Ask the Vet: Flea and Tick Medication Advice

 Ask The Vet

Q: Are some types of flea/tick medication better than others, and are they harmful to my pet?

Brent, Miami

A: Hi Brent!

Thanks for the question! This is a complicated issue; so let’s get to it.

For the first part of your question, the long and short of it is, “No.” Each medication has pros and cons depending on your needs, your lifestyle and your pet. We can break this down a number of ways, but I find it easiest to split things into several categories.

First, how is the medication administered? Second, what parasites does it protect against, and how does it do so? Third, does your pet have any complicating factors—such as allergies—or do you live in an area with a higher then normal burden of fleas and/or ticks?

How Is The Medication Administered?

Most available flea/tick medications are topical, meaning they’re applied to your pet’s skin. Some topical medications are absorbed into the skin and spread through the glands, and some simply stay on the surface. This is an important thing to note if you take your pet to the park a lot or give a lot of baths. Most products claim to be just as effective after a bath or swim in the lake, but there is some debate about this, and it’s likely that no one knows the real answer. This isn't to say an occasional bath means you need to reapply topical products, but they may not be the best fit if you are bathing your pet daily.

There are also oral products. The obvious benefit is they don’t wash off. The drawback is that some have gastrointestinal (GI) side effects. Most of these are mild and consist of vomiting, diarrhea, and decreased appetite, all of which are temporary and benign in most cases.

What Parasites Does It Protect Against, and How Does It Do So?

Flea and Tick PreventionThe second thing to look at is how the medicine actually works. Regarding flea prevention, the active ingredients in these products work different ways. Some kill fleas, but only after they bite. Others sterilize female fleas so they cannot produce eggs (but they can still bite your pet). Some cause a malfunction of the flea egg development cycle, keeping eggs and larvae from maturing into adult fleas, and still others kill fleas on contact. Most pet owners won’t notice any major differences in the protection provided by any of these products. They all prevent flea infestations from getting worse when used appropriately.

Does Your Pet Or Where You Live Have Any Complicating Factors?

Some pets have allergies to certain types of medication or have conditions that necessitate the use of one product type over another. A pet allergic to fleabites would benefit more from a product that killed fleas on contact, minimizing the effect of their condition from a flea infestation (although contact from a single flea is sometimes enough to produce a profound allergic reaction in these patients, necessitating a more advanced regimen from a veterinary dermatologist). If your pet had a bad experience from a particular product—topical or oral—switching to another is probably the way to go.

Yes, Medications CAN Be Harmful

Regarding your question about these medicines being potentially harmful to your pets, the answer—as is the case with any medicine—is, “Yes.” If used inappropriately, flea/tick prevention medications can be harmful. Using a flea/tick product recommended by your veterinarian, and in the appropriate weight range for your pet, will have a minimal chance of causing an issue, and an even smaller chance of causing a significant problem.

That said, a few instances come to mind:

  1. Products containing Ivermectin. Ivermectin is the active ingredient in a heartworm preventative medication (Heartguard and others), and has been linked to toxicity in specific breeds (Collies are the poster child for this). However, this is usually at very high doses often used for treating specific parasite infestations. This is due to a hereditary gene mutation called the ABCB1 mutation or MDR gene mutation. Because it’s hereditary, it can occur in many dogs. If you’re worried your pet may have this gene mutation, you can have it tested with a special genetic test to see if he or she is positive, but this is usually not necessary. Again, most flea/tick products that contain Ivermectin or drugs like it (often that protect against heart worm) contain a very small amount of this chemical, and are not toxic.
  2. Recently, it has been recorded that dogs receiving comforts (a tick preventative) and Ivermectin (the heartworm preventative) can be at an increased risk for toxicity. Again, this is generally only at higher doses of Ivermectin or in patients with the ABCB1 gene mutation.
  3. Some products contain permethrin or pyrethrins often found in insecticides. Both are very toxic to cats! Never apply a product labeled for use in dogs to a cat. Not only are these drugs toxic, but because dogs are generally much bigger then cats, the doses of these chemicals in the dog-labeled product are quite large and can be lethal if applied to your cat.
  4. Some animals will have allergic reactions to these medications, mostly manifesting as either skin reactions (rash, redness, loss of hair, itchiness, etc.) from the topical products, or, GI side effects (nausea, vomiting, diarrhea, etc.) from the oral products. If your pet experiences one of these complications, they are usually self-limiting, and discontinuing use of the offending product likely solves the issue.
  5. More severe reactions can lead to anaphylaxis (similar to a human stung by a bee) and are a medical emergency. This can involve dysfunction of the cardiovascular system, swelling of the larynx, difficulty breathing, and other problems. These types of reactions are extremely rare, but could occur.
  6. More complicated than “allergic” reactions are “idiosyncratic” reactions. These are very fast occurring, very severe allergic reactions that can occur in any pet without warning. Again, this is true of any medication but extremely rare in these products.

Cat Flea and Tick PreventionSome prescription-only products have generic over-the-counter versions. Theoretically, they should perform the same when compared to the most similar prescription product (i.e. the active ingredient—fipronil—in Frontline [available here at] is found in some generics like PetArmor).

Several people have noted flea/tick “resistance” in their area to some products (mostly Frontline/fipronil based products). To my knowledge, there is no official documentation of this occurring, but if a fipronil-based product is not meeting your needs the solution is as simple as discussing alternatives with your veterinarian and switching products at your pet’s next check-up.

If you’re unsure about what product is best for your pet, ask your primary care veterinarian. He or she is there to work with you toward many happy and healthy years with your pet, and is the best source for knowledge. If your pet has unique needs, your vet will research to make sure you have chosen the right product.

Don’t trust anecdotal evidence that you see online or hear from other pet owners. While it is possible they experienced side effects from one or more of these products, this doesn’t take into account correct product use. Furthermore, it is unlikely that a severe reaction in their pet will ever happen to yours, even using the same product. The risk is there, but it is extremely low.

Whew that’s a lot! The short version is: most prescription products are not toxic to the overwhelming majority of pets when used appropriately as prescribed by a veterinarian. Small side effects can occur and are treatable, and can be circumvented by changing the product used for your pet’s next dose. You will likely be satisfied with any combination of flea/tick/heartworm prevention protocol you choose.