Theophylline and enrofloxacin in a dog with chronic bronchitis
10-year-old Cocker Spaniel with chronic bronchitis controlled with low-dose prednisolone and extended-release theophylline at a standard dose. Enrofloxacin was added 4 days ago for bronchopneumonia. Today he is restless, panting, has vomited three times, and has tremors and sinus tachycardia at 210 bpm.
| Parameter | Result | Reference |
|---|---|---|
| Temperature | 39.2 °C | 38.0–39.2 |
| SpO₂ on room air | 96% | > 95 |
| Potassium | 3.4 mmol/L | 3.5–5.1 |
| Glucose | 105 mg/dL | 70–120 |
| Thoracic radiographs | Improved alveolar pattern | — |
What is the most likely cause?
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A. Worsening bronchopneumonia with hypoxaemia and sepsis
SpO₂ is normal, temperature has barely risen and the radiograph shows improvement: this does not support progression of the pneumonia.
B. Neurotoxicity from enrofloxacin alone at a standard dose
Fluoroquinolones can lower the seizure threshold at high doses, but alone they do not explain the vomiting and tachycardia; here the problem is their effect on another drug.
C. Adverse effects of prednisolone at anti-inflammatory doses
Panting can be caused by corticosteroids, but not tremors, repeated vomiting or a sudden-onset tachycardia of 210 bpm after a change in treatment.
D. Theophylline toxicity: enrofloxacin inhibits its metabolism
Correct. Enrofloxacin (and ciprofloxacin) inhibits the hepatic cytochrome that metabolises theophylline, whose concentration rises above its narrow therapeutic range: restlessness, vomiting, tachyarrhythmias, tremors and even seizures. Stop or reduce the theophylline or change the antibiotic.
lightbulb Key takeaway
Theophylline has a narrow therapeutic range and many interactions: fluoroquinolones, cimetidine and some macrolides increase its concentration. If you must combine it with a fluoroquinolone, reduce the theophylline dose and, where possible, measure serum concentrations.
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Updated: November 1, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.