Table 1. Characteristic of the included studies.

Author (year)	Country/region	Database	Median follow-up	Study type	Sample size	No. of aspirin users	No. of aspirin non-users	Median age	Adjusted variable	NOS score
Cardwell et al, 2014	Northern Ireland	National Cancer Data Repository	NR	Nested case–control study	9089	2387	6711	NR	Including surgery, chemotherapy, radiotherapy, statin use, metformin use, and comorbidities, including myocardial infarction, cerebrovascular disease, congestive heart disease, chronic pulmonary disease, peripheral vascular disease, peptic ulcer disease, renal disease and rheumatological disease, and diabetes	7
Liao et al, 2012	US	Nurses' Health Study and the Health Professionals Follow-up Study	14.75	Cohort study	964	561	403	68	Disease stage, initially included age, sex, year of diagnosis, aspirin use after diagnosis, tumour location, tumour differentiation, body mass index, microsatellite instability status, sex	8
Reimers et al, 2014	Netherlands	PHARMO	NR	Cohort study	999	180	794	NR	Sex, age, comorbidity, year of incidence, histological grade, stage, and chemotherapy	7
McCowan et al, 2012	UK	The Health Informatics Centre	4.22	Cohort study	2990	1340	1650	73	Age, sex, social class, stage of disease diagnosis	8
Bastiaannet et al, 2012	Netherlands	PHARMO	3.5	Cohort study	4481	3305	1176	69	Sex, age, comorbidity, year of incidence, grade, stage and treatment	6
Walker et al, 2012	UK	General Practice Research Database	NR	Cohort study	13 994	2619	1365	NR	Age, gender, smoking, BMI, alcohol use and comorbidity	8
Chan et al, 2009	US	Mailed questionnaire	11.8	Cohort study	1279	549	730	60.7	Age, sex, date of diagnosis, stage of cancer, site of primary cancer, histological grade of cancer, aspirin use, smoking, BMI, cancer in a parent or sibling	6

Abbreviations: BMI=body mass index; NR=not reported.