Inclusion Criteria:  Participants will have all of the following:  pathologically-confirmed advanced adenoma (defined as any one of >/= 10mm diameter, >/= 3 adenomas, high-grade dysplasia, tubulovillous or villous adenoma) 5 diagnosed at first colonoscopy in the National bowel screening programme within the previous 6 months; no residual colorectal adenomas; next colonoscopy planned within 5 years; willing and able to comply with all trial requirements, including treatment and assessments; signed written, informed consent.  Exclusion Criteria:  Participants will have none of the following:  currently taking selenium supplements (including in multivitamins) or within the last 6 weeks; previous history of colorectal adenoma, colorectal cancer or familial colorectal cancer syndrome; other significant cancers within the last 5 years; concurrent medical conditions that, in the opinion of the investigators, would compromise either participant safety or the integrity of the data (e.g., malabsorption); male participants with a female partner of childbearing potential or pregnant, and unwilling to remain abstinent or use effective contraception (including barrier contraception with a pregnant partner).