Nexgard Side Effects: How Long Does The Effects Last and Stay in Dog’s System?

chewable medicine
(Last Updated On: July 28, 2023)

Fleas and ticks can be a common problem with pets. In addition to the obvious inconvenience, they can also transmit deadly diseases to your little pet and even yourself. Therefore it comes as no surprise that there are many options available in today’s market, all in the name of keeping fleas and ticks at bay.

In this article, we will have a look at NexGard for dog side effects. It’s an FDA-approved product designed as a beef-flavored digestible chew. If your pet is little, or if it likes making its own decisions, you’ve probably struggled with the spot-on treatment.

Whether it’s Nexgard side effects like itching, seizures, lethargy, tremors or impacts on urination, keep reading to know more about side effects of nexgard.


What is NexGard Usually Used For?

NexGard is actually designed solely for dogs, claiming to have the ability to kill adult fleas before they lay eggs. It’s essentially a preventative measure. In addition, another thing worth mentioning is that the treatment also kills ticks, all in its chewable FDA-approved form.

According to NexGard, their product is safe for puppies as young as 8 weeks old, weighing 4 pounds or even more.

But how does it actually work? According to the official website, NexGard is made by Merial, the creators of the widely-known Frontline. Unlike topical treatments, this medication contains an afoxolaner, otherwise known as an insecticide and acaricide which is absorbed into your dog’s bloodstream.

If these words confuse you, you’re not alone. The bottom line is that the product both prevents and treats flea infestation and ticks.

If you’re anything like me, though, you’re probably already frowning whilst trying to understand if this product is safe or not. Most owners want to be fully aware of what their pets are eating, drinking and playing with, so let’s take a closer look at what NexGard is all about.

Nexgard Side Effects in Puppies and Dogs

While Nexgard side effects are quite rare, there are some cases where they actually do occur. If you’re planning on looking at the company’s official website, you will clearly see that the reported side effects of nexgard are listed multiple times within a page.

Can nexgard cause diarrhea in dogs?

These NexGard adverse reactions include:

  • Diarrhea,
  • Irritation
  • Drowsiness
  • Vomiting
  • Lack of appetite.

And just to make matters worse, the effect of the treatment in female dogs which are breeding or lactating has not yet been researched.

Needless to say, you should read all labels before buying any new product. Also, the best advice I can give you is to consult with your veterinarian before administering any kind of pet treatment, regardless of brand!

Allergies to Nexgard

So let’s go back to those potential NexGard problems. If you’ve already administered the medication to your dog and notice any allergic reactions or side effects (e.g. vomiting, lethargy), your dog may be suffering from NexGard Allergies; these occur when the active ingredient we’ve mentioned before –afoxolaner- is not suitable for your dog, despite it being considered safe.

But did you know that symptoms can vary from dog to dog? This means that it’s absolutely crucial to be informed about what products cause what adverse reactions, giving you enough time to spot the changes and call your vet.

You are the best judge of your dog’s usual behavior, so you’ll definitely be able to spot any differences in behavior. Trust your instincts!


If your dog begins to show signs of a reaction after giving them Nexgard, it is important to immediately stop giving it to them. Take your dog to the veterinarian, and bring the packaging of the flea and tick medication with you. Your veterinarian will take the necessary steps to help your dog, but they need to have the packaging of the medication to determine what may be causing the reaction in your dog.

It may take some time before your dog feels well again, but be sure not to give them the Nexgard if it is what caused their reaction. Your veterinarian may recommend another method to help prevents ticks and fleas.


The best treatment for the adverse effects Nexgard may have on your dog is to stop administering the product to them. There are no immediate treatments that can help get the substance out of your dog’s system, so it may take some time for the effects of Nexgard to finally wear off.

There are some treatment methods that can help alleviate some of the symptoms your dog is experiencing, but they are not meant to be complete solutions to the issues your dog may be facing as a result of Nexgard.

Two treatments that can help are topical medications that can help alleviate any skin irritation that may be a reaction to Nexgard and alternative preventatives that can help keep your dog free of parasites, like fleas and ticks. This second one is a method you should seek if your dog has had an adverse reaction to Nexgard.


Make sure to monitor your dog’s condition after you stop giving them Nexgard or any flea/tick medication that contains similar ingredients. You want to seek out other preventative measures to ensure your dog is not plagued by fleas and ticks, but you want to find protection that does not have ingredients that are as bad.

It is important to still protect your dog from fleas and ticks as they can carry severe diseases, such as Lyme disease.


What NexGard Says

While Nexgard promotes its use to prevent ticks and fleas, they recognize that the product has side effects on the dogs it is used on. This recognition of side effects is done on the package insert for Nexgard.

This insert includes two sections of the different side effects the product may have on your dog, but many owners find that Nexgard has placed more tame side effects at the top of the list, such as vomiting, while more severe side effects are located lower on the list, such as seizures.

The insert primarily focuses on the vomiting side effect when this is just your dog’s natural defense system purging the substance from its body. They then encourage the continued use of the product after this reaction, but it is better to listen to what your dog’s body is saying rather than the packaging.

