Table 1.

The recent randomized and placebo-controlled clinical trials using medicinal agents for the prevention of CRC.

Reference	Agent	Target Lesion	No. of Subjects	Observation Period	Preventive Effects
2006 Bertagnolli [71]	Celecoxib (200 or 400 mg twice a day)	Sporadic colorectal adenomas	2035 subjects; placebo (679) or 200 mg (685) or 400 mg (671) of celecoxib group	Either one and three years	The estimated cumulative incidence of adenomas by year 3 was lower in those receiving 200 mg (risk ratio 0.67 [95% CI: 0.59–0.77]) and 400 mg celecoxib (risk ratio 0.55 [95% CI: 0.48–0.64]).
2006 Arber [72]	Celecoxib (400 mg/day)	Sporadic colorectal adenomatous polyps	1561 subjects (628 in the placebo and 933 in the celecoxib group)	Either one and three years	The cumulative rate of adenomas detected through year 3 was lower in the celecoxib group; relative risk 0.64 (95% CI: 0.56–0.75).
2013 Ishikawa [73]	Aspirin (100 mg/day)	Polyps in patients with familial adenomatous polyposis (FAP)	34 subjects with FAP (17 each in the aspirin and placebo groups)	Six-ten months	The increase in mean diameter of polyps tended to be greater in the placebo group compared to the aspirin group.
2014 Ishikawa [74]	Aspirin (100 mg/day)	Colorectal adenomas and adenocarcinomas	311 subjects (159 in the placebo and 152 in the aspirin group)	Two years	The subjects treated with aspirin displayed reduced colorectal tumourigenesis; adjusted OR 0.60 (95% CI: 0.36–0.98).
2016 Higurashi [75]	Metformin (250 mg/day)	Sporadic colorectal polyps	151 subjects (72 in the placebo and 79 in the metformin group)	One year	The prevalence of total polyps and adenomas in the metformin group was significantly lower; (total polyps) risk ratio 0.67 (95% CI: 0.47–0.97), (adenomas) risk ratio 0.60 (95% CI: 0.39–0.92).