The relative oral bioavailabilities of ciprofloxacin and ofloxacin when they were coadministered with water or an enteral feeding product (Ensure) were assessed in 13 healthy volunteers. The area under the concentration time curve from time zero to infinity and the maximum concentration of drug in serum for both drugs were reduced by Ensure in comparison with those by water (P < 0.01). However, Ensure reduced the percent relative bioavailability of ciprofloxacin (72% +/- 14%; range, 52 to 96%) significantly more than ofloxacin (90% +/- 8.3%; range, 74 to 105%) (P < 0.005). Coadministration of Ensure significantly diminished ciprofloxacin and ofloxacin absorption, but ciprofloxacin absorption was reduced significantly more than ofloxacin absorption.