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"Why is my flea treatment not working?" Our vet offers some answers

Flea treatment not working
(Image credit: Getty)

The problem of a flea treatment not working is a pretty common scenario: you’ve bathed your pet, used a flea comb and applied the best flea treatment for dogs or the best flea treatment for cats, but still the critters persist. Fleas are frustrating to get rid of, and can make you and your pet miserable, but what’s the best way to get rid of fleas?

What is the flea lifecycle and why does it matter?

Fleas have a similar life cycle to the caterpillar and butterfly. The adult flea lives mostly on your pet, although they’ll jump back off and spend some time in your house. They lay eggs on your pet, which fall off into the carpet, but they may also lay eggs between the floorboards, in cracks in the tiles, and under the sofa — anywhere dark and hidden. These eggs hatch into caterpillar-like larvae, which feed on dust, debris, and poo from the adults. 

Once they’ve grown large enough, the larvae find a quiet corner to pupate, spinning a camouflaged cocoon from carpet fibres and dust. In the pupa they transform from a caterpillar-like legless larvae into an adult flea with long legs for jumping, and an appetite for animal blood. They may then hatch or if they sense conditions aren’t right, they’ll wait in the cocoon, protected, for up to 12 weeks. None of the flea products on the market can kill flea pupae!

There are three important things to note here:

  • Fleas go through four life stages
  • Only a very small proportion of the infestation is the adult fleas on your pet
  • None of the flea-killing products can get through the cocoon and kill the pupa

What does this mean for treating fleas on my pet?

The life cycle is key to treating fleas effectively. Firstly, it’s important to understand that most flea products contain an adulticide – a chemical to kill the adults. Some – but not all – will also contain ingredients that will kill the eggs/larvae, or an insect growth regulator, which prevents the fleas from moving through the life cycle. 

If you only kill the adults, you need to treat the infestation for long enough for all the eggs to hatch into larvae, all the larvae to pupate, and all the pupae to hatch – that’s 5-6 months!

Secondly, it’s important to remember that the cocoon stage is immune to all known insecticides… and it can survive in that stage for 12 weeks if conditions aren’t right.

Even if you kill the adults, the eggs, and the larvae, nothing will kill the pupae in their cocoons. So you need to treat your pet for long enough for all the pupae to hatch into adults so the adulticide can kill them – that takes at least 3 months! If you break in treatment at any point during this time, the pupae can hatch, feed, and start laying eggs again, starting your infestation from scratch.

The five mistakes people make when they say their flea treatment is not working

1. Using flea shampoo

One really common scenario is a frustrated pet parent who uses flea shampoo, then applies, for example, a topical flea treatment for cats, to kill the fleas. Many of the spot-on flea products use the natural oils of the coat to spread over your pet. So, if you’ve just washed your pet and stripped the natural oils, they’ll be less effective. Check the packaging for specific recommendations, but most say you cannot apply within 48 hours of a bath, and you shouldn’t bathe your dog for at least 48 hours after application, either.

2. Not treating all your pets

There’s no point in treating your dogs if the adult fleas can feed on your rabbit, or on the neighbour’s cat that sleeps on your sofa sometimes! The minute you forget a day of your dog’s treatment, or use a sub-standard product, they’ll catch fleas again. Make sure you treat all the animals in the house so the adult fleas have nowhere safe to go! If you have a visiting cat, please check with their owner before applying flea treatment, to avoid double-dosing.

Flea treatment not working

(Image credit: Getty)

3. Not waiting long enough

We’ve hit on this a couple of times already, but just to reiterate – in my experience, the single most common reason for flea treatment failure is not understanding how long it takes to get rid of an infestation. Pet parents often have unrealistic expectations, especially if they buy a product from the vets – but the truth is, the pupae can wait 12 weeks to hatch, which means you need to treat for 12 weeks, without a gap, to beat them.

4. Using bad, out-of-date, or fake products

Sometimes things are too good to be true. If the flea treatment seems a lot cheaper than you’re used to, ask yourself why. Could the product be out of date? Did it come from a reputable source? Does it actually claim to kill fleas, or does it just repel them? If you’ve used a sub-standard product, it stands to reason the result might not be what you want. There are also fakes circulating, so you might have done your best but still been caught out. Whilst an adulticide appears to be cheaper, remember that you’ll need to treat for twice as long to get rid of a flea infestation! PetsRadar has buying guides to all type of effective flea treatment including the best flea collar for dogs and the best flea collar for cats, along with the best flea medication for dogs.

5. Not treating the house

If your product only contains an adulticide, you still need to kill the eggs and larvae in the house. One way of doing this is with a house spray or flea bomb. Many of these products last for several months in the environment, so only need to be applied a couple of times a year. It’s easy to forget this step, but using flea spray takes the time to treat the infestation from 6 months to 3 months!

Help! I've done all this and the flea treatment still isn't working!

It’s time to go and see your vet. Don’t be embarrassed – we deal with fleas all the time and it is no reflection on your home or ability to care for your pet! Your vet will be able to work out what’s gone wrong and help you to get rid of the fleas. If your pet has seen the vet within the last 6-12 months, they may even not need to be seen – and if they do, a ‘flea check’ is often free!


So now you know the biggest mistakes made when people think their flea treatment is not working, hopefully you can avoid making these mistakes yourself! You’ll also know the importance of treating your pet for several months without a break in order to ensure the infestation has gone!

Dr Joanna Woodnutt MRCVS

Dr Joanna Woodnutt is an experienced vet with an interest in companion animals. She recently left full-time practice to work as a relief vet and write articles about pets.