Pet Drugs Side Effects & Contraindications


Side Effect: “a secondary or peripheral effect, particularly an undesirable effect of a drug or therapy”

Contraindication: “a factor that means the administration of a drug or therapy is inadvisable”

The drugs that your pet is prescribed, as well as improving and hopefully curing your pet's illness, will have effects on other areas and other body systems as well and this can sometimes lead to side effects. All licensed or authorised drugs have been through testing to prove their safety and ascertain potential side effects and contraindications but as with people, each individual animal may react slightly differently to a drug so should be closely monitored. Your vet will discuss the risks of the drug with you and decide if the potential benefits of the drug outweigh the potential side effects.

Side effects can vary greatly in presentation and severity. Some examples of potential side effects are listed below;

  • Lethargy
  • Sedation
  • Vomiting
  • Diarrhoea
  • Increased thirst
  • Increased hunger

 Other types of side effects can also be seen such as allergic or anaphylactic reaction to specific medications, commonly penicillin-based antibiotic in people and occasionally vaccine reactions which can range in severity from mild lethargy to severe breathing difficulties.

Side effects can also be seen with the route of administration. There can be some pain, swelling or minor bleeding at injection sites and manually getting cats to swallow tablets can cause damage to the throat so should be done gently or with water to aid swallowing.

The following are a few examples of specific known drug side effects but if you have any concerns about a medication your pet is on then you should discuss it with your vet.

Non Steroidal Anti Inflammatory Drugs (NSAIDs): These are drugs used commonly as pain relief, often in the long term control of arthritis but these drugs can cause vomiting and diarrhoea and can also have effects on the liver and kidneys if used long term.

Antibiotics: Many antibiotics can cause diarrhoea but some have more specific side effects.

  • Tetracyclines: these antibiotics can cause staining of the tooth enamel in young animal and so are contraindicated in animals less than one month old.
  • Fluoroquinolones: these antibiotics can cause damage to the cartilage of growing animals so are contraindicated in cats < 8 weeks and dogs < 18mths old.

Acepromazine (ACP): This is a drug used as a sedative and premedication before anaesthesia. It can lower the seizure threshold so should be avoided in epileptic dogs and used with care in Boxers who are particularly susceptible and can faint if given too high a dose. It can also lower blood pressure so this should be monitored after administration.

Steroids: These are used as anti-inflammatories and immunosuppressive. In the short term they can cause increased thirst and hunger and in the long term can have effects in the liver and metabolism.

There are many other examples but licensed drugs have been tested for their safety and although some have narrower safety margins than others the majority are very safe. You should be aware and mindful of side effects but you should also consider the benefits of the drug prescribed and follow your vet’s advice.


There will be certain situations in which some drugs should be avoided. The following are examples

  • Young Animals: fluoroquinolone antibiotics potentially causing cartilage damage as mentioned above.
  • Pregnancy and Lactation: Steroids should not be given to pregnant animals as they can cause abortion.
  • Dehydration: NSAIDs should be avoided in dehydrated animals as that can cause kidney problems.
  • Concurrent disease: Many drugs, including trimethoprim antibiotics, should be reduced or avoided in kidney disease.
  • In combination with other drugs: Steroids and NSAIDs should not be given at the same time as that can cause gastrointestinal ulceration.
  • In certain species: Rabbits should not be given the antibiotic clindamycin orally as it can cause severe, potentially fatal diarrhoea.

Drugs may also be contraindicated as the relevant testing may not have been done so the drug company does not have adequate information about those situations.

Information on all licensed drugs can be found on their data sheet, a document produced by the drug company detailing the uses, dose rates, contraindications and side effects of the drugs. These are compiled into a compendium and published each year by the National Office of Animal Health (NOAH). The NOAH compendium of Data Sheets for  Animal Medicines is also available online for easy reference. The British Small Animal Veterinary Association (BSAVA) produce a formulary which as well as listing licensed drugs, also includes details of unlicensed medicines. This is revised regularly and is currently in its 6th edition.

The take home message is to be aware of potential side effects but not to be scared of using potentially beneficial drugs and if you think your pet is experiencing a reaction to a drug then you MUST contact your vet immediately.

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