Vets Prescribe Human Drugs to Pets, But Don't DIY

When your dog starts limping or your cat comes down with a sniffle, it's natural to worry. For many families, pets are more than just animals - and we want them to have a standard of medical care similar to our own.

But it can still be surprising when the vet prescribes a medication that looks identical to something in your own bathroom cabinet.

Many human medicines are safe and effective for pets when used under veterinary guidance. But others can be harmful due to differences in how animals process drugs. So sometimes, pets need their own medicines.

So let's examine the differences between drugs for humans and animals - and why you shouldn't just give a pet your own medications.

Pet and human medicines explained

In Australia, the Therapeutic Goods Administration approves and regulates drugs for humans. The Australian Pesticides and Veterinary Medicines Authority APVMA does the same for pet drugs.

While both agencies consider efficacy and safety in deciding whether to approve a product, the APVMA also considers environmental impact. For example, medicines given to animals - especially when given at scale on farms - can leach into waterways, affecting aquatic life and water quality.

The market for animal medicines is smaller than that for humans, making them less cost-effective to develop. Sometimes, no medicine exists for an animal condition and vets may need to use a human medicine.

For certain diseases and conditions, vets are legally permitted to dispense human medicines for pets through a process called off-label prescribing.

There are also medicines approved for both humans and pets. They include classes of antibiotics, antidepressants, corticosteriods (anti-inflammatory drugs), antiparasitics and chemotherapy drugs.

For example, doxorubicin is a chemotherapy drug used in humans to treat cancers including those of the lungs and bone. In dogs, it is commonly used to treat lymphomas, melanomas and cancers of the bone , among others. In both humans and dogs, doxorubicin is used to treat mammary gland (breast) cancer.

Similarly, ivermectin can be used to treat parasite infections such as scabies in humans and animals.

Beware the safety issues

While many drugs are shared between humans and pets, not all are safe. In fact, some common household medications can badly harm or kill animals.

The painkillers ibuprofen and paracetamol are toxic to both dogs and cats. They can cause damage to the animal's stomach and kidneys, and may kill them.

This is because dogs and cats break down medicines different to the human body. For example, the proteins in a cat's liver are different from the human liver, so they can't break down paracetamol. It can damage their red blood cells and reduce their body's ability to carry oxygen.

And the situation can differ between animals. The flea and tick medication permethrin , for instance, is safe for dogs but highly toxic to cats - potentially causing tremors, seizures and death.

And pets are far more sensitive to drug dosages than humans, so even small quantities of the wrong medicine can be fatal.

Animal-only medicines

Pets may also be given medicines no longer used for humans, or one specifically developed for animals.

Carprofen is a non-steroidal anti-inflammatory drug previously used in humans but is now only prescribed for dogs. A vet might prescribe it for pain or inflammation.

We don't use it in humans anymore because it's expensive to manufacture. But it's still used for dogs because it's effective, and alternatives such as paracetamol and ibuprofen aren't suitable for them.

Typically, medicines are developed for pets only when they address a condition specific to animals.

For example, humans don't usually suffer from heartworm , but infection in pets is common. The arsenic-based drug melarsomine was designed specifically for animals and treats heartworm in adult dogs.

And of course, humans should not take medication prescribed for their pet .

While pet medicine may look similar to yours, there may be differences in formulation or dose that can cause side effects or toxicity in humans.

What to remember

If your pet is sick or injured, never give them a drug out of your own medicine cabinet - even if the vet has previously prescribed them the medication.

Take your animal to the vet. They will advise on the most appropriate treatment and dose, so you don't do your pet further harm.

Nial Wheate in the past has received funding from the ACT Cancer Council, Tenovus Scotland, Medical Research Scotland, Scottish Crucible, and the Scottish Universities Life Sciences Alliance. He is a fellow of the Royal Australian Chemical Institute. Nial is the chief scientific officer of Vaihea Skincare LLC, a director of SetDose Pty Ltd (a medical device company) and was previously a Standards Australia panel member for sunscreen agents. He is a member of the Haleon Australia Pty Ltd Pain Advisory Board. Nial regularly consults to industry on issues to do with medicine risk assessments, manufacturing, design and testing.