Advantage II Flea Treatment – Neurological Side Effect

Bayer’s Advantage® II flea treatment for cats contains unlisted inert ingredients that may cause permanent neurological damage to your cat or dog (vetinfo).

Advantage® II for cats contains an active ingredient which acts on the nervous system of the flea. It is called: midacloprid, an insecticide. It is interesting to me that it attacks the flea’s nervous system.

This chimes with at least two comments that I have noticed on this product. One is on this website and the other on Amazon. Here are the comments:

The first couple of days after applying this product to my cats it seemed to be working great. Especially for my cat who seems to be very sensitive to fleas. However today I watched one of them fall off a fairly tall piece of furniture twice (something he is on every day and has never fallen off of before) and strangely like he went limp or something. Then a little while later I watched as my other cat who was just standing on the floor fell with her back legs slamming together and her front paws trying to get a grip. Then I read on a few different websites that it has been found that in RARE occasions this product can have neurological effects….(Amazon).

My 4 year old cat cocoa is an indoor cat but she gets out sometimes i recently changed her liter to all natural liter and gave her a dose of small cat advantage 2 which was dose number two of three but instead of every month i give her every couple months never noticed her being allergic to it but after letting her inside one day the day after i noticed she was hiding out on top of a bin i lifted her up and she waslimp for the next couple of days she wouldnt eat or get up when she did it took a lot of energy i noticed a line of fur missing no wound and nowher belly near her nipples is reddish brown i have an appt in a couple days but it is killing me not knowning please help…(this website).

Each cat caretaker needs to check this out themselves but it appears that the insecticide can also attack the nervous system of the cat. The website states in reference to its use on dogs:

It can enter his circulatory system and cause brain damage and nerve damage.

That statement supports the comments of the two users quoted on this page. Although, the website states it is one of the safest treatments for fleas on dogs.

Now, as far as I am concerned if a potential, albeit rare, side effect is this serious, it is unusable. It is about risk but it is also about the upside and downside of using drugs like these and insecticides are dangerous chemicals. 

The vetinfo website adds an important extra bit of information about this drug. They say some of the inert ingredients in the product are not tested for safety because they don’t have to be under the law. Also, even if these inert ingredients are toxic they don’t have to list them. I’ll quote the site verbatim for the punch line:

Such undisclosed ingredients might be unsafe for use on your pet and the neurological damage caused by them may be permanent.

Has anyone more knowledge about this flea treatment? When I see “neurological damage” and “permanent” in one sentence as a possible side effect it puts me off buying it. Does it do the same thing to you?

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Michael Broad

Hi, I'm a 71-year-old retired solicitor (attorney in the US). Before qualifying I worked in many jobs including professional photography. I have a girlfriend, Michelle. I love nature, cats and all animals. I am concerned about their welfare.

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  • Used advantage 2 on my cat (trusted the vet). He has lost half of his fur and has severe neurological problems. He can barely walk or stand. Will this improve? It has been about 6 weeks now. If not I will probably have to put him down. There is no warning about this on the box. Are there any class action suits out there against Bayer? I'm mad as hell

    • I used this on my cat yesterday and today he's been showing neurological signs like twitching and jerking and his fur is also falling out. He seems to be unsteady in his gait and I'm so worried about him. I've never seen this reaction before as I've used it on my other cats with no problems. Is your kitty any better?? Why don't they list this as a potential side effect??? I'm also mad as hell!

    • Thanks for commenting. A vet will advise on recovery. I'd have thought your cat would recover at least substantially but I am speculating. I don't believe there is a class action. There should be. Good luck to you both.

  • It can certainly cause neurological damage. One of my cats started having balance problems as a result. Luckily she's been improving with time.

  • Otherwise healthy cat now has front paw muscle tremors when laying down and stretching out his arms, I know due to Advantage II application. Now I'm fearing that it'll get worse. I'm never putting it on him again. Has anyone experienced the same, did it get better, did it get worse does it stay the same? Hes 4 yrs old. If a cat licks this he ingests it then it gets into his bloodstream and into a central nervous system. The cat's central nervous system and an insect central nervous system is different but still it's a pesticide anyway you cut it!!

    • Thanks for sharing Carolyn. So sad though. These flea treatments are potentially very dangerous.

  • My cat had a seizure within 24 hours of applying Advantage II. He did not lick any part of it and it did not run down any part of his body after applying. He had a grand mal seizure every 4 days even after starting phenobarbital. Cost me a few thousand dollars in the specialty hospital. He never had any neurological symptoms of any kind before using this. Once the thirty days of the med was out of his system he never had another seizure. There is no doubt at all in my mind that this flea medication is what caused his seizures. I just thank God it didn't seem to cause any permanent problems.

  • My cat loves to jump off everything, so she has hurt her paw in the past. I just took her to the vet because all of a sudden her gate was different. I know she jumped pretty hard down off a stool onto the vinyl floor, so I took her to the vet yesterday. The vet said she is acting like a neurological or nervous system issue. I couldn't think of anything, other than perhaps she hurt her back jumping. But now, I am wondering if the ADVANTAGE II I just administered a week ago has poisoned her. I did catch her scratch at the area I placed it with her paw and then was licking her paw. They are running several blood tests to try and determine if a viral or bacteria attacking her system. I will mention this to the vet. Does anyone know if a blood test would reveal if affected by the ADVANTAGE? My heart is just breaking to see my feisty active baby girl now walk a few steps and then lay down. I can't afford an MRI and spinal tap.

