Purpose: To show the clinical and radiographic results of intrabony peri-implantitis lesions treated nonsurgically with adjunctive systemic metronidazole with a mean follow-up of 50 months. Materials and Methods: Subjects diagnosed with peri-implantitis (probing depth ≥ 5 mm with concomitant bleeding on probing and/or suppuration) with radiographic evidence of intrabony defects > 2 mm were included in this study. Implants affected received one session of nonsurgical mechanical debridement with ultrasonic and steel curettes. Systemic metronidazole was immediately prescribed for 7 days. Clinical and radiographic variables were registered at baseline and at the end of follow-up. Results: Eighteen patients and 25 implants were included in this investigation. At baseline, the mean radiographic bone level and intrabony component were 4.52 ± 2.14 mm and 3.93 ± 1.51 mm, respectively. After a mean follow-up of 54 (range: 12 to 108) months, the mean radiographic bone level reduction was 2.6 ± 0.21 mm, and the intrabony component reduction was 2.85 ± 0.37 mm (P .05). A mean probing depth reduction of 4.66 ± 1.33 mm was observed (P .05). Conclusion: Within the limits of this study, nonsurgical treatment of peri-implantitis with the adjunctive administration of systemic metronidazole has shown potential effectiveness in terms of probing depth and radiographic defect reduction after a mean follow-up of 54 months. Keywords: metronidazole, nonsurgical peri-implant therapy, peri-implantitis, radiographic bone loss