This advice comes after the government recently issued an urgent warning about fake veterinary products being sold online
One person who has fallen victim to this is a man named Alan from Preston, who thought he was giving his cat Smokey his usual flea treatment.
As reported by BBC News , Smokey was violently ill, vomiting and refusing food or water within hours of applying the dose of what he thought was Frontline Plus.
He was found to have a blockage in his intestines, and emergency surgery was needed to save his life.
Speaking to BBC's Morning Live, Alan showed the vet the medication packaging, which looked almost identical to the regular Frontline Plus box, but it was found to be counterfeit.
He added: "It's horrific and I had absolutely no idea there was such a thing as fake pet medication."
According to Helen Barnham at the Intellectual Property Office (IPO), the online seller who Alan bought the medication from managed to sell 211 batches of suspected counterfeit pet medications and supplements.
She said: "If it's not from a vet or a trusted seller then please be vigilant."
Alongside this, the PDSA urges pet owners to source prescriptions from reliable companies listed on the Veterinary Medicines Directorate .
The eight signs to look out for that could mean pet medicine is fake include:
PDSA Vet Nurse Shauna Walsh said: "Counterfeit, or fake veterinary medicines can pose a serious threat to our pets' health and wellbeing. While legitimate medications play a vital role in keeping our pets healthy, fake medicines can cause severe harm or even be fatal.
"Fake medicines may contain dangerous substances, or incorrect ingredients that can make pets extremely ill. Consumption can lead to symptoms like twitching, swelling, breathing difficulties, vomiting, diarrhoea, collapse, coma and even death.
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"We always recommend that you only give your pet medication which has been prescribed by your vet.
"When fulfilling a prescription online source them from reputable companies that are on the Register of online retailers, brought to you by the Veterinary Medicines Directorate (VMD).
"If you are concerned about your pet, or suspect they are reacting badly to any medication, contact your vet immediately."