How Safe are the Newest Flea Preventatives?


What do you think of Comfortis? I just started
giving it to my Labs and wondered about the safety
of it. It is doing a good job on the fleas.

Tulsa, OK

Two popular new flea preventatives have come onto the market recently. One, Comfortis, is an oral pill taken once monthly available only for dogs. Unlike topical spot-on flea preventatives, Comfortis does not force dogs to endure a greasy patch of skin each month. Also, a small number of dogs have local skin reactions after topical treatments are applied. Comfortis avoids this problem as well.

The safety profile of Comfortis appears to be good. However, I don’t have much personal experience with the medicine. Most of my patients do fine with Frontline, Advantage, or Revolution so I have not had much interest in rocking the boat. However, I am happy to have another option for future use.

The other popular new flea preventative is ProMeris. Promeris is a topical product that kills fleas and ticks and is available for both dogs and cats. It was released with great fanfare to the veterinary community. However, like Comfortis, it has yet to become as widely used as the more established flea and tick medicines.

Shortly after the release of ProMeris, I received an urgent e-mail from a client. It had been forwarded around the world by dozens of people, but my client assured me that the source was a friend of a friend who was absolutely reliable. The e-mail claimed that ProMeris is very dangerous and may cause horrible side effects in dogs and people.

Having learned a while back that sometimes one encounters false information on the internet, I did what any scientifically-inclined individual would do under such circumstances. I looked up ProMeris on

The result was that the claims of severe reactions can be neither proved nor disproved. From the snopes page:

Just about any product used with animals, no matter how generally safe, can produce adverse reactions in some cases due to misuse . . . or unusual sensitivity to one or more ingredients in individual animals.

I couldn’t have said it better myself. The general safety profile of ProMeris appears to be good, and I have yet to personally encounter any animals who have had severe adverse reactions to it.

Although I do not yet have extensive experience with Comfortis or ProMeris, I most certainly have a great deal of experience with fleas. Fleas are vile, bloodsucking, disease-spreading, misery-causing pests. Have I mentioned that fleas once killed a third of the people living in Europe? Fleas spread the Bubonic plague.

So, my assessment of the newest flea preventatives is this: they almost certainly are safer and generally more healthy for pets than fleas.

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