Indiana University-Purdue University Indianapolis (IUPUI) / Objective: The aim of this study is to investigate the effects of cinnamon water extract on nicotine-induced Streptococcus mutans biofilm. This study utilized S. mutans biofilm assays with varying concentrations of nicotine/cinnamon water extract levels. Design: A preliminary experiment was carried out to confirm the most likely effective concentration of cinnamon water extract on S. mutans biofilm. Then a 24-hour culture of S. mutans UA159 in microtiter plates was treated with varying nicotine concentrations (0-32 mg/ml) in TSBS at the same time with or without the optimum cinnamon water extract concentration. A spectrophotometer was used to determine total growth absorbance and planktonic growth. The microtiter plate wells were washed, fixed and stained with crystal violet dye and the absorbance measured to determine biofilm formation. Results: The results indicated that cinnamon water extract was able to inhibit biofilm formation significantly (p<0.05) at 5 mg/ml cinnamon water extract, therefore, 5 mg/ml of cinnamon water extract was recognized as the MIC for S. mutans biofilm formation. When combined with nicotine, cinnamon water extract sub-MIC (2.5 mg/ml) demonstrated a significant inhibitory effect (p<0.05) in biofilm and total absorbance measures at high concentrations of nicotine (8 mg/ml and above). In addition, cinnamon water extract showed a significant effect (p<0.05) at very low concentrations of nicotine (0.25 and 0.5 mg/ml) in all measures (biofilm, planktonic and total absorbance). However, at low concentrations of nicotine (2 and 4 mg/ml), there was a significant increase (p<0.05) in biofilm growth, whereas planktonic growth was significantly (p<0.05) decreased at the same concentrations. Conclusion: These results provided more evidence regarding the negative effects of nicotine and also demonstrated the positive influence of cinnamon water extract in reducing nicotine-induced biofilm formation, which needs be confirmed by in-vivo studies.