over the years having read a lot about these sorts of drugs I can honestly say that we, as cat and dog owners, need to be extremely cautious in giving them to our treasured animal companions.
They should be avoided wherever and whenever possible. Certainly, based on the terribly sad stories about Trifexis, cat owners should read the labels and avoid products containing the above insecticides.
Some websites do hint at the dangers of these drugs. I don’t think vets always give clear warnings on their use.
Elanco, the veterinary division of Eli Lilly and Co sells Trifexis. It is said that they are behaving irresponsibly. Who can disagree? The big pharma companies…Elanco says:
“What we continue to say is there is no link established between Trifexis use and death… Reports are not an indication of cause.”
Sources: Various but primarily http://vitalanimal.com/fire-your-vet-trifexis/.
Conclusion: avoid these, check labels and challenge your vet.