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We have previously demonstrated that subgingival levels of nitrate-reducing bacteria, as well as the in vitro salivary nitrate reduction capacity (NRC), were diminished in periodontitis patients, increasing after periodontal treatment. However, it remains unclear if an impaired NRC in periodontitis can affect systemic health. To determine this, the effect of nitrate-rich beetroot juice (BRJ) on blood pressure was determined in 15 periodontitis patients before and 70 days after periodontal treatment (i.e., professional mechanical plaque removal, oral hygiene instruction, and subgingival instrumentation), as well as in a healthy control group of 15 individuals. Additionally, subgingival and tongue samples were taken to analyse the bacterial composition with Illumina sequencing of the 16S rRNA gene. In healthy individuals, the systolic and diastolic blood pressure (SBP and DPB) decreased significantly (both P < 0.01) 90 min after BRJ intake, but not in periodontitis patients. However, after periodontal treatment, this blood pressure-lowering effect was recovered (P < 0.05 for SBP; P < 0.01 for DBP). Lower levels of salivary nitrate after identical doses of BRJ intake indicated a potentially higher NRC in healthy individuals (P < 0.05). Periodontitis-associated bacteria decreased in tongue and subgingival samples after periodontal treatment (P < 0.01). In contrast, nitrate-reducing bacteria were associated with health in both habitats, but increased only in subgingival plaque after periodontal treatment (P < 0.001). This is the first study showing that periodontitis could limit the blood-pressure lowering effects of nitrate reduction by the oral microbiota. We propose that an impaired NRC represents a potential link between periodontitis and systemic conditions, which should be confirmed in future randomized controlled trials. Future work should also aim to determine if nitrate prebiotic supplementation and/or tongue cleaning could improve the treatment of periodontitis and its associated comorbidities., Competing Interests: Competing interests: The authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: A.M. and B.T. Rosier are coinventors in a pending patent application owned by the FISABIO Institute, which protects the use of nitrate as a prebiotic and certain nitrate-reducing bacteria as probiotics. The remaining authors declare no competing interests., (© 2025. The Author(s).)