Some of you may have personal experience with the once-popular canine heartworm preventative, ProHeart 6. It was marketed from June 2001 to September 2004, until the FDA recalled it after thousands of adverse reaction reports crossed its desks. And now, almost four years later, it's making a comeback (that's the plan, anyway).

According to the FDA, 5,552 ProHeart 6 reactions were reported after dogs received this every-six-month injection. 500 dogs died, though definitive causality in these deaths was not established.

Fort Dodge, the manufacturer, claims a total of 18 million doses were sold during this period, implying that the risk of reaction is one per every 3,242 injections.

I saw two of them. One was not terrible, just some mild muscle pain and a low-grade fever for two days. The other was comparably horrendous, LOTS of pain and a high fever for three to four days. Fluids and pain relievers treated both cases successfully. I’m not so sure we reported the first reaction (it was so mild and he wasn’t my patient) but I’m sure we reported the second one.

The generic name for ProHeart 6 is moxidectin. Fort Dodge stands by it. It’s been used in Europe and Australia for over a decade with relatively few complaints. The Aussies even use it in its ProHeart 12 form as a once-a-year heartworm preventative.  Beats heartworms, they say.

Here in the US, about 250,000 of our dogs are infected with heartworms every year. That’s in spite of the widespread use of heartworm preventatives.

Why? Because some of us refuse to seek preventative medical attention for our pets. And the rest of us who manage a heartworm-positive status for our pooches? We simply forget to give the meds as prescribed.

Fort Dodge says it seeks to cut into the latter group’s prevalence with an every-six-month approach. And it’s true, client noncompliance  is probably the biggest source of heartworm disease in this country these days.

Some vets like it, too, since it requires that owners come in and see the vet twice yearly (though most of us don’t charge for an exam at the same time).

But I’m largely unmoved. My position with respect to ProHeart 6 stands: The drug has its place. But it’s a very limited one in my practice.


1-Noncompliance is a major source of heartworm disease but by far the largest subset of this group includes those who refuse any preventatives at all.

2-Noncompliance in my area (where year-round heartworm meds are a must) means owners don’t come back in a timely fashion for their ProHeart 6 shot. (That’s what happened when we carried it, despite our reminders.)

3-The risk/reward axis is mighty high with ProHeart 6 relative to alternative means of heartworm prevention.

Then why would any vet use ProHeart 6?

Plenty won't. Yet despite my reservations, I likely will. That’s because some of my clients will swear there’s no other way they’ll consider using heartworm preventatives (yes, I swear I have clients like this). And, remember,  I live in Florida.

Fort Dodge is making sure any of us who plan to use this product at least understand their basic rules. Veterinarians can only access ProHeart 6 through a limited distribution approach. An online seminar must be completed before a veterinarian can place an order.

And that makes sense to me. Though I seriously doubt that any reactions were caused by veterinarians administering an injection incorrectly, as the company infers may have been the case in the past, setting a bar (even a low one like this) helps ensure we vets know what we’re in for when we use it.