In children, stone formation after ceftriaxone (CTRX) therapy by increasing calcium excretion was showed in the literature. In this study, we investigated the effect of CTRX, cephalothin (CP) and ampicillin (AS) therapy on urinary calcium excretion in adults. 180 participants included in the study who divided into six equal groups. The groups were; (1) CTRX therapy in stone free patients, (2) CTRX therapy in patients who have urinary stone; (3) CP therapy in stone free patients, (4) CP therapy in patients with urinary stone, (5) AS therapy in stone free patients, (6) AS therapy in patients with urinary stone. The patients received 2 g/day intravenous CTRX, CP and AS for 5 days in all groups respectively. There were no significant differences in demographic characteristics and blood biochemistry between the groups. Before and 5 days after the antibiotic therapies, the participants were evaluated by 24-h urinary calcium to creatinine ratio. Results were compared between the groups statistically by ANOVA and Tukey test. After drug therapies in group 2 and 4, the excretion of calcium to creatinine ratio in 24-h urine was more than the other groups. We found that both groups of two drugs therapy with or without stones (groups 1, 2, 3, 4), have significantly increased calcium to creatinine ratio in 24-h urine (p < 0.05). We did not find statistically difference in groups 5 and 6, after AS therapy. As a result of the study, we suggest that the patients who have taken antibiotic therapy with CTRX or CP, have an increased risk for the urolithiasis. In addition, we think that these drugs should be used carefully especially in patients with urolithiasis.