You want to save money whenever you can so when you see the usual brand of flea treatment for your cat listed at half the normal price, you click “buy”.

It arrives and you apply it to your pet, but they fall ill and you have to rush to the vet for treatment.

It emerges that the medicine you bought was a fake and that it contained dangerous chemicals that triggered vomiting and seizures . You are lucky that the cat survived, but have ended up with a large vet bill.

Pet owners have been warned when buying flea treatments online after toxic pesticide traces were found in counterfeit products.

The Veterinary Medicines Directorate (VMD), the UK government agency that oversees pet medication, says the fakes often lack proper active ingredients, making them ineffective.

But in the worst cases, they contain harmful substances that can cause vomiting, muscle tremors, breathing difficulties, seizures and even death.

Last year, the VMD cited the case of a cat that had to have extensive surgery after its owner used what turned out to be fake Frontline flea treatment.

Charlotte Inness, a vet who founded the online pharmacy VetMedi.co.uk, says that a “grey market” for animal medicines exists where unregulated websites and social media accounts sell the fake medications to pet owners hoping to save money.

A three-month dose of flea medication typically costs about £20, but the counterfeit versions are being sold for less than half that amount, she says.

“Best-case scenario? You’ve wasted your money. Worst case? You are dealing with avoidable suffering or the sudden loss of a beloved family member,” says Inness.

The VMD says there has been an increase in the number of people reporting fake treatments since it issued an alert last year. It has contacted a number of eBay sellers and retailers as a result.

What it looks like

The advert or listing offers a well-know brand at a much lower price than you usually pay. The “grey” websites selling these products will often ask for payment via wire transfer, a common tactic by criminals to avoid chargebacks on credit and debit cards.

They often use stock photos of genuine products. It is only when customers get the medication in the post that they can check whether it is fake.

When you receive the fake medication, close inspection reveals a lack of a VMD logo.

The packaging may have spelling mistakes and the logos may be blurred, with the details sometimes in a foreign language. In one case highlighted by the VMD, a counterfeit version of the Frontline flea treatment used the word “gatti” (Italian for “cats”) on the packaging.

The product may lack a batch number and expiry date, both of which legitimate products include. The fakes may also smell of white spirit and paraffin. Genuine flea treatments are odourless.

What to do

If you suspect that something you have bought is counterfeit, report it to your local trading standards.

You can also report suspicious veterinary medicines or retailers to the VMD enforcement team. You can check whether a medication is authorised via the VMD’s product information database.

A spokesperson for Boehringer Ingelheim, which produces the Frontline treatment, said it works with the VMD to have counterfeit listings removed. “We would advise customers to use our approved information on where to buy it,” the spokesperson added.

Inness says you should get your animal checked by a vet if it suffers hair loss or has a seizure following treatment with suspect medication.