Tehran University of Medical Sciences Journal of Craniomaxillofacial Research 2345-5489 12 2 2025 09 23 79 85 EN Somayeh Hosseini Tabatabaei Oral and Dental Disease Research Center, Faculty of Dentistry, Zahedan University of Medical Sciences, Zahedan, Iran; Department of Restorative Dentistry, Faculty of Dentistry, Zahedan University of Medical Sciences, Zahedan, Iran. Atie Ghorbani Faculty of Dentistry, Zahedan University of Medical Sciences, Zahedan, Iran. Amir Hossein Khazaei Oral and Dental Disease Research Center, Faculty of Dentistry, Zahedan University of Medical Sciences, Zahedan, Iran. 2025 06 04 2025 07 05 Introduction: The purpose of this study was to investigate the effects of 15% carbamide peroxide, orange juice, and Cola on the microhardness of composite resin restoration material.  Materials and Methods: In this in vitro study, forty disk-shaped composite samples were prepared and randomly classified into four groups (n=10); the artificial saliva (control), bleaching agent (15% carbamide peroxide), orange juice, and Cola. Vickers microhardness was measured on the surface of the samples before and after immersion for 6 and 48 hours.  Results: The microhardness values of the 15 % carbamide peroxide, Orange juice and Cola groups were significantly lower after 48 hours compared to the artificial saliva group (P=0.003, P=0.002, P=0.001, respectively). However, these differences were not statistically significant after 6 hours of immersion (P=0.068). When comparing the microhardness values of these groups over time, as expected, these measures significantly decreased, except for the 15 % carbamide peroxide group in which the mean microhardness value did not significantly decrease from baseline after 6 hours immersion (P=0.106). However, there was a significant difference after 48 hours compared to baseline and 6 hours immersion (P=0.001, P=0.004).  Conclusion: This suggests that 15 % carbamide peroxide gel can be employed as a bleaching agent in cases with composite restorations for a limited amount of time without significant deterioration of the microhardness.  https://jcr.tums.ac.ir/index.php/jcr/article/view/546 https://jcr.tums.ac.ir/index.php/jcr/article/download/546/445