Flea Spot-Ons and Swimming: What a UK Study Found in the Water
Four weeks after a flea spot-on, a single swim still washed enough insecticide off a dog to exceed safe levels for a pond, a UK study estimated.
Written by thecanineledger
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In a UK study, researchers put 49 dogs treated with one of two common flea spot-ons into a tub of water for about five minutes. The insecticide was found in every sample, even 28 days after treatment (Perkins et al., Veterinary Record 2025).
While the amount washed off fell over time, the authors estimate that one swim could still push a small pond above safe levels for water life a month later. They say current label advice on swimming does “not provide adequate environmental protection.”
→ Ticks on UK Dogs: What a 12,000-Dog Check Found
1. The study
The dogs. 49 healthy dogs, volunteered by their owners. Each was treated with one of two spot-on products, then “swam” once in a tub, on day 5, 14 or 28 after treatment.
The risk model. The researchers modelled a large dog entering a small pond, then compared the predicted concentration with published safe levels for freshwater life.
Funding. The UK’s veterinary medicines regulator funded the work and helped review the draft. The authors declare no competing interests.
Our note: The regulator that funded this study also licenses these products.
2. What they found
| Days after treatment | Share of applied insecticide washed off (first product) | (second product) |
|---|---|---|
| 5 | 4.0% | 10.1% |
| 14 | 0.8% | 1.8% |
| 28 | 0.4% | 1.4% |
Above safe levels. For both insecticides, the modelled pond concentration stayed above safe thresholds for at least 28 days after a single swim.
Smaller than the drain. Across all UK dogs, the authors estimate swimming releases less insecticide than bathing, bed washing and hand washing do through the drains. But they say “localised impacts may still occur”, especially in ponds used by many dogs.
3. What the authors recommend
No swimming for four weeks. They suggest treated dogs stay out of rivers and ponds for the product’s full four weeks.
Treat by risk. They support targeting flea treatment at dogs at higher risk or with a confirmed infestation, rather than treating every dog all year.
Our note: We don’t advise on balancing flea control against these findings, because it depends on your dog and household. Ask your vet.
Our note: The authors say little is known about the environmental risks of tablet flea treatments.
→ Flea and Tick Tablets and Seizures: What the FDA Warning Means
4. What the study can’t tell you
A tub, not a river. Five minutes in a tub of 20°C water may not match real swimming.
A model, not a measurement. Pond levels were calculated, not measured, and assumed the insecticide spread evenly.
Early days missed. Swimming is allowed sooner than day 5 for one product, so the authors say they may have underestimated wash-off.
Effects on the dog. The study didn’t test whether swimming makes the treatment work less well.
Small slips. The paper gives the label’s swimming wait for the second product as four days in one place and five in another.
Our note: We don’t name products or quote labels. Check your product’s label for swimming advice.
5. What we could not verify
| Commonly said | What we found |
|---|---|
| “After a couple of days, swimming is fine” | Not for the water. Insecticide still washed off at four weeks. |
| “Dog swimming is the main source of river pollution” | No. The authors estimate drains release more. |
| “Swimming washes off the treatment” | Not tested. The study measured what reached the water, not whether protection dropped. |
Sources
- Perkins R, Glauser G, Goulson D. Swimming emissions from dogs treated with spot-on [drug name omitted] or [drug name omitted]: assessing the environmental risk. Vet Rec 2025: 49 dogs, University of Sussex. Funded by the UK’s veterinary medicines regulator
Always confirm with your vet before acting on anything on this page.