Ciprofloxacin × Theophylline

Ciprofloxacin strongly inhibits CYP1A2 – the main theophylline metabolic route. Theophylline plasma levels rise 2- to 3-fold. Theophylline has a narrow therapeutic window, so any rise is clinically significant.

Nausea, tachycardia, tremor, insomnia, agitation. At levels above 30 mg/L: seizures and ventricular arrhythmias. Older patients and chronic kidney disease: earlier symptoms.

Avoid the combination. Alternative antibiotics: a cephalosporin or levofloxacin (weaker CYP1A2 inhibition). If ciprofloxacin is needed, halve theophylline and check level at 3–5 days.