Wong 2012a.

Methods	RCT
Participants	Country: China. 12 villages in Linqu County, Shandong Province. 1024 participants with H. pylori infection and advanced gastric lesions (severe chronic atrophic gastritis, intestinal metaplasia, indefinite dysplasia, or dysplasia); mean age 53.0 (range 35 to 64) years, 46.4% men. Method to confirm H. pylori infection:13Carbon‐urea breath testing. Histology was also performed. 100% participants with preneoplastic lesions at baseline Study period: 2002‐2009
Interventions	Anti‐H. pylori treatment and/or COX‐2 inhibitor or placebo in a 2x2 factorial design: 1. Omeprazole 20 mg, amoxicillin 1 g, clarithromycin 500 mg + placebo twice daily for 1 week (n = 255) 2. Placebo (n = 258) 3. Omeprazole 20 mg, amoxicillin 1 g, clarithromycin 500 mg twice daily for 1 week + celecoxib (n = 255) 4. Celecoxib + placebo (n = 256) Follow‐up: 5 years
Outcomes	Gastric cancer: histological examination at 5 years. Regression or progression of advanced gastric lesions.
Notes	2x2 factorial design, in the main analysis we did not include data from the 2 arms that used celecoxib, only data for H. pylori eradication only vs placebo only (n = 513). The celecoxib arms were included in a sensitivity analysis. Eradication rate: 63.5%
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomised treatment assignments were generated blindly by Westat Inc, (Rockville, MD, USA) after eligibility was determined.
Allocation concealment (selection bias)	Low risk	Central randomisation was used.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Double‐blind, placebo‐controlled trial. All placebos were identical in number, size, and colour to the original medications. Both participants and investigators were blinded to treatment assignments.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Endoscopists and pathologists were blinded.
Incomplete outcome data (attrition bias) All outcomes	Low risk	234/258 (90.7%) vs 233/255 (91.4%) participants had follow‐up gastric biopsy data and the authors reported outcomes for these, losses to follow‐up with reasons were balanced and provided, 89.7% completed the repeat upper endoscopy and histology.
Selective reporting (reporting bias)	Low risk	Reported prespecified outcomes.
Other bias	Low risk	No other risk of bias was noted.