OBJECTIVE To determine the effect of Ensure on the relative oral bioavailability of gatifloxacin in healthy volunteers. METHODS Single-dose, randomized, crossover study. METHODS University-affiliated research center. METHODS Twelve healthy volunteers (six men, six women) aged 18 years or older with no clinically significant abnormal findings on physical examination or in medical history. Intervention. Subjects consumed 120 ml of study liquid-water or Ensure-every 30 minutes for five doses. With the second dose, subjects ingested a single gatifloxacin 400-mg tablet that had been uniformly crushed and mixed into the study liquid. RESULTS Serial blood samples were collected for 48 hours, and gatifloxacin concentrations were determined by high-performance liquid chromatography. Pharmacokinetic data were analyzed by using noncompartmental methods. Maximum serum concentration (Cmax) and area under the serum concentration-time curve from zero to infinity (AUC0-infinity) were tested for bioequivalence after log-transformation of the data. Comparison of parameters for gatifloxacin administered with water versus those with Ensure showed that Cmax (4.35 +/- 0.90 vs 2.41 +/- 0.58 mug/ml, p<0.0001) and AUC(0-infinity) (42.4 +/- 10.1 vs 31.3 +/- 8.3 mg*hr/L, p<0.0001) were significantly decreased with Ensure, and bioequivalence was not achieved for either parameter. The geometric least squares mean ratio was 0.553 (90% confidence interval [CI] 0.501-0.611) for Cmax and 0.730 (90% CI 0.664-0.802) for AUC0-infinity. The median time to reach Cmax was significantly prolonged when gatifloxacin was administered with Ensure versus that with water (2.5 hrs vs 1.0 hr, p=0.006). CONCLUSIONS The Cmax and AUC0-infinity of gatifloxacin were significantly decreased when the drug was administered with Ensure. The clinical significance of these findings will depend on the offending pathogen and its susceptibility to gatifloxacin.