Is NexGard Safe for Dogs? A Complete Safety & Side Effects Guide
Is NexGard safe for dogs? We break down the science, side effects, dosage by weight, and when to call the vet — plus safer alternatives for flea and tick prevention.
Is NexGard Safe for Dogs? The Quick Verdict
NexGard (afoxolaner) is an FDA-approved oral flea and tick chewable that has been prescribed to over 100 million dogs worldwide since its 2013 launch. Clinical trials and post-market surveillance show adverse reactions in approximately 2-4% of dogs, most commonly mild and self-limiting (vomiting, diarrhea, lethargy). However, dogs with a history of seizures or neurological conditions require veterinary evaluation before use. NexGard is not a food — it is a prescription medication (isoxazoline class) that requires a veterinary prescription and should never be dosed without professional guidance.
If you've landed here wondering about NexGard's safety profile, you're asking the right question. As a medication your dog will take monthly for potentially years, understanding its risks, benefits, and proper administration matters far more than it does for a one-time treatment. Below, we unpack the veterinary science, real-world data, and practical guidance you need to make an informed decision with your veterinarian.
What Is NexGard and How Does It Work?
NexGard is a beef-flavored, chewable tablet containing afoxolaner as its active ingredient. Afoxolaner belongs to the isoxazoline drug class, which also includes fluralaner (Bravecto), sarolaner (Simparica), and lotilaner (Credelio). These compounds work systemically — meaning they enter your dog's bloodstream and kill parasites when they bite.
Here's the mechanism: afoxolaner targets the nervous system of fleas and ticks by inhibiting their GABA-gated chloride channels and glutamate-gated chloride channels. This causes uncontrolled neural activity in the parasite, leading to death. The drug doesn't affect mammalian GABA receptors in the same way, which is why it's generally safe for dogs — but individual sensitivity varies.
| Attribute | Detail |
|---|---|
| Active Ingredient | Afoxolaner (isoxazoline class) |
| Manufacturer | Boehringer Ingelheim (originally Merial) |
| Administration | Oral chewable, once monthly |
| Minimum Age | 8 weeks old |
| Minimum Weight | 4 lb (1.8 kg) |
| Kills Fleas | Within 8 hours, 100% within 24 hours |
| Kills Ticks | Lone star, American dog, deer tick (48-72 hours) |
| Prescription Required | Yes (veterinary prescription) |
| Average Cost (2026) | $22-$32 per chewable; $250-$380 for 12-pack |
NexGard Dosage by Dog Weight
NexGard is dosed based on your dog's body weight, and your veterinarian will select the appropriate chewable size. The dosing is not adjustable by splitting tablets — each chewable is formulated for a specific weight range.
| Dog Weight | Chewable Size | Afoxolaner Dose | Color Coding |
|---|---|---|---|
| 4-10 lb (1.8-4.5 kg) | Small | 11.3 mg | Orange |
| 10.1-24 lb (4.6-10.9 kg) | Medium | 28.3 mg | Green |
| 24.1-60 lb (11-27.2 kg) | Large | 68 mg | Blue |
| 60.1-121 lb (27.3-54.9 kg) | Extra Large | 136 mg | Purple |
Critical note: If your dog weighs over 121 lb, your veterinarian will combine chewables to achieve the correct dose. Never give a smaller dog a chewable meant for a larger weight class — the dose concentration difference is significant and can increase the risk of neurological side effects.
Documented Side Effects: What the Data Shows
In the pivotal field study submitted to the FDA for NexGard's approval, 615 dogs received afoxolaner at the recommended dose. Here's what the adverse event data actually showed:
| Side Effect | Frequency | Typical Duration | Severity |
|---|---|---|---|
| Vomiting | ~2.0% of dogs | Self-limiting, 12-24 hours | Mild |
| Diarrhea / loose stool | ~1.5% of dogs | Self-limiting, 24-48 hours | Mild |
| Lethargy | ~0.8% of dogs | 12-24 hours | Mild |
| Dry/flaky skin | ~0.5% of dogs | Variable | Mild |
| Seizures / tremors | Rare (post-market reports) | Requires veterinary intervention | Moderate to Severe |
| Loss of appetite | ~0.5% of dogs | 24-48 hours | Mild |
In 2018, the FDA issued a safety alert regarding the isoxazoline drug class, noting that seizures, muscle tremors, and ataxia (loss of coordination) had been reported in some dogs. This doesn't mean NexGard causes seizures in most dogs — the events were rare — but it does mean that dogs with a pre-existing seizure disorder or neurological history should be evaluated carefully before starting any isoxazoline product.
Who Should NOT Take NexGard?
While NexGard is safe for the majority of dogs, certain populations need special consideration or should use an alternative flea and tick preventive:
- Dogs with a seizure history: The FDA's 2018 alert specifically flagged isoxazolines for dogs with epilepsy or prior seizure events. Your veterinarian may recommend a non-isoxazoline alternative such as a topical (e.g., Frontline Plus, K9 Advantix) or a collar (Seresto).
- Puppies under 8 weeks: NexGard is not approved for puppies younger than 8 weeks old.
- Dogs under 4 lb (1.8 kg): No approved dosage exists below this threshold.
- Dogs with known afoxolaner sensitivity: If your dog had an adverse reaction to a previous isoxazoline dose, all drugs in this class should be avoided.
- Breeding, pregnant, or lactating dogs: NexGard has not been evaluated for safety in breeding, pregnant, or nursing dogs. Your veterinarian will likely recommend an alternative with established reproductive safety data.
