Objectives/hypothesis: The advice given to patients regarding the shelf life of antimicrobial ear drops is based on little or no evidence. We aimed to determine appropriate recommendations for the shelf life of common antibiotic-containing topical otic solutions.
Study design: Prospective experimental design.
Methods: Ear drops containing gentamicin and ciprofloxacin were analyzed. Their effectiveness was assessed on agar plates inoculated with both Staphylococcus aureus and Pseudomonas aeruginosa. Bottles were also assessed for contamination once opened. In addition, drops used for 1 week on patients with culture-positive otorrhea were analyzed for contamination of both the bottle teat and the drop solution.
Results: Neither antibiotic showed deterioration in effectiveness against either microorganism over a 4-month period, as assessed by the size of inhibitory zones on inoculated agar plates. Of drops returned after use on infected patients, none of the cultures from the teats of the bottle grew any organisms. The drop solution itself also showed no contamination after having been opened for 4 months and used by a patient.
Conclusions: There appears to be no evidence that antimicrobial ear drops containing either gentamicin or ciprofloxacin lose their efficacy against either S. aureus or P. aeruginosa over at least 4 months from opening, or that the bottle contents become contaminated after use in patients with culture-positive otorrhea. It is likely these findings apply to other antimicrobial drops and for other organisms. This allows us to advise patients to keep unfinished drops for potential future use with beneficial economic implications.