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Natural Supplements · May 27, 2025

Your dog’s flea treatment might be doing more harm than good

Seizures, contaminated waterways, and toxic effects on skin cells. Here’s why I think it’s time to take natural flea and tick control seriously.

Golden tan dog on living room floor beside a flea spray bottle, natural flea treatment

By Beryl Shuttleworth, BSc Hons Biochem

Last reviewed: 2026-07-22

Natural options like neem oil and essential oils genuinely do repel and kill fleas and ticks, the science backs that up, though they wear off faster than conventional products and we don’t yet have as much real-world data on pets. That said, the synthetic alternatives aren’t without their own serious problems: isoxazolines have been linked to seizures and even death in some dogs, fipronil and imidacloprid are building up in our waterways and harming aquatic life, and permethrin can be fatal to cats. There’s no perfect answer here, it’s about weighing the risks on both sides.

Common misconception: Natural flea and tick treatments don’t really work, only chemicals get the job done

I get why this one sticks around. Conventional spot-ons and oral treatments have mountains of trial data behind them, and the results really are impressive, fluralaner, for example, knocks out 99.2-99.9% of fleas over twelve weeks [12]. Next to that, plant-based options can look a bit hopeful.

But “not as thoroughly tested” is not the same as “doesn’t work,” and the evidence for some natural ingredients is genuinely solid.

Take neem seed oil (Azadirachta indica). It contains over 100 active compounds, including a group called limonoids that interfere with how insects grow and reproduce. In controlled studies, neem oil at concentrations of 20-100% killed tick larvae within 24-27 hours, and the more concentrated it was, the faster it worked [6]. There’s also early evidence it can help with fleas, mange mites, and other skin parasites in dogs, though larger, longer-term trials are still needed before we can say anything definitive [10].

Essential oils are another area worth taking seriously. Oils rich in compounds like thymol, carvacrol, eugenol, and cinnamaldehyde have shown real insect- and tick-killing activity in lab studies [7]. Lemon eucalyptus oil and its derivative PMD (p-menthane-3,8-diol) are registered as biopesticide repellents in the US, PMD is actually the only plant-based repellent the CDC officially recommends for public use [8]. Cedar, cinnamon, mint, and lemongrass oils have all performed comparably to DEET in some lab repellency tests [8]. The honest catch? They don’t last as long, typically somewhere between 30 minutes and two hours, and how well they hold up in real-world use on dogs and cats isn’t fully mapped out yet [7].

Here’s where I land on this: natural treatments are not a straight swap for a high-efficacy synthetic, and I’d never claim otherwise. They work mainly by repelling parasites and disrupting their life cycle rather than killing them outright, and they need more frequent application. But “they don’t work” just isn’t what the evidence says. Used regularly, as part of a joined-up approach, they have a real place, especially for owners who have good reason to be wary of the neurological side-effects documented with isoxazolines [3], or who are concerned about the environmental persistence of fipronil and imidacloprid in rivers and waterways [11].

How natural and conventional flea & tick treatments work

Conventional chemical treatments are designed to be toxic to parasites whilst leaving your pet (largely) unharmed, but how they do that, and at what cost to the wider environment, varies quite a bit.

Fipronil and imidacloprid are the active ingredients in many popular spot-ons. At the doses on the label, they’re considered safe for dogs and cats, but both are now recognised as persistent pollutants that find their way into rivers and streams, where they harm aquatic creatures like copepods and mussels. Laboratory studies have also flagged toxic effects on human skin cells, which has led to their agricultural use being restricted in the EU and UK [1, 11].

Permethrin belongs to the pyrethroid family and works by interfering with the nervous system of insects. Dogs handle it reasonably well because their bodies clear it quickly, but cats absolutely cannot, permethrin spot-ons are contraindicated in cats because the risk of serious poisoning is real and well-documented. Even in dogs, exposure can cause tremors, seizures, drooling, and vomiting [2].

Isoxazolines, afoxolaner, fluralaner, sarolaner, and lotilaner, are the newer oral chewables. They work fast and work well: kill rates of over 97% against fleas and over 94% against ticks, with a single dose lasting 12 weeks or more. That’s genuinely impressive efficacy. The concern is that the FDA has formally warned veterinarians and pet owners that this class can trigger neurological effects, muscle tremors, loss of coordination, and seizures, in some animals. A survey of 1,325 dogs treated with isoxazolines found that 80.1% showed adverse effects, with serious events including seizures and death reported in 4.55-14.79% of animals [3, 4].

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Natural alternatives work differently, and it’s worth being clear about what they can and can’t do.

Essential oils from plants like lemon eucalyptus, thyme, and cedarwood have shown genuine repellent activity against ticks in laboratory and field studies. The honest caveat is that they don’t last long, typically 30 minutes to 2 hours, and we don’t yet have a complete picture of how well they perform when applied to companion animals out in the real world [7].

Oils of cedar, cinnamon, mint, and lemongrass have come out favourably in laboratory repellency tests, sometimes matching or even outperforming DEET. That’s worth knowing, but these are repellents, not killers. They discourage parasites from landing; they don’t eliminate an infestation [8].

