How Long Does Flea Treatment Take to Work on Dogs and Cats?
Flea treatment often seems not to be working — but understanding how it works explains why seeing new fleas after treatment is normal.
How Flea Treatments Work
Most modern flea treatments (spot-ons and oral tablets) kill adult fleas by affecting the flea's nervous system when it bites the treated animal. They do not repel fleas — fleas must bite first.
Time to Kill Adult Fleas
- NexGard, Simparica (oral): kills adult fleas within 2–4 hours
- Bravecto (oral): kills adult fleas within 2–4 hours
- Frontline, Advantage (topical): kills adult fleas within 24 hours
Why You Still See Fleas After Treatment
The flea life cycle is the key. Only 5% of a flea infestation lives on the pet — 95% is in the environment as eggs, larvae, and pupae. These stages are not killed by pet treatments. They hatch continuously and jump onto your pet, where they die after biting — but you may see them before they die.
The Full Cycle Takes 3 Months
Breaking a full flea infestation takes approximately 3 months of continuous treatment on all pets in the household, combined with regular vacuuming and washing of bedding. Environmental sprays (containing insect growth regulators) significantly speed this up.
Treating the Home
- Vacuum thoroughly twice weekly (concentrate on skirting boards and sofa bases)
- Wash all pet bedding at 60°C weekly
- Use a household flea spray containing IGR (insect growth regulator)