Do Pet Prescriptions Have Side Effects? Everything You Should Know About Your Pet's Medications

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We are often quite conscious of the various side effects of the medications we take, but have you ever considered the side effects of your pet prescriptions? If your cat or dog is taking anything, it is a good idea to know about any secondary effects. Here are some of the most commonly taken pet prescription medications, and a list of their side effects.

We have all seen those medication ads on TV where it seems like half of the air time is taken up listing all the side effects. Granted, most of the time the side effects are rare, and when they do occur, they are not as serious as the thing they are treating, but it can be a bit disconcerting to think that you can’t just close the door on one ailment without opening the window for another. Pet prescription medications can have side effects, too.

Here are side effects to watch for while your pet is on any of these popular medications. If you notice any problems, be sure to call your vet.

Ivermectin (Heartworm Treatment)

The active ingredient in Heartgard, Tri Heart, and a number of other heartworm medications, this popular drug is used to keep the ever looming threat of heartworms at bay. It does, however, have a few possible side effects of its own.

Also, ivermectin should not be given to Australian Shepherds or most breeds of Collie, as they all suffer from a genetic predisposition to this drug, known as ivermectin toxicity. Check with your vet regarding ivermectin toxicity if you think your dog might be at risk.

Fipronil (flea and tick treatment)

Used in many popular flea and tick topicals (Frontline Plus, Sentry, PetArmor), this antiparasitic treatment is known to be effective at quickly killing all existing flea and tick life on your pet, as well as keeping them away. As for side effects and things to look out for, there are only a few, but they are still worth noting.

  • Irritation at site of application
  • Should be used with caution in geriatric, debilitated, or already sick pets
  • Cannot be given to dogs or cats under 8 weeks of age

While the side effects of fipronil are not life threatening, if you notice that your pet is showing signs of skin irritation, stop treatment and consult your vet. Also, if the solution is consumed orally or if it gets in their eyes, take them to the vet immediately, as this could have severe adverse effects. Be careful to not let your pet lick the area of application, as this could make them ill.

Imidacloprid and Permethrin

Another popular parasite preventative (used in K9 Advantix and Advantage), imidacloprid is a medication that kills fleas in all stages of life (egg, larvae, pupae, adult). Another topical treatment, the only known side effect (when used properly) is skin irritation. However, like with fipronil, should the medication get into the eyes or mouth of your pet, your vet should be consulted ASAP, as this could lead to serious complications.

Often used in conjunction with imidacloprid to offer a wider spectrum of protection, permethrin is an insecticide designed to keep ticks, mosquitoes, and other annoying (and potentially disease spreading) bugs away. This chemical can be found in popular pest control treatments like K9 Advantix. While effective at keeping insects off your pet, this potent pesticide has a few possible side effects, most of which are uncommon.

Also, permethrin is not to be used in cats, as they have a strong adverse reaction to the medication, often resulting in death. Do not give your cat any flea or tick treatment designed for a dog, as it could contain this chemical. Also, like every other topical pesticide, avoid letting your dog lick the applied area.


A commonly used non-steroidal anti-inflammatory drug (NSAID) for the treatment of arthritis and joint pain in aging dogs, carprofen (also known as Rimadyl or Novox) can be found in the medicine cabinet of many a dog parent. However, because of the very nature of this type of medication, there are a number of possible side effects, such as:

Proin (Phenylpropanolamine)

Used to treat urinary incontinence in dogs, Proin works by mimicking the effects of adrenaline or epinephrine, which helps manage the constriction of the blood vessels. The possible side effects for this medication are:

  • Restlessness
  • Irritability
  • Hypertension
  • Anorexia
  • Tremors
  • Rapid heart rate
  • Arrhythmias
  • Urine retention

Vetmedin (Pimobendan)

For the treatment of congestive heart failure in your pet, Vetmedin can help by expanding your pet’s blood vessels, allowing blood to flow more easily, and taking some of the strain off their overworked heart. An inodilator, this medication has a few possible side effects, such as:

More on Pet Medication

How to Safely Buy Budget Pet Meds
Do Cheap Pet Meds Do The Trick? Generic Vs. Brand Names
How Do Broad Spectrum Pesticides Like Flea and Tick Meds Work?

This information is for informational purposes only and is not meant as a substitute for the professional advice of, or diagnosis or treatment by, your veterinarian with respect to your pet. It has, however, been verified by a licensed veterinarian for accuracy.

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