Wong 2004.

Methods	RCT
Participants	Country: China. 7 villages in Changle County, Fujian Province. 2423 healthy participants recruited for a screening endoscopic study. Participants without endoscopic lesions and positive for H. pylori infection (n = 1628) were randomised. Those with peptic ulcer were excluded. Mean age 42.2 (range 35‐65) years, 54.0% men. Method to confirm H. pylori infection: histological examination and rapid urease testing. 37.7% participants with preneoplastic lesions at baseline (gastric atrophy, intestinal metaplasia, dysplasia). Study period: 1994 to Jan 2002
Interventions	1. Omeprazole 20 mg, amoxicillin/clavulanic acid 750 mg, metronidazole 400 mg twice daily for 2 weeks (n = 817) 2. Placebo (n = 813) Follow‐up: 7.5 years
Outcomes	Incidence of gastric cancer: gastric cancer in participants with or without precancerous lesions at baseline was the secondary outcome. Histological examination at 7.5 years or, if diagnosed before 7.5 years, review of clinical records and pathology specimens by 3 blinded clinicians.
Notes	This was the first study targeted at a general population (less than 40% with precancerous lesions); a few discrepancies were seen between Wong 2002 abstract and Wong 2004 full publication. Inconsistent sample size between the full publication followed up at 7.5 years (817 vs 813) (Wong 2004) and the conference abstract followed up at 7 years (819 vs 809) (Wong 2002). We used data from the full publication.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomisation generated by computer.
Allocation concealment (selection bias)	Low risk	Randomisation was performed by drawing a sealed envelope that contained a pre‐assigned random treatment generated by computer.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Double‐blind, placebo‐controlled study, endoscopists were blinded and participants were followed by blinded clinical team, likely participants were blinded as well.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Endoscopists and histologists were blinded, clinical team in Hong Kong who reviewed the gastric cancer cases were blinded.
Incomplete outcome data (attrition bias) All outcomes	Low risk	735/817 vs 703/813 followed at 7.5 years, losses to follow‐up with reasons were balanced between 2 groups (7.7% vs 11.4%); 62% had follow‐up endoscopy, but those who refused endoscopy were followed up in clinics.
Selective reporting (reporting bias)	Low risk	Reported prespecified outcomes.
Other bias	Unclear risk	Inconsistent sample size between the full publication followed up at 7.5 years (817 vs 813) (Wong 2004) and the conference abstract followed up at 7 years (819 vs 809) (Wong 2002).