Practical guidance: Choosing the right flea and tick protection for your pet

There’s no single “right answer” when it comes to flea and tick prevention, and anyone who tells you otherwise isn’t giving you the full picture. What works brilliantly for one dog may be completely wrong for another, and some products that are fine for dogs are genuinely dangerous for cats.

Conventional spot-ons and chewables, the ones your vet is most likely to reach for, do work. Products based on fipronil and imidacloprid have been used for years and are effective month-to-month. The newer isoxazoline chewables (think fluralaner, afoxolaner, sarolaner, lotilaner) last longer, up to 12 weeks per dose, and in controlled studies have hit flea-control rates of over 99%. That’s genuinely impressive. But they’re not without controversy. The FDA flagged in 2018 that isoxazolines can cause neurological effects in some animals, including tremors and seizures, and a survey of over 1,300 dogs found that serious adverse events, including seizures and death, were reported in a meaningful proportion of treated animals. That doesn’t mean you should never use them, but it does mean you should go in with your eyes open, especially if your dog already has any kind of neurological history.

Permethrin-based products are another one to flag. Fine on dogs at the right dose, but genuinely toxic to cats, keep them away from any permethrin spot-ons entirely.

On the natural side, there are some genuinely useful options, though I’ll be straight with you: the evidence base is thinner, and most of the strongest results come from lab studies rather than field trials on real pets. Neem oil (from *Azadirachta indica*) has shown real acaricidal activity against tick larvae, with concentrations of 20-100% killing larvae within 24-27 hours in studies, and it has centuries of traditional use in Ayurvedic medicine. There’s also early evidence it may help with mange mites and fleas in dogs, though more clinical work is still needed. Essential oils, particularly those rich in thymol, carvacrol, and eugenol, and oils like cedarwood, lemon eucalyptus, and clove, show promising repellent and insecticidal activity in lab settings, with some performing comparably to DEET in tick-repellency assays. The honest caveat: their effect doesn’t last as long as synthetics (typically 30 minutes to 2 hours), and we don’t yet have robust field data on how well they work when applied to a dog or cat going about their day. Diatomaceous earth works differently, it’s purely physical, scratching and drying out the tick’s outer layer, and lab studies show it can kill tick nymphs within hours. Its effectiveness drops in humid conditions, and the quality of the product matters a great deal.

A few things that should always factor into your decision, whatever route you go: your pet’s age (young and elderly animals need extra care), their species (cat or dog, this really matters), any existing health conditions, and what other medications they’re on. If you’re unsure, talk it through with your vet before you commit to anything.

What we’d recommend


Herbal Pet Tick and Flea Chews

A natural way to help deter ticks and fleas, without synthetic chemicals or the neurological side effects some pets experience.

See the product

Frequently asked questions

Are conventional flea and tick treatments like fipronil and imidacloprid safe for my dog?

They pass the required safety checks, but I wouldn’t call them risk-free. Both fipronil and imidacloprid are classified as persistent, mobile, and toxic chemicals, they hang around in water and harm aquatic creatures like copepods and mussels. Lab studies have also flagged toxic effects on human skin cells. They work, no question, but “meets the safety threshold” is a lower bar than most owners realise. It’s worth thinking about your dog’s individual health and lifestyle before reaching for a monthly chemical treatment as a default.

Can isoxazoline flea treatments (Bravecto, Credelio, Simparica) cause seizures in dogs?

Yes, and this isn’t scare-mongering. The FDA formally warned vets and owners back in 2018 that isoxazolines (that’s afoxolaner, fluralaner, sarolaner, and lotilaner) can trigger muscle tremors, wobbliness, and seizures in some dogs and cats. A survey of 1,325 treated dogs found 80.1% had some kind of adverse reaction, with serious events, seizures and deaths, reported in 4.55-14.79% of cases. The pre-approval trials painted a much rosier picture than what post-marketing data later showed.

Is permethrin safe to use on dogs for flea and tick control?

It can be used on dogs, but there’s one rule you absolutely cannot break: keep cats well away from any dog treated with permethrin. Cats can’t process it the way dogs can, and exposure, even from grooming a treated dog, can be fatal. In dogs, a wrong dose or misapplication can cause tremors, seizures, drooling, and vomiting. Dogs handle pyrethroids at the right dose because their bodies clear the chemical much faster than insects do, but getting the dose and species right is non-negotiable.

Do natural essential oils actually repel ticks and fleas on dogs?

The research is encouraging, though it’s upfront about what it doesn’t yet know. Oils of lemon eucalyptus, thyme, cedarwood, and clove all show real repellent activity in lab and field tests, and cedar, cinnamon, mint, and lemongrass have matched DEET in some lab repellency studies. The honest catch: they don’t last long, typically 30 minutes to two hours, and how well they perform on an actual dog out on an actual walk isn’t yet fully established. If you go this route, you’ll need to reapply regularly.

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