Side Effects May Occur

and loving dog. Starting November 8


Concerning your article, “Reflections on Heartworm,” WDJ August 2002:

Last summer in July, we adopted Casey, an 11-month old, 13-inch Beagle. All was well until we gave her the first monthly Heartgard medication. Casey was fine for about 24 hours, after which she developed diarrhea and became lethargic. We took her to our veterinarian, who told us that this incident was just coincidental, and that the medication couldn’t possibly be the cause of the reaction. He gave her an antibiotic and Casey recovered over a period of about a week.

A year later, just about a month ago, we debated about whether we should give her another Heartgard. Foolishly we opted to do so, trusting our veterinarian’s judgment that the medication was necessary. We got the same reaction, only more severe! We are certain that Casey survived only because she is young and strong. Now we know she should never have this medication again, and we are looking for a new veterinarian.

-Luke and Mariana Thompson
Coral Springs, FL


My beloved pet of two years is a Miniature Schnauzer named Jack. I almost lost him. We went to the vet for his yearly booster about four weeks ago and the vet suggested I get the ProHeart 6 injection for Jack instead of the monthly chewable since I forgot to give him a dose.

Exactly two hours later, Jack began throwing up, itching uncontrollably, and trying to escape from the house. I called the vet at 7:25 p.m. (five minutes before the office was supposed to close) and after consulting the vet, the nurse told me that it couldn’t be a shot reaction because they gave him a shot of Benadryl. She told me to wrap him in a towel and call them in the morning.

Five minutes later, the vomiting was more violent. I called the office back and their answering machine was on. I rushed him to the emergency vet and they gave him fluids and another shot of Benadryl. Jack was depressed and avoided me for about a week. The emergency vet told me, “Don’t be quick to blame the ProHeart injection for your dog’s reaction. However, I have seen several other cases just like Jack’s and coincidentally, they all had just received the ProHeart 6.”

I realize that heartworm prevention is better than getting heartworm, but I will give him the monthly chewable and only during mosquito season.

-Paige Michalski
via e-mail


I gave a Heartgard chewable to my four-year-old Maltese, Pookie, on November 1, 2001. He had just had his annual physical and was healthy, a very playful, energetic, and loving dog. Starting November 8, he slowly became lethargic, didn’t want to play, had one episode of diarrhea, didn’t want to walk, and stopped eating. On November 12, he lost his balance, had two seizures and he died that evening at an emergency veterinary clinic, going into cardiac arrest while having a blood transfusion and while I was holding him. His platelet count was very low and the diagnosis was immune mediated thrombocytopenia.

This has been such a terrible loss and experience, and I still can’t believe that my healthy sweet little dog died. There were no warnings on the package, like death or serious illness being a possible side effect.

-Barbara Marsden
Pasadena, CA


I am so sorry to hear accounts like these. We would never advocate that the preventives for heartworm should not be used; clearly, they have their uses.

However, we’d like to see dog owners and veterinarians regard them with more caution. Many vets are unconvinced that the medications that they use so frequently without problems can cause some dogs’ illness, and by discounting this possible link, precious time is wasted that could be spent treating the animal for poisoning.

We’ve said this in a number of articles about the potential dangers of using toxic pesticides on dogs; serious illness or death is a possible side effect of all of them. Many dogs tolerate the use of these products without problems; some probably experience mild side effects that are never associated with the pesticides; and a few do suffer serious illness.

So, even if your veterinarian doesn’t do it, it is vitally important that YOU report your loss to the FDA’s Center for Veterinary Medicine. They maintain a database of “adverse drug experiences” suffered by animals. There is a form you can fill out online ( or you can call (888) FDA-VETS. Both reporting systems are confidential.

While the Adverse Drug Experience reports don’t “prove” links between a health problem and a drug, they can help highlight areas where further study or caution is needed. –Editor


As a former user of black walnut, I feel compelled to share with you what I learned from my holistic veterinarian. He informed me that black walnut can have very astringent effects on the lining of the digestive tract, and that over a period of time can actually harden the gut, inhibiting the nutrient transfer process. Black walnut contains a high level of tannins, which can ultimately interfere with absorption of vital nutrients. He discouraged the long-term use of this herb extract as a preventative for heartworm, instead encouraging immune system support and proper nutrition. He is also an advocate of a topical mosquito repellent, Buzz Away (made by Quantum, or 800-448-1448). This is a combination of cedarwood, eucalyptus, lemongrass, and peppermint oils, and is available in a pump spray and towelette form.

