Common Veterinary Drug Interactions Reference
Clinically important drug interaction patterns to know before combining medications in dogs and cats.
NSAIDs
| Combined with | Risk |
|---|---|
| Corticosteroids | GI ulceration, perforation - AVOID concurrent use |
| Other NSAIDs (incl. aspirin) | Same - 5-7 day washout when switching NSAIDs |
| ACE inhibitors / diuretics | Acute kidney injury - monitor renal values |
| Nephrotoxic drugs (aminoglycosides, contrast) | Additive AKI risk |
Opioids / sedatives
| Combined with | Effect |
|---|---|
| Other CNS depressants | Additive respiratory depression - reduce doses |
| MAOIs (selegiline) | Serotonin syndrome with tramadol, meperidine |
| SSRIs (fluoxetine) | Serotonin syndrome with tramadol, fentanyl |
Antibiotics
| Combination | Effect |
|---|---|
| Fluoroquinolones + theophylline | Theophylline toxicity (cardiac, CNS) |
| Fluoroquinolones + warfarin | Increased bleeding risk |
| Doxycycline + dairy / antacids | Reduced absorption (chelation) - give 2 hrs apart |
| Macrolides (erythromycin, clarithromycin) + cisapride/digoxin | CYP3A inhibition - increases serum levels |
| Trimethoprim-sulfa + cyclosporine | Hyperkalaemia, renal injury |
Cardiac drugs
| Combination | Effect |
|---|---|
| Digoxin + furosemide | Hypokalaemia worsens digoxin toxicity - monitor K+ |
| ACE inhibitors + potassium-sparing diuretics (spironolactone) | Hyperkalaemia - monitor K+ |
| Diltiazem + beta-blocker | Excessive negative inotropy / bradycardia |
Other clinically important
| Combination | Effect |
|---|---|
| Metronidazole + warfarin | Increased bleeding (CYP inhibition) |
| Phenobarbital + many drugs | CYP induction - increased metabolism of co-administered drugs |
| Ivermectin + p-glycoprotein inhibitors (ketoconazole) | CNS toxicity in MDR1 mutant dogs (Collies, Shelties) |
| Activated charcoal + any oral drug | Reduced absorption - give other drugs 2+ hrs apart |
Not exhaustive - always check the formulary and product insert before combining drugs in real patients. When in doubt, separate doses by hours or pick alternative drugs.
Frequently asked questions
What's the recommended washout between NSAIDs?
Standard 5-7 day washout when switching between NSAIDs, or between an NSAID and corticosteroid. Aspirin requires longer (10-14 days) due to irreversible platelet COX-1 inhibition.
Is tramadol safe with fluoxetine?
Risky - both have serotonergic activity and the combination has caused serotonin syndrome in dogs. If the patient needs both, choose alternative analgesia (gabapentin, NSAID with appropriate washout) or stop the SSRI under cardiology / behaviour guidance.
Which dogs are at risk from ivermectin?
Dogs with the MDR1 mutation (Collies, Australian Shepherds, Shelties, Long-haired Whippets, Old English Sheepdogs). Heartworm preventive doses are safe; higher mange or treatment doses cause severe CNS toxicity. Genetic testing is widely available.