Many people in the UK use mouthwash on a regular basis. Recently, a longitudinal study conducted in Puerto Rico that monitored overweight and obese adults over a three-year period (which included periodontal and oral hygiene assessments) concluded that those using mouthwash twice daily or more at baseline had an approximately 50% increased risk of developing prediabetes/diabetes combined, compared to those who used mouthwash less than twice daily or not at all. The proposed mechanism to explain this is that mouthwash has antibacterial effects in the oral cavity, yet oral bacteria play an important role in the salivary nitrate-nitrite-nitric oxide pathway, and reduced levels of nitric oxide are associated with insulin resistance as well as adverse cardiovascular effects such as hypertension and impaired vascular function. However, methodological limitations in the study bring into question the generalisability of the findings. In this article, the important role of oral bacteria in the production of nitric oxide is discussed, and the findings of the Puerto Rican study are considered in detail. It is important that dental professionals are aware of emerging research on this topic as patients frequently ask for advice on use of mouthwash as part of their oral hygiene regime.