Are there side effects to the medication used to prevent cat heartworms?

For the most part, these preventives are very well tolerated. Sometimes you’ll see local site reactions where you apply the topical; they'll lose hair and get itchy. On occasion, I've had owners report cats that just don't feel well after the topical is applied. If we suspect that the heartworm preventative is causing some lethargy or depression, we'll switch products.

In my experience, I'm going to say no, because again, most of the medications I'm using to prevent are combination medicines that also do fleas, ticks, ear mites, heartworms, intestinal worms, and those kinds of things. I use things like Revolution, Revolution Plus, and Advantage Multi for Cats. Those are all things I've used. Full disclosure in answer to that question, yeah I have seen some cats that don't like it when you put it on. They act very irritably, and they may even get a little patch of hair loss in that area because you're putting it on the back of the neck. I've seen those kinds of side effects, but it's not specific to the heartworm preventative that's in it. Many of those products may be alcohol-based because that helps them to absorb through the oil layer in the skin and be absorbed in the bloodstream. I think more side effects have to do with the liquid itself or the carrier that the drug is in, and perhaps not the drug itself.

Any time you put a chemical in or on an animal, you always have the risk of there being a reaction to that animal, but it's not something that I routinely see in cats.

Most cats do just fine with the medication used to prevent heartworm disease. Sometimes it can cause a little skin irritation if you use a topical treatment, or it can cause a little GI upset. However, these are rare occurrences and the medications are generally safe to use.