9 human medicines that can be fatal to pets.

tablets imageA recent article in the Veterinary Times (yes, we have our own newspaper!) reported that almost 10% of pet owners admitted using human medicines to treat their pets. Now, a lot of people certainly do think that if you can buy a drug “over the counter” it is automatically safe for pets… but this is NOT true!

Yes, it is true that drugs sold over the counter for human use are relatively safe for humans (but only relatively – in 2014, there were 200 human fatalities due to paracetamol poisoning in England and Wales alone). However, dogs and cats are not human – they respond to medications differently for two reasons:

  1. Body size – the safe dose of a medication is expressed as “mg/kg” – in other words, the amount of the active ingredient per kg of body weight. So, the maximum safe dose of a substance for a 70kg human is going to be about 20 times higher than it is for a 3.5kg cat.
  2. Biological differences – humans are generally better at dealing with poisons than any other mammal species. A useful rule of thumb used by medical and veterinary toxicologists is that “if it’s poisonous to humans, it’ll kill pets; but if it’s safe for humans… it may still kill animals”.

Any medication will have potential side effects (if it doesn’t, that probably means it isn’t actually doing anything), and if the dose is high enough, these side effects may be fatal. Cats and dogs, just like us, rely on their livers to convert drugs into different chemical forms so they can be excreted (usually in urine, or in bile in the faeces). If these processes aren’t fast enough, or complete enough, the drug, or it’s breakdown products, build up in the animal’s system, and may be harmful or even fatal.

The other issue is legal – it is a criminal offence for any person other than a vet to give human medicines to animals. Even vets are only permitted to do so if there is no other available and suitable option.

So, let’s look at the most commonly misused and dangerous human drugs…


According to the survey, nearly 4% of the British public give antihistamines to their pets. Now, antihistamines do have a place in the treatment of some allergic conditions in dogs and cats; however, the dose is really important! Excessive consumption in humans is likely to lead only to drowsiness.

In dogs and cats, however, it can lead to vomiting, drooling, wobbliness, convulsions and then a coma.


Astonishingly, despite years of education, nearly 3% of pet owners would give paracetamol to their dog or cat. The problem with this medicine is that the liver can only deal with so much of it at a time – and if too high a dose is taken, it is fatal (and in a very unpleasant way).

In dogs, there are licensed forms that are safe to use, but only at doses that are much, much lower than in people. Overdose leads to liver failure, damage to the blood cells (so the gums turn brown), difficulty breathing, kidney failure, and death.

In cats, there is no safe dose. Even very small amounts will lead to blood damage, facial and paw swelling, liver and kidney failure, difficulty breathing and then death – recovery once symptoms appear is unlikely.

Ibuprofen and Aspirin

Ibuprofen and Aspirin are also commonly misused painkillers (just over 3% of owners give them), They are part of a family of drugs called non-steroidal anti-inflammatory drugs or NSAIDs – other medications in the group include diclofenac and naproxen. Some of these drugs (such as carprofen or meloxicam) are commonly used in veterinary medicine, but the human forms are usually more toxic to pets.

In both dogs and cats, poisoning may lead to vomiting and diarrhoea (often with blood), kidney and liver failure, and sometimes convulsions. Aspirin can also lead to an abnormal rise in body temperature, difficulty breathing, and abnormal bleeding.

Opiate Painkillers

A lot of “extra strength” painkillers also include codeine, a morphine-based drug. Other medicines in this family include tramadol, fentanyl, oxycodone, and methadone. Although they are sometimes used by vets, the dose has to be carefully monitored.

In dogs, overdose leads to drowsiness, vomiting, wobbliness, potentially coma, difficulty breathing and death from suffocation.

Cats suffer similarly, but are much more sensitive to the effects of many opiates than dogs are.

Other potentially toxic medicines include…

Caffeine pills (which cause stomach upsets, wobbliness, seizures, and heart problems).

Human-strength Glucosamine Joint Supplements (which lead to a glucosamine overdose – in dogs this is associated with liver failure).

High-Strength Iron Supplements – these are found in a lot of body-building supplements. In dogs, excessive iron leads to stomach upsets, dehydration, abnormal bleeding, kidney and liver failure, coma and death.

Psoriasis Creams (containing vitamin-D) are sometimes used by well-meaning owners to soothe itchy skin. However, unlike (most!) humans, dogs won’t leave creams on their skin – they lick it off. If eaten, these creams can cause vitamin-D poisoning, leading to kidney failure, muscle spasms, convulsions, difficulty breathing, heart failure and death.

The bottom line is, never, EVER give human medicines to your pet unless one of our vets has specifically advised you to – and then only use the exact dose recommended. If you think your dog or cat may have had access to medication, call us anytime day or night so we can take care of them – don’t wait for symptoms to begin!

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