Background. Metronidazole treats obligate anaerobic bacterial and protozoal infections, with an elimination half-life of around eight hours. The long elimination half-life, the favorable ratio of steady-state serum levels to minimum inhibitory concentration, and the presence of active metabolites lead to consideration of metronidazole use at 12-hour dosage intervals. This systematic review aimed to compare the clinical outcomes of twice-daily and thrice-daily metronidazole dosing. Methods. We used the Preferred Reporting Items for Systematic Reviews and Meta-Analyses checklist. PubMed, Scopus, Science Direct, the Cochrane Database of Systematic Reviews, and the Cochrane Central Register of Controlled Trials to systematically identify all relevant studies published up to June 16, 2023. Results. The final analysis included two published retrospective cohort studies; a single site (n = 200) and a multisite study (n = 85) of ‘good’ quality, as measured by the Newcastle-Ottawa scale. The reported baseline characteristics of the 8-hour and 12-hour dosing groups were comparable, and neither study identified significant differences in primary and secondary clinical outcomes. Meta-analysis of the need to escalate antibiotic therapy also showed no statistically significant differences using the Mantel-Haenszel fixed-effect method (95% CI: 47.6% lower to 6.4 times higher risk, p = 0.342) and inverse-variance method (RR: 1.87, 95% CI: 0.52-6.65, p = 0.336). Conclusions. Dosing metronidazole every 12 hours is as effective as every 8-hour dosing for anaerobic infections. Healthcare systems may consider the adoption of every 12-hour metronidazole dosing with continued evaluation of patient outcomes.