Background: Use of multivitamins may reduce the risk of colorectal adenoma, but the duration of use needed is unclear. Methods: We prospectively examined years of multivitamin use and risk of colorectal adenoma among 43 641 women who had a first endoscopy between 1991 and 2007 in the Nurses' Health Study II. Use of multivitamins was assessed through biennial questionnaires since 1989. Results: We documented 2277 colorectal adenoma cases. Reporting multivitamin use at any time during the study period compared with never reporting its use was associated with a reduced risk of adenoma (multivariable relative risk (RR)=0.86, 95% confidence interval (CI): 0.76–0.97). There was no clear trend with duration of multivitamin use: years of use compared with never use, ⩽4 years (RR=0.84, 95% CI: 0.74–0.96), 5–9 years (RR=0.89, 95% CI: 0.77, 1.02), 10–14 years (RR=0.86, 95% CI: 0.74, 1.01), 15–19 years (RR=0.85, 95% CI: 0.70, 1.02), and 20–26 years (RR=0.80, 95% CI: 0.64, 1.01); (P trend=0.87). The strongest associations (years of use vs never user) were for size of adenoma: large (⩾1 cm) <4 years (RR=0.75, 95% CI: 0.58–0.96) and in alcohol users (⩾1.4 g per day) 20–26 years (RR=0.67, 95% CI: 0.49–0.91). Conclusion: Our findings suggest that use of multivitamins is associated with lower risk of colorectal adenoma, even with relatively short duration of use
