White 2002Low.

Methods	Randomised, double‐blind, placebo‐controlled trial with parallel group design (two intervention groups).
Participants	Country: United Kingdom. Number of participants randomised: 100, mean age 63, 58% males. Inclusion criteria: patients with Barrett's oesophagus on long‐term (> 12 months) proton pump inhibitors (PPI) treatment attending for surveillance endoscopy. Exclusion criteria: pregnancy or lactation, previous gastric surgery, serious cardiovascular, respiratory, renal or neurological diseases, history of alcohol or drug abuse or use of non‐steroidal antiinflammatory drugs.
Interventions	The participants were randomly assigned to receive: group 1: vitamin C 100 mg, vitamin E 200 mg, n = 50; group 2: placebo, n = 50. Participants were supplemented and followed 12 weeks.
Outcomes	The primary outcome measure was: changes in putative markers of DNA damage in gastric tissue following supplementation with vitamins C and E.
Notes	Plasma vitamin C and E were measured to assess patient compliance. Seventeen participants failed to attend for endoscopy and 11 participants were not compliant with the trial medication, leaving 72 participants for final analyses. Overall, 14 participants in each group were lost to follow‐up. Additional information about all‐cause mortality obtained from the authors.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Sequence generation was achieved using computer random number generation.
Allocation concealment (selection bias)	Low risk	Allocation was controlled by a central and independent randomisation unit, so that intervention allocations could not have been foreseen in advance of, or during, enrolment.
Blinding (performance bias and detection bias) All outcomes	Low risk	The trial was described as blinded, the parties that were blinded, and the method of blinding was described, so that knowledge of allocation was adequately prevented during the trial.
Incomplete outcome data (attrition bias) All outcomes	Low risk	The numbers and reasons for dropouts and withdrawals in all intervention groups were described.
Selective reporting (reporting bias)	Low risk	Pre‐defined, or clinically relevant and reasonably expected outcomes are reported on.
Other bias	Low risk	The trial appears to be free of other components that could put it at risk of bias.