Faecal excretion of ciprofloxacin after a single oral dose and its effect on faecal bacteria in healthy volunteers.

High concentrations of ciprofloxacin have been shown to persist in the faeces of volunteers for several days after a week of oral treatment with this drug, which was also found to have a prolonged effect on aerobic Gram-negative intestinal bacteria. To determine whether a shorter course of ciprofloxacin would have the same prolonged effect, we treated ten healthy adult volunteers with a single oral dose of 750 mg ciprofloxacin and found that this was not followed by any significant changes in the counts of anaerobes or streptococci, but that there was a mean decrease of 2.5 log10 cfu/ml in the counts of faecal Enterobacteriaceae, which lasted for a full week. We attributed this to the persistence of high faecal ciprofloxacin concentrations for several days in all the volunteers. We did not observe any significant increase in the MICs of ciprofloxacin for faecal Enterobacteriaceae, or any faecal overgrowth of staphylococci, fungi, Pseudomonas aeruginosa or Clostridium difficile.