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What’s the safest flea treatment for cats? Our vet answers

safest flea treatment for cats
(Image credit: Getty)

It’s common to worry about drug side effects on your pet, and it’s likely you’ve found yourself wondering what the safest flea treatment for cats is. The problem is, that’s not an easy question to answer. Flea treatments for cats vary – they’re made of different drugs, at different concentrations, and used in different ways. In addition, some drugs will be riskier if a cat has a particular underlying condition, whilst others might be safer in certain ages, or in pregnancy. 

Of course, when in doubt be sure to consult your veterinarian who can provide you with personalised advice about your pet's needs, including ways to prevent fleas, treating any sore skin from flea bites, and whether flea medicines vs collars is the best course of prevention for your feline.

What are common side effects of flea treatment medication?

As with all medications, sometimes reactions to flea treatments happen. Depending on the product, side effects vary. However, the side effects most often noticed after flea treatments include:

  • Drooling (thought to be from licking spot-ons)
  • Hair loss (usually at site of application or collar)
  • Vomiting (after tablet flea treatments)

However, even though these are the ‘most commonly seen’ side effects, they’re generally very rare. All adverse reactions are ranked on a scale from ‘common’ (less than 1 in 10 cats) to ‘very rare’ (less than 1 in 10,000 cats), and for most flea treatments, side effects are rare or very rare.

Which flea products should you never use on cats?

Of course, there are some drugs that cats simply shouldn’t have. One of these is permethrin, an insecticide often used in flea treatments. Flea products with permethrin are usually clearly marked with a warning not to use on any feline members of the household, but it’s worth checking the ingredients before you give any flea product to your cat.

Some feline flea products may include very small doses of permethrin. This small dose is safe in most cats, but there are risks with overdosing or in ill cats. Check carefully whether these can be given to cats before use and if in any doubt, don’t use.

safest flea treatment for cats

(Image credit: Getty)

What’s the safest flea treatment for cats?

So, now we know which flea treatment you should never give to cats, let's look at those that are safest. Assessing the safest flea treatment for cats is difficult. We looked at the datasheets to discover the side effects for each of the common active ingredients in flea treatments for cats:

  • Fluralaner (spot on) – hair loss, itching, or redness at application site (2 in 10 cats), tremors and inappetence (less than 1 in 100 cats) and vomiting/drooling (around 1 in 250 cats).
  • Lotilaner (tablets) – self-resolving vomiting (less than 1 in 10,000 animals).
  • Imidacloprid (spot on) – self-resolving drooling (if licked, no frequency data given), incoordination, tremors, and lethargy (less than 1 in 10,000 animals treated).
  • Selamectin (spot on) – temporary hair loss at application site (less than 1 in 1000 animals), temporary irritation at the site of application (less than 1 in 10,000 animals), reversible neurological signs including seizures (less than 1 in 10,000 animals).
  • Fipronil and fipronil/S-methoprene (spot-on or spray) – self-resolving drooling (if licked, no frequency data given), temporary skin reactions such as redness, itching and hair loss at application site (less than 1 in 10,000 animals), and isolated reports of neurological symptoms, vomiting and respiratory signs.
  • Nitenpyram (tablet) – hyperactivity, panting, excessive grooming, yowling and itchiness lasting less than an hour (less than 1 in 10,000 animals treated), muscle tremors, wobbliness, and seizures (less than 1 in 10,000 animals treated).
  • Spinosad (tablet) – temporary vomiting in the first 48 hours (less than 1 in 10 cats), diarrhoea and inappetence (less than 1 in 10 cats), lethargy, uncoordination and drooling (less than 1 in 100 cats), seizures (less than 1 in 1000).
  • Lufenuron (injection or oral suspension) – itching, vomiting, and diarrhoea (less than 1 in 10,000 cats).
  • Flumethrin/imidacloprid (flea collar) – temporary itchiness, redness and hair loss (less than 1 in 100 cats), eczema, inflammation and skin lesions (less than 1 in 1000 cats), change of food intake, drooling, vomiting, and diarrhoea (less than 1 in 1000 cats).

From this information, the product with the least serious and least frequent side effects is likely the lotilaner tablets, with the lufenuron injection coming a close second. However, there are a couple of things to consider here:

  • I’ve only looked at one or two examples of each active ingredient to compile this list – for some active ingredients there are many different brand names that may have different side effects.
  • Side effects may be down to the suspension, excipients, or non-active ingredients or user error (as with drooling after licking), not the active ingredient.
  • This doesn’t take into account how long the product has been on the market, and therefore how many adverse reaction reports it has picked up. Whilst all products are tested for safety before being made available, useful information from the huge numbers and variety of cats in the ‘real world’ will not have been collected yet in very new products.
  • Individual pets may react differently, or the side effects may be more debilitating if on top of other illnesses your individual pet may have.
  • These products may not all be equally as effective, or suitable for every situation. Some of these products shouldn’t be used on kittens, lactating queens, or cats that suffer from seizures. Others may not be suitable for cats that like to roam or who are difficult to tablet. Lastly, some fleas may be resistant to some products making local knowledge essential when choosing the correct flea treatment to use.

safest flea treatment for cats

(Image credit: Getty Images)

How can I make sure my cat’s flea treatment is as safe as possible?

There are some things you can do to make a flea treatment as safe as possible. 

Firstly, use a prescription product advised by your veterinarian. They’ll be able to weigh up your cat’s individual risks based on their age, weight, and other conditions. They may even say that flea treatment is not needed year-round in your area. 

Secondly, check the best before date on products before applying – out of date products may not be safe to use. You should also ensure that you’ve stored the products in the conditions stated on the packet, as heat and light can cause some medications to stop working or become unsafe.

Ensure you read the packaging and follow all instructions for use. Some products need applying differently to others, so check before each application that you know exactly where and how to apply and how much to give.

You should never mix products, use a flea treatment designed for or prescribed for a different animal, or use a different dosage than the one that was prescribed. 

What should I do if I notice a reaction to a flea medication in my cat?

If your cat is showing symptoms that you suspect are related to their flea medication, you should call your vet for advice. Even if no treatment is needed, your vet will usually ask you for further information – this will be used by the vet to report the adverse effect to the drug manufacturer. 

Collecting this information allows drug manufacturers to keep an eye on their products and collect the ‘real world’ information to make sure their safety advice is up to date. The drug manufacturer may want further information about the side effects your cat has experienced as part of their investigation, or may even ask for tests to be done to determine exactly what caused the symptoms in your cat.

Tabby cat hiding under bed

(Image credit: Getty Images)


Side effects from medications are generally rare, but it’s sensible to find the safest flea treatment for cats, if you can. Your vet is the best person to advise you on what’s safest for your cat’s individual circumstances and they can also provide you with further information about getting rid of fleas or treating other animals in the house.

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.