NexGard vs Simparica: What's the Difference?

NexGard vs Simparica: The Core Difference

NexGard (afoxolaner) and Simparica (sarolaner) are both monthly isoxazoline-class chewable flea and tick treatments for dogs. They're different drugs made by different manufacturers, but their mechanism of action is similar: they kill fleas and ticks systemically by acting on the parasite's nervous system. The practical differences are subtle but worth understanding before choosing.

Note: This article compares standard NexGard and standard Simparica. Both products also have expanded versions — NexGard Spectra and Simparica Trio — that add heartworm and intestinal worm coverage. If you want all-in-one protection, see our NexGard Spectra vs Simparica Trio comparison.

Active Ingredients

NexGard contains afoxolaner (made by Boehringer Ingelheim). Simparica contains sarolaner (made by Zoetis). Both are isoxazolines — a drug class that kills external parasites by overstimulating their nervous system, causing paralysis and death.

Side-by-Side Comparison

Which Acts Faster?

Simparica has a meaningfully faster onset. It starts killing fleas within 3 hours and ticks within 8 hours. NexGard starts killing fleas within 8 hours and ticks within 48 hours. For households with active flea infestations where you need rapid relief, Simparica's faster action is a genuine advantage. For a broader look at tick kill speeds across all products, see our tick prevention for dogs guide.

Puppy Age Restriction: An Important Difference

NexGard is approved from 8 weeks of age (minimum 4 lbs). Simparica requires dogs to be at least 6 months old. This is a significant distinction for puppy owners — if your dog is under 6 months, NexGard is the appropriate choice between these two. Simparica Trio (the expanded version) also has the 6-month minimum age restriction.

Tick Coverage

Both NexGard and Simparica protect against 5 tick species: black-legged ticks, American dog ticks, brown dog ticks, lone star ticks, and Gulf Coast ticks. Coverage breadth is essentially equivalent.

Safety & Side Effects

NexGard (afoxolaner) and Simparica (sarolaner) are both isoxazoline-class parasiticides. The FDA has issued a safety communication noting that isoxazoline products have been associated with rare neurologic adverse events — muscle tremors, loss of coordination (ataxia), and seizures — including in some dogs with no prior seizure history. Both drugs remain FDA-approved and are considered safe for the vast majority of dogs, but it's worth discussing your dog's full medical history with a vet before starting either product, especially if your dog has ever had a seizure.

The most commonly reported side effects for both are mild and short-lived: vomiting, diarrhea, lethargy, or reduced appetite in the first day or two after dosing. Neither product has the moxidectin-related MDR1 breed considerations that come up with Simparica Trio or NexGard Spectra, since standard NexGard and Simparica don't include a heartworm-prevention ingredient.

Missed a Dose?

Both NexGard and Simparica are dosed monthly, so the guidance is the same for either: give the missed dose as soon as you remember, then resume your regular monthly schedule from that date. Since neither product includes heartworm prevention, a short delay is lower-stakes than it would be with Simparica Trio or NexGard Spectra — but don't let gaps become a habit, since flea and tick coverage lapses the moment the prior dose's window closes.

Real-World Scenarios

You just found fleas on your dog and want them gone fast: Simparica's 3-hour flea kill gives you the quickest relief of the two.

You have a puppy under 6 months old: NexGard is approved from 8 weeks; Simparica requires 6 months, so NexGard is the only option between the two for young puppies.

You want heartworm coverage in the same chew: neither standard product includes it — you'll want NexGard Spectra or Simparica Trio instead. See our full comparison of the all-in-one versions.

Which Is More Widely Used?

NexGard has historically been the market leader for monthly oral flea and tick treatments in the USA, with a longer track record. Simparica is newer but has rapidly gained popularity due to its faster speed of kill. Both are considered safe and effective. To understand how to buy either without a prescription, see how to buy pet medications from Australia.

Price

NexGard and Simparica are comparable in price from Australian retailers. Both are significantly cheaper through Australian sources than from US vets or US-based pharmacies. See why pet medications are cheaper from Australia for a full explanation.

Which Should You Choose?

For puppies under 6 months: choose NexGard — it's approved from 8 weeks, Simparica is not.
For fastest flea relief: choose Simparica — it starts killing fleas within 3 hours.
For broadest parasite coverage: consider Simparica Trio or NexGard Spectra — see our NexGard Spectra vs Simparica Trio comparison for the full breakdown.

Frequently Asked Questions

Is NexGard or Simparica better for dogs with a history of skin allergies?
Neither ingredient is an allergen in the traditional food-allergy sense, but any new medication can occasionally trigger a reaction in a sensitive individual dog. If your dog has a history of drug sensitivities, mention it to your vet before starting either.

Can I use NexGard or Simparica on cats?
No. Both are formulated and dosed specifically for dogs. Afoxolaner and sarolaner are available in cat-specific products from the same manufacturers, but the dog chewables should never be given to cats.

Do I need to withhold food before or after giving the chew?
No fasting is required. Both are palatable chews meant to be given with or without food, though giving them with a meal can help reduce the small chance of an upset stomach.

What if my dog spits out part of the chew?
If you're not confident your dog swallowed the full dose, contact your vet or the manufacturer's support line for guidance rather than guessing at a partial re-dose.

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