bravecto flea treatment side effects

Even Pets Have Adverse Drug Reactions: Serious Problems With Popular Flea Treatment

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A controversial flea treatment is getting a lot of bad publicity online – I just wish we knew that before we gave it to our dog.

In July my dogs were itching like mad, but my family were hesitant about giving them chemical flea treatments. Since we all felt bad about how much they were itching, we decided it was best to do something about it.

At the vet’s office, they couldn’t recommend the relativity new drug, Bravecto highly enough. Released in 2014, this drug kills fleas and ticks which can cause a range of health issues in both animals and humans. Due to its extremely long half-life it also stays in high enough concentrations in an animal’s blood to continue to do so for up to three months, according to Merck, the pharmaceutical company who manufactures Bravecto.

The name Merck will be familiar to many readers. They manufacture the controversial HPV vaccine Gardasil, which has allegedly caused many severe reactions in young women and girls. The drug has been implicated as a cause of severe neurological symptoms, chronic fatigue and even death. Merck also manufactures the Male Pattern Baldness/Benign Prostatic Hyperplasia drug Propecia, which has a list of claims made against it from patients who say the drug causes long term sexual pain and severe sexual dysfunction that continues long after usage is stopped.

It seems the controversy that Merck is shrouded in also extends into the veterinary world.

Two days after our three dogs were given Bravecto, one of them – our nine year old Border Collie cross, Willow – started exhibiting some unusual symptoms. At first she seemed nervous and afraid, then she began to appear very hesitant to walk up the stairs. By the next day she couldn’t properly control her back legs, which were now very weak. By that afternoon she was very lethargic and unresponsive – she seemed worryingly unwell.

After a quick google search I found two Facebook pages, Is Bravecto Safe?‘ and Does Bravecto Kill Dogs?. These pages are filled with people who say Bravecto harmed their dogs, including stories about dogs that exhibit ataxia (limb weakness and incoordination), often affecting the hind legs primarily. Since this is exactly what my dog was displaying, it seemed quite plausible that Bravecto was the cause of her suffering.

As well as ataxia, people on these pages also complain that Bravecto causes skin diseases, autoimmune disorders, seizures, vomiting, bloody diarrhea, blindness – even organ failure and death. Sadly, there are many stories of dogs that have died after administration of Bravecto and owners believe the drug is to blame.

When we took our dog to the vet they were quite skeptical about the possibility that Bravecto had harmed her, but they agreed she was ataxic and reported the possible adverse reaction to Merck nonetheless. We also reported the reaction to the Department for Environment, Food and Rural Affairs (DEFRA).

Days after our first visit to the vet, my mum spoke to the head vet at the practice. He informed her that he didn’t think Bravecto had harmed our dog and that the ataxia she was still exhibiting was likely a result of something else altogether – an underlying condition or perhaps a brain tumor. He felt that with the information available to him, Bravecto was one of the safest flea treatments he’d ever prescribed. He also said that reports online from consumers were merely anecdotal and didn’t accurately represent side effects caused by Bravecto.

After doing some reading I found a report from The Federal Office of Consumer Protection and Food Safety (BVL) in Germany stating that “adverse drug reactions to Bravecto® have been reported since introduction to the market, which partly lead to severe neurological symptoms, e.g. trembling, ataxia, seizures, epilepsy.”

Why was my vet unaware of this? At what point do you stop dismissing an abundance of anecdotes about a drug causing very specific neurological side effects? Why would a healthcare professional resist the blatant possibility that a drug they’ve prescribed could do harm?

In part, these questions can be answered by the actions of Merck themselves.

In light of the bad publicity Bravecto has received, Merck has set up a website called ‘Bravecto Facts’ that aims to ‘set the record straight and correct social media myths’ about Bravecto. On this site Merck claim that they care about the health of our animals and that there’s ‘no question’ in their ‘confidence in the safety […] of Bravecto’.

It seems that investigation from regulatory bodies such as the German BVL isn’t enough to make Merck question the safety of Bravecto – but could it be that Merck are resisting criticism of Bravecto despite knowing that the drug is not as safe as they insist it is?

It would seem so. Several people have contacted me personally to tell me that Merck has offered to pay towards vet bills incurred after possible adverse reactions to Bravecto, but only after signing a waiver that says they won’t pursue legal action against Merck in the future. This doesn’t inspire confidence in Merck’s claims that Bravecto is a safe flea treatment – it speaks of a company who are aware that their drug may cause enough damage to incur major legal costs in the future, but are doing their best to avoid that possibility.

Many, including myself, would argue that Merck should be held accountable for the damage we believe Bravecto has inflicted upon our pets – time will tell whether they will be.

Thankfully, although my dog is still somewhat ataxic, she has improved a lot in the last three weeks. Unfortunately, the same can’t be said for many dogs whose owners suspect adverse reactions to Brevecto.

If you suspect your dog has had an adverse reaction to any drug, please report it to your vet and the appropriate governing body in your country.

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Photo by Alvan Nee on Unsplash.

This article was published originally on August 16, 2017. 


  1. Have a soon to be 2 GSP diagnosed with autoimmune disease. Gave her Bravecto 2x in her short life. Wondering now if I harmed her. She probably won’t live to be 3. Very sad!

  2. About 2 years ago our vet prescribed Bravecto for our then 10 year old Jack Russell.
    Not sure how long after that but she developed a tremor in her front legs and stopped coming down stairs and steps.
    A different vet diagnosed a non-specific degenerating neurological condition due to her age. We didn’t associate Bravecto with this – therefore did not mention it to the vet.
    She continues to have the tremor and now occasionally stumbles as her right front leg gives way.
    It is very upsetting to think we may have inadvertantly caused this condition.
    We will inform the original vet about this association with Bravecto

  3. I’ve been using Bravecto as a Breeder for several years since it’s beginning
    I have never had a problem with any of my dogs due to it’s use and my dogs have been flea and tick free
    Total of 7 dogs that have had this given twice a yesr

    • Personally, I suggest you stop using bravecto in light of the information in this article, from countless pet owners and the fact the authorities are investigating this drug. My dog has had issues with her back legs ever since being given bravecto all those months ago. Not having had any problems yet does not mean that you won’t have any in the future and it doesn’t mean that other pet owners won’t either.

      • I know this is as written in 2018 but I’m hopeful for a response. I have my dog Bravecto 11 weeks ago, 2 days after administering the flea medication my dog stopped using his back legs. I nursed him back to health, now less than 2 months later it has happened again. He has to be walked with a support harness and a towel under him to support his back legs as he drags them.

        I read that Bravecto stays in a dogs system for 12 weeks. Did your dogs get better after the time period the Bravecto is good for? In other words, next week makes 12 weeks since I gave my dog this horrible medication, could I see a major improvement?

  4. Thank you Ben for bringing this serious issue to the attention of the public. Regards Carol
    Visit my FB page Bravecto Nexgard Comfortis Simparica Trifexis & others Do They Kill Dogs? I am trying to share information on these drugs and how to detox your dogs and keep them flea and tick free naturally.

    • Many Border collies (among other dog’s breeds) have the MDR1 defective gen mutation, and are thus highly sensitive to some antiparasitic drugs. Check it online. That is probably your dogs case.

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