-Ann Schmidt
Nantucket, MA


I enjoyed “Living with a Difficult Dog” (September 2002) and the accompanying sidebar that encouraged owners to rule out physical causes – an often overlooked cause of aggression.

At four years old, our female Shar-Pei/mix started being aggressive. She lunged at someone who petted her, and a few weeks later bit a small boy who was running by. We took her to our trainer for a refresher course, but after two more incidents, we thought we might have to put her down.

While looking her over one night, I happened to check her mouth. She flinched and jumped as I ran my fingers around her gums. I got her to lay still while I peered inside. Imagine my horror when I found four broken molars with pink roots exposed, causing excruciating pain! An outstanding veterinary dentist told us that the hard knuckle bones Calypso loved to chew were the cause of her problems; she had ground her teeth down to the roots. Her problems were solved with several root canals and stainless steel crowns.

Five years later, Calypso is the sweetest, best-loved dog in the neighborhood. She eats raw turkey necks and chicken backs to keep her teeth in good condition, and at nine years old, her veterinarian always exclaims, “What great teeth, and what a sweetheart!”

-Donna Philburn
via e-mail


  1. Heartworm medicine will never be given to my dogs again—not the remaining doses this year or ever. It is killing my older collie. My dogs previously received Heartgard; which they were able to tolerate with few side effects. This year we were given Interceptor Plus. We didn’t notice much right away except for some lethargy and a lot of loose stools. About three weeks after the first dose, our older Collie began having seizures. He couldn’t walk, dragged his legs, walked into walls, paced in circles… After several vet visits and blood tests, nothing was found indicating anything wrong. Approximately 60 days after the first dose, we almost had our dog back, so we were told it was okay to give him the second dose. Less than three weeks later, the same symptoms came back with a vengeance. We now know that Collies are particularly suspectible to heartworm medicine—our other girl is 25 lbs. heavier than he is and only suffered the lethargy and loose stools. He is at the bottom of the weight range for the dose. We took him in today—we have one week to try to see if he comes out of what I would call poisoning. We are feeding and watering him by hand. Our vet says there is a 90% chance he will not come back. No more poison in this house!

  2. One month ago we lost our most precious and beloved dog Griffin. After reading many articles about the pro heart 6 I know this is what caused his death. However, I feel the vet caused it more than just the injection. She gave him 2 other injections at the same time. One was the Bordetella and the distemper/parvo/LEPTO. She never told me or gave me any paperwork that showed what adverse reactions the pro heart had caused, if I knew that I would NEVER of had her give it to him. As I have read further regarding this I also read that your dog or cat should NOT have any other injections at the same time. He was the best little buddy I could ever have. He became my support dog after an accident I had which injured my back. When I have a bad day it’s intensified because he’s not here to give me the support that I need. It’s still so hard, I just want my heart to stop hurting and feeling like it’s been torn out of my chest. It makes me sick to think that someone that is supposed to be loving, compassionate and give everything to keep your animals healthy may be the one responsible for the death of my dog. I rescued him about 10 years ago at the age of approximately 2. He was full of life. Last year, June 2020 he had a bladder stone removed and he was acting like something was wrong and my husband and I was worried he had another stone. I made him an appointment, took him in and when the vet called me she said, “he’s a little anxious” . I told her that we think he has a stone again. I had to ask her to do an x-ray and it was confirmed that he had 2 bladder stones. There is so much more to his final few months, but it would take too much to address. All I know is that not only should Pfizer take this PROHEART OFF THE MARKET AGAIN (the fda took it off a few years ago, but allowed it back if they followed the guidelines they set forth and the FDA allowed it back. You can find an article on the proheart or Pfizer website) but MY DOGS VET SHOULD AND WILL BE HELD ACCOUNTABLE IF FOUND THAT SHE OVERDOSED MY DOG. I CANNOT REPLACE HIM, BUT I CAN STAND UP FOR HIM AND ANY OTHER ANIMAL THAT HAS LOST THEIR LIVES DUE TO THE NEGLIGENCE OF THEIR VET. I’m not saying she is at fault, but I’m going to find out and if she is I will do anything and everything to see she looses her license. THIS SHOULD NEVER HAVE HAPPENED AND I HOPE IT HASN’T HAPPENED TO ANOTHER ANIMAL. ONE IS ONE TOO MANY. I MISS HIM SO MUCH AND MANY DAYS I DON’T KNOW HOW IM GOING TO MAKE IT ANOTHER DAY WITHOUT HIM. I LOVE YOU GRIFFIN ♥️


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