Are Flea & Tick Medications Actually Safe for Your Dog?

How Do These Drugs Actually Work?

NexGard (afoxolaner), Simparica (sarolaner), Bravecto (fluralaner) and Credelio (lotilaner) all belong to a class called isoxazolines. When a flea or tick bites your dog, it ingests the drug in your dog's blood. The drug then blocks two types of chloride channels in the parasite's nervous system — GABA-gated and glutamate-gated channels — causing uncontrolled electrical firing until the parasite dies.

The key claim from manufacturers is selective toxicity: invertebrates (insects, ticks) have different receptor subtypes than mammals, so the drug targets parasites while leaving your dog relatively unaffected. This is largely true — but as we'll see, not perfectly true.

The FDA Warning You Probably Never Heard About

In September 2018, the US Food & Drug Administration issued a formal alert requiring all isoxazoline flea and tick products to update their labels with a neurological adverse events warning. The reported reactions included:

• Muscle tremors
• Ataxia (loss of coordination, wobbly walking)
• Seizures — including in dogs with no prior seizure history

This doesn't mean these drugs are dangerous for most dogs — the majority tolerate them without issue. But it does mean the "completely safe" narrative isn't quite accurate. Adverse reactions are rare, but they are real and documented.

The Gene Mutation That Changes Everything

This is the part most owners — and some vets — don't know about. A gene called MDR1 (also called ABCB1) controls a pump that removes certain drugs from the brain. Dogs with a mutation in this gene cannot clear drugs like moxidectin (found in Simparica Trio) efficiently from their brains, leading to toxic accumulation and severe neurological reactions — even at standard label doses.

Washington State University's Veterinary Clinical Pharmacology Lab has been researching this mutation for over two decades. Their testing programme shows the following prevalence in herding and related breeds:

What About Natural Alternatives? The Honest Evidence

You'll find dozens of natural flea remedies suggested online — garlic, neem oil, apple cider vinegar, essential oils, diatomaceous earth. The appeal is obvious: no FDA warnings, no synthetic chemicals. But the evidence behind these varies enormously. Here's what the research actually shows:

🌿 Neem Oil

Evidence: Moderate. Neem contains azadirachtin, a compound that disrupts insect hormone systems and acts as a repellent. Multiple peer-reviewed studies show repellent and mild acaricidal activity against ticks and fleas. It won't kill a heavy infestation but may help as a topical spray in a low-pressure environment. Diluted in a carrier oil, it's generally safe on dog skin — but avoid the eyes and mucous membranes. PubMed search →

🪨 Food-Grade Diatomaceous Earth (DE)

Evidence: Effective as an environmental treatment — not reliable on dogs. DE is a powder made from fossilised algae (diatoms) that physically damages flea and tick exoskeletons, causing them to dehydrate. A 2021 review in Molecules confirmed DE has genuine activity against Ixodida (ticks and fleas) and other arthropod pests — particularly in dry indoor environments like carpets, dog bedding, and floor cracks. On a dog's coat, it loses efficacy when damp and raises inhalation concerns with the fine silica dust. Best used as an environmental treatment between washes, not applied directly to the dog. Review study →

🌿 Essential Oils — Fleas and Ticks

Evidence: Promising for fleas; modest for ticks. A 2025 critical review of 25 studies (published in Insects) found that essential oils from cinnamon (Cinnamomum spp.), clove (Syzygium aromaticum, active compound: eugenol), and sweet basil showed the most pronounced insecticidal and repellent effects against the cat flea (Ctenocephalides felis). For tick repellency, a separate study found geraniol (from rose geranium and lily-of-the-valley oil) showed activity comparable to low-dose DEET against Ixodes ricinus in lab conditions. All essential oils require frequent reapplication and work best in low-pressure environments only. ⚠️ Safety warning: not all essential oils are safe for dogs — tea tree oil and pennyroyal are toxic. Cinnamon and clove oils must be heavily diluted before any skin contact, as undiluted concentrations cause burns and mucous membrane irritation. EO review (fleas) →  Geraniol study (ticks) →

🧄 Garlic — Why We're Including It Here

Garlic is one of the most frequently suggested natural flea remedies in online dog communities, based on the idea that it makes a dog's blood smell or taste unpleasant to fleas. We're including it in the comparison table below because so many owners have tried it or asked about it. But here's what the science says:

• No evidence it repels fleas at any safe dose. No peer-reviewed study has shown garlic to be an effective flea preventative in dogs.
• Real toxicity risk at any consistent dose. Garlic — like all Allium species — contains N-propyl disulfide, which oxidises haemoglobin in red blood cells, causing Heinz body haemolytic anaemia. Dogs are significantly more sensitive to Allium toxins than humans. The dose that theoretically might affect fleas (entirely unproven) overlaps dangerously with the dose that begins causing measurable red blood cell damage in dogs. Studies on Allium toxicity in dogs →
• Some dogs seem unaffected by small amounts for years. But "my dog seems fine" is not proof of safety — early anaemia can be subclinical, and effects are cumulative and dose-dependent.

🪮 Flea Combing + Environmental Hygiene

Evidence: Genuinely effective as a complement. Daily flea combing (especially around the neck, base of the tail and groin) doesn't prevent fleas but catches infestations early. Combined with weekly vacuuming of carpets and floors, and washing bedding on a hot cycle, this significantly reduces flea burden. In very low-risk environments — indoor urban dogs with minimal outdoor exposure — this mechanical approach alone may be sufficient.

Honest Risk vs Benefit Comparison

Garlic is included here because it is one of the most widely suggested "natural" options online — see the section above for the full explanation.

Option	Flea/Tick Efficacy	Risk to Dog	Best For
Isoxazolines NexGard, Simparica, Bravecto	✅ Very high (99%+)	⚠️ Low for most; higher for MDR1 breeds	High tick/flea pressure areas; most breeds
Older spot-ons Frontline, Advantage	🟡 Moderate (declining resistance)	✅ Lower neurological risk	Dogs with seizure history or MDR1 mutations
Natural alternatives Diatomaceous earth, neem, cedar	❌ Low — not clinically proven	✅ Minimal toxicity risk	Low-risk environments; as a complement only
Garlic	❌ No evidence it works	❌ Real risk of hemolytic anaemia	Not recommended
No prevention	❌ None	❌ High risk of tick-borne disease (Lyme, Ehrlichia, Rocky Mountain Spotted Fever)	Only very low-risk indoor dogs

The Bottom Line

Isoxazoline flea and tick medications are effective and generally safe — but "generally safe" is doing a lot of work in that sentence. The FDA warning is real. Adverse neurological events do occur. And for Collies, Australian Shepherds and other herding breeds with the MDR1 mutation, the risk calculation is genuinely different.

This isn't an argument against using these medications. Tick-borne diseases like Lyme, Ehrlichia and Rocky Mountain Spotted Fever are serious and sometimes fatal — the disease risk in a tick-heavy area almost certainly outweighs the drug risk for most dogs. But it is an argument for being an informed owner rather than just accepting a monthly chew as automatically safe.

Three things worth doing:

1. If you own a herding breed, ask your vet about MDR1 testing before starting isoxazolines
2. If your dog has a history of seizures or neurological issues, discuss alternatives with your vet
3. If you see tremors, wobbling or unusual behaviour after starting a new flea medication, contact your vet immediately and mention the drug