Based on the size of the RERI estimate, the synergy between periodontitis status and CRP levels was more prominent in the participants aged ≥ 60 years (RERI, 1.30; 95% CI, 0.40–2.53) and those with hypertension (RERI, 0.76; 95% CI, -0.21 to 1.97) than that in the total population (Fig 3B). The gene discussed is CRP; the disease is hypertensive disorder.