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Aims To compare the pharmacokinetics of amoxicillin (AMX) in obese and nonobese subjects, given as single dose 875-mg tablets. Methods A prospective, single-centre, open-label, clinical study was carried out involving 10 nonobese and 20 obese subjects given a dose of an AMX 875-mg tablet. Serial blood samples were collected between 0 and 8 hours after administration of AMX and plasma levels were quantified by liquid chromatography-tandem mass spectrometry. The pharmacokinetic parameters (PK) were calculated by noncompartmental analysis and means of the 2 groups were compared using Student t-test. Analysis of correlation between covariates and PK was performed using Pearson's correlation coefficient. Results Ten nonobese subjects (mean age 30.6 ± 7.12 y; body mass index 21.56 ± 1.95 kg/m2 ) and 20 obese subjects (mean age 34.47 ± 7.03 y; body mass index 33.17 ± 2.38 kg/m2 ) participated in the study. Both maximum concentration (Cmax ; 12.12 ± 4.06 vs. 9.66 ± 2.93 mg/L) and area under the curve (AUC)0-inf (34.18 ± 12.94 mg.h/L vs. 26.88 ± 9.24 mg.h/L) were slightly higher in nonobese than in obese subjects, respectively, but differences were not significant. The volume of distribution (V/F) parameter was statistically significantly higher in obese compared to nonobese patients (44.20 ± 17.85 L vs. 27.57 ± 12.96 L). Statistically significant correlations were observed for several weight metrics vs. AUC, Cmax , V/F and clearance, and for creatinine clearance vs. AUC, Cmax and clearance. Conclusion In obese subjects, the main altered PK was V/F as a consequence of greater body weight. This may result in antibiotic treatment failure if standard therapeutic regimens are administered.