What The Studies Say

Nexgard chewables were found to be safe for most dogs as long as they are 8 weeks of age or older. It is recommended, however, to avoid giving Nexgard to your dog if your dog has a history of seizures. According to the Food and Drug Administration, the benefits of Nexgard outweigh the negative side effects. Many studies observed no adverse reactions during their duration.

Independent Studies Reveal More

Many independent studies have looked into the effects isoxazoline-based flea, and tick chewables have on dogs. The most recent of these studies was published in 2020. 

Through an examination of a survey done by veterinarians, pet owners, and others who care for pets, many adverse side effects of Nexgard were reported. These effects included vomiting, depression, diarrhea, seizures, and death. The final two discussed here are side effects that are not given much significance on the insert in Nexgard packaging. Some owners lost their beloved pets due to this product, and it is interesting to see seizures and death reported more from owners who use Nexgard than reported from Nexgard themselves.

How Fast Does NexGard Work?

In terms of how long it takes for the treatment to actually work, NexGard acts pretty fast, killing 100% of fleas within 24 hours. Keeping the chemistry lesson to a minimum, I’ll only tell you that the medication works by over-stimulating the nervous system of both ticks and fleas.

You may still see your little furry friend itching or scratching after 24 hours. That’s usually just a habitual reaction which will stop after 2 to 3 days.

A great thing which may put owners at ease is that it can be taken alongside other treatments such as heartworm medication.

Don’t forget though: just like with any other flea treatment, different strengths are required for different sized dogs. If you think that might pose a problem for you, your vet will make sure that the right dosage will be administered for your dog. If applied correctly, it will provide 30 days of effective protection against both fleas and ticks!

NexGard Vs Frontline

Most of you might not have been aware of NexGard until now. But you have probably at least heard of Frontline. It may then surprise you to know that both products share the same maker. But what are the obvious differences between the two?

Main distinction is that NexGard is a chewable version of the spot-on treatment, and it kills 100% of fleas within 24 hours, as opposed to the 2-4 hours advertised by Frontline.

Moreover, Frontline has been tested in lactating female dogs and is considered safe to use, whereas NexGard still hasn’t answered that question.

In comparison, some of you might have problems with the topic treatment as you won’t be able to wash your dog 2-3 days before and after applying the medication. The last notable difference is that the oral medication actually requires a prescription from the doctor, so it’s not widely available like Frontline. However, this may actually be a blessing in disguise, as you won’t have to worry about applying the wrong dosage. You win some, you lose some, right?


To sum up, fleas and ticks can be a serious problem if not controlled. However we are quite lucky to be nowadays able to choose from a variety of solutions including oral and topical treatments.

If in the past you struggled with applying spot-on remedies, you may want to opt for the chewable version of the medication.

Just like with any other product, it’s important to be aware of the side effects of nexgard, as we’ve seen with the potential NexGard problems. Whatever treatment you choose, just remember that your veterinarian will surely lend you a helping hand in case you’re confused about which way to go. 7 Flea Collars To Buy


  • Johnny Earwood says:

    Well I gave it to all 3 of my dogs everything was going good up till the 13th that’s when the 1st dog started to have a dry cough then about 4 days later another one then a few days after that yep the 3rd one started it too.Any idea’s.

    • Jennifer says:

      Interesting. I have come to realize that one of my dogs develops a dry cough after I give her nexgard too. Lasts about 10 days and then goes away. I’m going to switch to something else.

  • Ken Mault says:

    How long do the side effects last in dogs who are having a reaction!

  • Jeanne-Marie Richards says:

    My healthy seven month old puppy was given Nexgard by my vet last Wednesday. She suffered liquid stools for two days, lethargy, and then on Saturday her third eye lid got stuck. She has been in hospital since Saturday and I am beside myself with worry. The hospital vet says her lymph glands are up. Tilly (my pup) is having a CT scan this morning, her eye is no better. Is this due to Nexgard? Will she get better? Has anyone else reported similar problems? My poor baby.

  • Martha Reyes says:

    My dog could not get comfortable. He kept moving and getting up at night.

  • Lillian Mccomber says:

    I started my cocker on nexgaurd last year around Feb., and about two months later she started having eye problems. She is now blind in her right eye and both of her eyes have puss that I have to remove daily and has severe dry eye and had a God awful infection that almost the completely ate through her nose and eye area. I have spent close to two thousand dollars trying to fix this. I think she got the dry eye disease from the nexgaurd and then she scratched her right because of the dryness of the eye and poked her eye. I am going to stop the nexgaurd for a couple of months and see what happens. She was on frontline before with no problems

  • Shirl S says:

    My Golden is experiencing petite mall seizures. They were increasing in frequency, until about a month ago (2 months after discontinuing due to winter months). They have almost disappeared. So I have to think it’s from the Nexgard.

  • Dawn says:

    I gave nexguard to 4 of my chichis and think it’s fair to say they are allergic to the nexguard. All 4 chichuauas have a terrible dry cough the 1st month lasted 4 days. 2nd pill yesterday july 4th and all day with cough. I called my vet and she also advised to stop. Great this med is expensive thank God it was just a cough and nothing else. Going back to natural products. Sad in Connecticut

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