Major Veterinary Drug Interactions in Dogs and Cats

veterinary drug interaction

Use Caution with Major Drug Interactions

When treating clients with multiple or complicated conditions, it is hard to keep track of all the potential veterinary drug interactions! That being said, it’s also extremely important to be aware of how drugs interact with one another. Some of them can cause worse symptoms than the ones the client came in with. The last thing you want is for an animal to leave your practice worse than when it came in.

Whenever you prescribe multiple medications, it is best to double-check (or triple-check) for potential interactions and to use caution with drugs that are known to have major drug interactions.

With that in mind, let’s review four important veterinary drug interactions – as well as a simple way to make this whole process easier.

Remember These Four Drug Interactions

Cimetidine, Sucralfate, Ketoconazole, and Metoclopramide are all veterinary drugs with major interactions. Even for experienced veterinarians, it is always a good idea to refresh your knowledge of important drugs as well as their side effects and interactions.


Cimetidine could be the drug needed to treat your client’s mast cell tumor, but it is absolutely critical to weigh the possible side effects and veterinary drug interactions! This drug was listed in the top ten drug interactions that dogs and cats experience.

Cimetidine is an anti-ulcer drug and a major P450 enzyme inhibitor. It can only be prescribed for cats and dogs as an extra-label drug to treat mast cell tumors, gastritis (inflammation of the stomach), esophagitis (inflammation of the esophagus), and gastric or esophageal reflux. Although cimetidine is often safe under the supervision of a veterinarian, there are certain precautions and drug interactions that are important for all veterinarians to be aware of.

Cimetidine decreases the clearance of many drugs which can lead to drug toxicity. Knowing the drug dosage will aid veterinarians and vet technicians in determining the significance of potential drug interactions. Depending on the dose of chloramphenicol, cimetidine may interact negatively and cause a reduction in the number of white blood cells in the body (leukopenia). Mixing cimetidine with metronidazole or lidocaine can lead to neurologic side effects, and theophylline and aminophylline combined with cimetidine may interact to cause theophylline toxicity.

There are many other drugs that interact with cimetidine including warfarin, propranolol, diazepam, and midazolam. Cimetidine should be used with caution if administrating to older clients or those with blood diseases.

Additionally, there are some alternatives to cimetidine if your client is using another drug that might interact negatively. Anitidine, famotidine, or nizatidine are all H2 blockers, but not P450 enzyme inhibitors at therapeutic conditions. They are recommended for use if a client is on multiple drugs.


Sucralfate drug is used to treat and prevent intestinal ulcers on a short term basis. While the drug can be very effective at treating ulcers in the mouth, esophagus, stomach, and intestines, it also has potential drug interactions with several medications. Because it is an aluminum-containing drug, it can cause complexities with many other drugs in the GI tract, which decreases drug absorption.

For fluoroquinolones and tetracycline/doxycycline, sucralfate doesn’t allow for the drugs to absorb enough to have an active effect. For veterinary drugs that block H2, sucralfate delays but does not decrease the actual amount of absorption.

There are several other possible drug interactions, and some veterinarians recommend not combining the medication with other oral drugs because of the potential veterinary drug interactions. Serious side effects from taking sucralfate are rare, but some cats and dogs experience an allergic reaction and/or constipation from taking sucralfate.


Ketoconazole is used as a veterinary drug to treat certain serious fungal infections in the body and works by stopping the growth of fungus. It has a high potential to interact with other veterinary drugs. Ketoconazole is best absorbed at an acidic PH and should not be combined with drugs that will prevent the client from having an acidic PH like Omeprazole, H2 blockers, or other antacids. Ketoconazole also inhibits p-glycoprotein, an important drug efflux transporter in the intestine, kidney, and biliary tree. This can decrease the proportion that another drug enters into the circulation (bioavailability) after ketoconazole is consumed.


Taking both Cyclosporine and ketoconazole can allow clients to take lower doses of cyclosporine. There are recommended doses for cyclosporine, depending on the amount of ketoconazole a client is taking. It is important to consider all the details of drug interactions, dosage, client allergies, and potential side effects when providing service. Nausea and vomiting are the most common side effects for Ketoconazole.


Finally, another important veterinary drug with major interactions to be aware of is metoclopramide. Metoclopramide is prescribed to help with the quick passage of food through the upper digestive tract. It is sometimes used to treat disorders of the upper gastrointestinal tract like acid reflux disease. It has more side effects than most veterinary drugs, and it may increase seizure activity in animals with epilepsy and cause sedation, hyperactivity, changes in behavior, constipation, diarrhea, and abdominal pain.

Metoclopramide is a dopaminergic (D2) antagonist and prokinetic agent with several important interactions. Increased absorption of acetaminophen, aspirin and alcohol overdoses can occur through interaction of the veterinary drugs. Enhanced tremor side effects are possible when metoclopramide interacts with phenothiazines (e.g. chlorpromazine, acepromazine) or selective serotonin reuptake inhibitors (e.g. fluoxetine), or with renal insufficiency.

Metoclopramide may interact with all of the following drugs: cimetidine, acetaminophen, aspirin, central nervous system depressant or sedative, derivatives of tetracycline, anticholinergic, anticoagulant, and diazepam.

How to Stay Informed about Drug Interactions?

Obviously, there are a lot of drug interactions to remember – and, unless you’re gifted with a photographic memory, you probably won’t be able to remember all of them. Chances are, you won’t even remember all of the veterinary drug interactions we’ve discussed in this article.

That’s why it’s important to be able to provide veterinary client education and to have quick access to veterinary drug information that you can communicate to your client so that they are able to make the most informed choice. Searching for all of your drug interaction information on Google and looking up articles such as this one is a slow and ineffective way to receive information about drug interactions, dosages, and side effects.

Fortunately, there’s a better way to stay informed about drug interactions- VPR Cloud.

VPR Cloud is an online and comprehensive veterinary pharmacy reference that provides constantly updated drug information. VPR Cloud will make keeping up-to-date on all the potential and major veterinary drug interactions much easier for you and your veterinary practice. If you want to find out about all that VPR Cloud offers, visit and sign up for a free 14-day trial!