    • Hi, were you able to figure out if Advantage II was the cause of your pets health issues? The exact same series of events has happened with my otherwise completely healthy, active cat, and feisty cat. Its been almost a month of symptoms (weakness in hind legs, loss of coordination in hind legs, strange gait in back legs). Took him to the vet and his bloodwork/x-rays all came back normal. Now they are telling me to get an MRI and spinal, but it is thousands of dollars. Hadn't occurred to me that it could be Advantage II, but I did give it to him for the first time about a week before symptoms started. Thanks in advance for any info or advice.

    • I presume you have stopped the Advantage treatment. I can't really help - only hope that the cause is not Advantage. I think a good vet should be able to test for nerve damage without an MRI or spinal tap. The very best of luck.

  • I wanted to add my brief experience with Activyl, quite startling. I read the side effects info on the flea med before applying it to my cat. It listed 3 prominent side effects that may occur, and I believe it said to discontinue use if they did.

    all i did was open the vial of Activyl with my cat 5' away from me and without me getting any closer than that and within 5 seconds she exhibited all 3 major listed side effects just from the air transmission of the energies or smell of the product.

    This actually shocked me, but it did happen and it 1) showed how substances can affect living creatures through via energetic or other properties 2) that the side effects listed were very accurate and indeed, the top 3 to look for, and 3) (most important) my cat was highly sensitive to this product and so I should never give it to her.

    However it may have some value in repelling fleas without being put on the cat, who I value way too much to subject to such an experiment.

    Also, I will never forget the limited reviews of Revolution I saw on a website. Most spoke of serious reactions and ill effects to their animal. Some liked it, said it was effective. One, most memorable said that it was very effective and fast acting against fleas, but also "killed the cat".

    Uh! A little too effective! What about essential oil and EMF treatments? Any input about those, guys and gals?

    • Thank you, Chris, for passing on your experiences. Your comment is very interesting for the fact that your cat reacted so dramatically so early on even before the treatment. She must have reacted to the vapour, in minute quantities, that emanated from the vial. This tells us how potent this poisonous insecticide is. Personally, I hate flea treatments and avoid them as best I can. They are ultimately poisons. Every day I flea comb my cat to make sure that he is free of fleas. If he has a flea I kill it with my own hands. Fortunately at the moment I don't have a flea problem and intend for it to remain that way.

  • I gave my 7 year old 3.8 lb yorkie Advantage Multi on Sunday on Monday she could not blink her eye. Her lids and mouth on left side were swollen. It has been 3 weeks with 3 trips to vet and 2 trips to eye doctor. She still can't blink. The vets laugh at me because they don't believe the Multi caused this. I am furious.

    • My God, what terribly bad luck. I feel for you and your Yorkie. Vets can be bloody infuriating. I wish you both the very best of luck. Thanks for sharing.

  • I just now put Advantage on my cat and then found this article by accident. Comforting.

    Up until now I applied diatomaceous earth to her every few days and clipped her back claws to limit the damage she can do to herself (fun!). It seemed to help a little, and I couldn't find any more fleas, but she still acts like she has them, so I resorted to Advantage. She's lost maybe 20% of her fur and she's miserable all the time. I've never let her outside once in seven years.

    The only consolation I can find is the knowledge that I really shouldn't have kids. This is heartbreaking enough with a cat, so I can't imagine how bad it would be to watch my hypothetical kid suffer from some ailment with no reliable treatment options.

    I'm flat broke and can't afford a vet. Here's hoping she doesn't die. Poor little Lois.

  • I, too, can empathize with all pet owners who've witnessed the devastating reactions that Bayer's pet products can elicit. Five years ago, after applying Advantix, I watched in horror as my dog, raced around in a paranoidal state, eyes glazed over, jumping, running into walls, crying from pain, panting uncontrollably then dragging his hind legs as they were paralyzed. If I had not gotten him to the Pet Emergency Hospital in time, he would have died. Two days spent there, and thousands of dollars later, they saved him and the "temporary paralysis" resolved. Praise The Lord! Two points I'd like to make. Just because your pet does not have a negative reaction after the initial dose does NOT mean you're safe. Second/third doses may cause a very nasty reaction. These drugs are filled with dangerous, poisonous pesticides. Calls to Bayer were a joke - they denied ever having had any complaints regarding reactions to their products. All you have to do is go on line and read for yourself what Bayer products have done to pets now, in the past, and will do in the future, including death, if someone does not get these "pesticides" off the shelves! My vet's office refuses to use or suggest using any of Bayer's products. Out of good conscience, they have treated enough pets in the past to understand how dangerous the reactions to these meds can be. They also can't stand to see their beloved "clients" suffering in so much debilitating pain.

    • Great comment. Thank you for visiting and sharing your experience. I am pleased you saved your dog. Must have been hell.

      • Michael, it was hell! However, I believe everything happens for a reason, if sharing my experience warns/helps just ONE pet owner from losing their precious baby, then it was all worth it! God is good!

        • Well, you have warned some people. I am sure there will be some dogs and cats who will benefit.

  • Hi, thankfully none of my cats had that stuff, but i do wander esp about Cassy as that's why she died of Neurology, but i think she died more of old age plus bladder infection. I would definitly not take that stuff. I mostly use Revolution that attacks worm and fleas.

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