Contact your vet or an emergency clinic if your dog exhibits any of the following after taking NexGard:
- Seizures or muscle tremors
- Ataxia (stumbling, loss of coordination, inability to stand)
- Persistent vomiting (more than 2 episodes in 12 hours)
- Facial swelling, hives, or difficulty breathing (signs of anaphylaxis)
- Extreme lethargy lasting more than 24 hours
- Loss of appetite for more than 48 hours
If you suspect a serious adverse drug reaction, you can also report it directly to the FDA's veterinary adverse event reporting system or call Boehringer Ingelheim's technical services at 1-866-637-4257.
NexGard vs. Alternatives: How Do Other Flea & Tick Products Compare?
NexGard isn't the only option. Understanding how it stacks up against other preventives helps you and your vet choose the best fit for your dog's health profile and your lifestyle.
| Product | Type | Active Ingredient | Duration | Cost/Dose | Seizure Risk |
|---|---|---|---|---|---|
| NexGard | Oral chewable | Afoxolaner | 30 days | $22-$32 | Low (class warning) |
| Bravecto | Oral chewable | Fluralaner | 12 weeks | $50-$70 | Low (class warning) |
| Simparica | Oral chewable | Sarolaner | 30 days | $20-$28 | Low (class warning) |
| Frontline Plus | Topical spot-on | Fipronil + (S)-methoprene | 30 days | $12-$18 | None documented |
| Seresto Collar | Collar | Imidacloprid + flumethrin | 8 months | $55-$70 (one-time) | None documented |
| K9 Advantix II | Topical spot-on | Imidacloprid + permethrin | 30 days | $12-$16 | None documented |
Key insight: Oral isoxazolines (NexGard, Bravecto, Simparica) share the same FDA class warning regarding neurological events. If seizure risk is your primary concern, a topical or collar-based product avoids the isoxazoline class entirely. However, oral products have the advantage of not washing off during swimming or bathing and eliminate the skin irritation some dogs experience with spot-on treatments.
Practical Tips for Administering NexGard Safely
Most dogs readily accept NexGard because it's formulated with beef flavoring. However, here are practical steps to ensure safe administration:
- Give with a full meal. Afoxolaner absorption is significantly enhanced when given with food. Feeding NexGard on an empty stomach can reduce efficacy and increase the likelihood of vomiting.
- Observe for 30 minutes post-dose. If your dog vomits within 2 hours of administration, contact your veterinarian — a replacement dose may be needed since the drug may not have been fully absorbed.
- Don't split or crush chewables. The active ingredient isn't evenly distributed throughout the tablet. Splitting can result in underdosing (reduced efficacy) or overdosing (increased side effect risk).
- Track dosing dates. NexGard must be given monthly. Set a phone reminder or use a pet health app. Gaps in coverage allow flea populations to re-establish rapidly — a single female flea can lay 40-50 eggs per day.
- Store properly. Keep chewables in their original packaging at room temperature (below 86°F / 30°C). Don't remove from blister packs until ready to administer.
Frequently Asked Questions About NexGard Safety
Can NexGard cause seizures in dogs?
Seizures have been reported as a rare post-market adverse event for all isoxazoline-class drugs, including NexGard. The incidence rate is low — the FDA issued a class-wide warning in 2018 based on accumulated reports, not because seizures are common. Dogs with pre-existing epilepsy or a seizure history are at higher risk. Always disclose your dog's full medical history to your veterinarian before starting NexGard.
Is NexGard safe for puppies?
NexGard is approved for puppies 8 weeks of age and older, weighing at least 4 lb (1.8 kg). Puppies younger or smaller than these thresholds should not receive NexGard. Your veterinarian can recommend age-appropriate flea and tick prevention for very young puppies, such as manual flea combing or Capstar (nitenpyram) for acute flea infestations.
Can I give NexGard and Heartgard together?
Yes. NexGard and Heartgard Plus (ivermectin/pyrantel) are commonly prescribed together and have been studied for concurrent use. Boehringer Ingelheim also markets NexGard Spectra, which combines afoxolaner with milbemycin oxime for heartworm prevention in a single chewable. Consult your veterinarian to determine which approach suits your dog's preventive care plan.
What if my dog eats another dog's NexGard?
If a small dog accidentally ingests a chewable meant for a much larger dog, the dose could exceed safe levels. Contact your veterinarian or the ASPCA Animal Poison Control Center at (888) 426-4435 (a consultation fee applies) for guidance. Monitor for vomiting, tremors, or ataxia and seek emergency veterinary care if neurological symptoms develop.
Does NexGard protect against heartworms?
No. Standard NexGard only covers fleas and ticks. It does not prevent heartworm disease or intestinal parasites. If you want heartworm coverage in an oral chewable, ask your veterinarian about NexGard Spectra or a separate heartworm preventive such as Heartgard Plus or Interceptor Plus.
How long does NexGard stay in a dog's system?
Afoxolaner has a half-life of approximately 15 days in dogs. The drug maintains effective blood concentrations for a full 30-day dosing interval, which is why monthly administration is required. It takes roughly 75 days (5 half-lives) for afoxolaner to be essentially eliminated from the body after the last dose.
This article is for informational purposes only and does not replace professional veterinary advice. Always consult your veterinarian before starting, stopping, or changing any medication for your dog. Your vet will determine the appropriate preventive based on your dog's health history, lifestyle, and regional parasite risks.
Dr. Hannah Wickes
All our authors care for dogs every day — read more of their work on the authors page.
