Unless you work in a medical or pharmaceutical field, chances are you might not know a lot about the medications that your vet prescribes to a poorly pet. It can be confusing to be given tablets when you’re not entirely sure about the whats and whys of the medication, especially as it’s difficult to take it all in when you’re worried about your pet. It can be even more confusing if you have to return to the vets and they give you some medication that’s a bit different to what you had before, and this time it works. So, why didn’t they just give you that in the first place?

How do vets decide what medication to give?

The choice of what medication to use in each scenario depends on many individual factors. Your vet will make choices depending on the animal they are treating, the condition, any concurrent medical factors and more.

Here are some of the points that a vet must consider each time they dispense medication.

The condition 

The medication prescribed needs to be effective for that health concern. For example, antibiotics treat bacterial infections, anti-inflammatories act against inflammation and chemotherapy targets cancers. Medications are also often very specific – “heart drugs”, for example, can range from medications which reduce fluid retention to drugs which target the action of the heart muscle itself.

The species, age and breed of the patient

Some drugs are toxic to certain animals, others just don’t work well. Some breeds have genetic quirks which make them more susceptible to side effects from certain products. Young animals may have their growth affected by some products, or be more vulnerable to adverse effects. 

Other health conditions 

Care must be taken when prescribing if the animal has a concurrent medical condition, or is on other medications. This includes pregnancy and lactation. 

Doses and formulations available

For example, tablets, syrups, liquids, injections… And of course how much of the active ingredient is in them. The choice will depend on the pet’s size, species and age.

Adverse reactions 

Virtually all drugs have some form of side effect, which will need to be discussed with the pet owner.

Individual factors 

Each pet and owner have differing needs. Some pets will tolerate tablets, others won’t. Pets may be on specialised diets, have allergies or other specific requirements. There may be financial constraints, time limitations or multiple carers. 

Medicines should all have a marketing authorisation (a “license”), which informs the user which conditions the drug treats, and in which species. According to the Veterinary Medicines Regulations (VMR), vets must follow a ‘cascade’, which is a decision-tree about suitability of drugs. Ideally, each medicine prescribed should have a licensed product to treat that certain health condition in that exact species. Sometimes, there is no drug licensed for that purpose, and so the cascade dictates that a vet may use a drug licensed for that condition in a different species, or for a different condition in the same species. If there is no such product, there are other options such as using human drugs or drugs licensed in other countries. 

Why don’t vets just use strong drugs?

The term ‘strong’ is often misused, especially in the context of pharmaceuticals. Strength can mean a higher dosage, a good efficacy or a change in type of medication and so is not a very specific term to use when prescribing drugs.

Vets will always prescribe the medicine which they feel is best suited for purpose, that will target the health condition appropriately without causing significant safety concerns. 

Some medical conditions have a very limited range of treatments available, which can make the decision-making process easier, but also means there is little support if the primary treatment fails, or is unsuitable. Other classes of drug, such as antibiotics, have a range of products. This allows for more choice, but also means that some products may not be entirely effective in specific scenarios.

Let’s look at some examples.

Example: antibiotics

There are multiple different types of antibiotics used to treat bacterial infections in pets. These antibiotics vary in many ways. They kill or immobilise bacteria in different ways, they access varying areas of the body, they target different bacterial species and they require dosing at different intervals throughout the day. 

Your vet may choose a different antibiotic for a urinary infection than a bite wound. This is because of the differences in which bacteria are likely to be causing these infections and which drugs will penetrate well to these areas of the body. Antibiotics may be prescribed based on an examination alone, or they may require samples taken and sent to a laboratory for culture, revealing which specific bacterial strains are causing the problem. Ideally, all potential infections would be swabbed and sent away for more specific treatment, but this is both more expensive and takes some time, so is not always the most appropriate action.

If your pet has an infection which doesn’t clear, a chronic or recurrent issue, the type of antibiotic may need to be changed, or an extended course of treatment given. This doesn’t mean that the incorrect medicine was prescribed, or a ‘weak’ drug used. Medical conditions are not static: they are constantly evolving and reacting to factors such as the body’s immune system, outside influences and bacterial mutations and growth. 

Example: anti-inflammatories

Another common type of drug is the non-steroidal anti-inflammatory (NSAID). These medications are used to reduce inflammation, control pain and treat fevers. There are multiple different types of NSAIDs, and your vet will prescribe one which is suitable. This may depend on your pet’s species, size, specific health condition and other factors such as compliance with tablets. Your pet may need to change to a different type if the medication is ineffective or is not well tolerated. Again, this is likely not because the initial product wasn’t strong enough, but because subtle differences in individual physiology can cause differences in how animals respond to certain drugs. 


The prescribing of medication is a complex issue, requiring expert knowledge. Although there may be multiple types of similar drugs, the classification of medicines into ‘strong’ and ‘weak’ is rarely appropriate and instead depends on a multitude of factors. Your vet will always prescribe the medication which they feel is most suitable for your pet, their specific needs and your lifestyle. If you’d like more information about the type of medication your pet has been given, or have specific concerns, your vet will be happy to guide you through the decision-making process. 

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