How Does NexGard Work?
QUICK ANSWER
NexGard is a chewable flea and tick preventative that contains afoxolaner, which enters your dog's bloodstream and kills fleas and ticks when they bite and feed. It kills fleas within 8 hours and ticks within 48 hours. It's given monthly and requires a prescription from your veterinarian.
NexGard is one of the most popular flea and tick preventatives on the market, but most owners give it to their dog every month without really knowing how it works. Here's the mechanism, the timeline, and what you should be aware of.
How does the active ingredient work?
NexGard's active ingredient is afoxolaner, which belongs to a class of drugs called isoxazolines. When your dog takes the chewable tablet, afoxolaner is absorbed into the bloodstream and distributed throughout the body tissues. When a flea or tick bites your dog and ingests blood containing afoxolaner, the compound overstimulates the parasite's nervous system, causing uncontrolled activity, paralysis, and death. It specifically targets receptors in the insect and arachnid nervous system that are structurally different from mammalian receptors, which is why it kills parasites but is considered safe for dogs at approved doses.
How fast does it work?
NexGard begins killing fleas within 4 hours of administration, with complete flea kill achieved within 8 hours. For ticks, the timeline is longer; most ticks are killed within 48 hours of attachment. This means the parasites do need to bite your dog first in order to be exposed to the drug. NexGard doesn't repel fleas and ticks; it kills them after they start feeding. This is an important distinction from topical repellents. A single dose provides protection for a full 30 days.
Is NexGard safe?
NexGard is FDA-approved and has been widely used since 2013. The most commonly reported side effects are mild and include vomiting, diarrhea, lethargy, and decreased appetite. The FDA has issued a class-wide warning for all isoxazoline products (NexGard, Bravecto, Simparica, Credelio) noting the potential for neurological adverse events including muscle tremors, ataxia, and seizures, particularly in dogs with a history of seizure disorders. If your dog has epilepsy or a history of seizures, discuss this with your vet before starting NexGard. For most healthy dogs, it's considered safe and effective.
Does my dog need a prescription?
Yes. NexGard is a prescription medication that must be obtained through a veterinarian. Your vet will confirm your dog's weight for proper dosing, check for any health conditions that might make the drug inappropriate, and often recommend a heartworm test before starting (since NexGard doesn't cover heartworm; a separate preventative is needed for that). It's available for dogs 8 weeks of age and older weighing at least 4 pounds.
NexGard works by making your dog's blood lethal to fleas and ticks that bite them. It's effective, convenient, and widely used, but it requires a vet prescription for good reason. Talk to your vet about whether it's the right fit for your dog's specific situation and health history.
More Dog How-to Questions
Mystery Question?
Mystery Question?
Mystery Question?