Table 1. Study overview (case-control studies included in systematic review).
| No | Reference | Region | Time frame of study | Design of research study | Gender | Case Definition (or at risk population) | Controls | Habit | Adjusted factors in analysis |
| 1 | Madani A, 2012[45] | India | 2005–2006 | CCP | M+F | Histopathologically diagnosed incident cases | Relatives, friends, care-takers of cases without cancer | ST NOS, areca nut | Alcohol, non veg diet, education, monthly household income, occupation & other tobacco types |
| 2 | Muwonge, 2008[46] | India | 1996–2004 | CCP | M+F | Histopathologically diagnosed incident cases | Randomly selected, without oral cancer | ST NOS, BQ without tobacco, areca nut/lime+tobacco | Education, religion, smoking & alcohol drinking |
| 3 | Thomas SJ, 2007[30] | Papua New Guinea | 1985–1987 | CCH | M&F | Clinically diagnosed incident cases | Without diagnosis of cancer, plus guardians of inpatients | BQ without tobacco | Age, sex, province, urban or rural residence, educationand income & smoking |
| 4 | Zanor, 2003[67] | India | 1993–1999 | CCH | M | Histopathologically confirmed incident cases | Non tobacco-related cancer patients, histologically confirmed cases | ST NOS, BQ without tobacco | Age, center, education level, alcohol consumption & smoking |
| 5 | Balaram P, 2002[48] | India | 1996–1999 | CCH | M+F | Incident cases by interview & oral examination | Admitted relatives, friends with cancer other than oral cancer, patients from outpatient clinic free from any malignant disease | ST NOS, BQ without tobacco | Age, gender, center, education, smoking & drinking habit |
| 6 | Chen PC, 2002[58] | Taiwan | 1994–1997 | CCH | M&F | Patients diagnosed with SCCC | Not mentioned | ST NOS, BQ without tobacco | Gender, age, smoking, HPV6 & 11 |
| 7 | Dixit R, 2000[49] | India | 1986–1992 | CCP | M&F | Cancer registry | Age-stratified, randomly sampled to follow age distribution of cases | ST NOS BQ without tobacco | Religion, educational status, bidi & cigarette smoking |
| 8 | Merchant A, 2000[57] | Pakistan | 1996–1998 | CCH | M&F | Biopsy proven primary cases | Orthopaedics and surgical ward patients without history of any malignancy | ST NOS, BQ without tobacco | Cigarette smoking & alcohol use. |
| 9 | Wasnik KS, 1998[50] | India | Not mentioned | CCH | M+F | Clinical, histopathologically and radiologically diagnosed incident cases | One control as non-cancer patient & others having cancer at another site | ST NOS, BQ without tobacco | Smoking and alcohol |
| 10 | Lu CT, 1996[59] | Taiwan | 1990–1992 | CCH | M+F | Patients diagnosed with cancer of oral cavity | Non-cancer cases, living in the same area as the case for at least 5 years and with similar educational background | ST NOS, BQ without tobacco | Alcohol drinking & tobaccosmoking |
| 11 | Hirayama, 1996[44] | India, Ceylon, Afghanistan, Thailand, Malaysia, Kasakh SSR and Uzbek SSR | 1963–1964 | CCH | M+F | Patients diagnosed with cancer of oral cavity | Patients from outpatient clinics of dental department and non cancer patients from same hospital | ST NOS, BQ without tobacco | Not mentioned |
| 12 | Ko YC, 1992[60] | Taiwan | 1992–1993 | CCH | M&F | Histopathologically confirmed | Non-carcinoma patients treated during same period in ophthalmology & physical check-up department | ST NOS, BQ with tobacco | Education & occupation |
| 13 | Nandkumar, 1990[15] | India | 1982–1984 | CCH | M+F | Population based cancer registry | Non-cancer cases | ST NOS, BQ without tobacco | Not mentioned |
| 14 | Sankaranarayan, 1990[68] | India | 1983–1984 | CCH | M+F | Hospital cancer registry | Non-malignant cases & those attending outpatient divisions of teaching hospitals of a medical college | ST NOS, snuff | Age, religion, bidi smoking, snuff & alcohol drinking |
| 15 | Sankaranarayan, 1989[51] | India | 1983–1984 | CCH | M+F | Patients with carcinoma of tongue | Non-malignant cases & those attending outpatient divisions | ST NOS, snuff | Age, religion, bidi smoking, snuff use & alcohol drinking |
| 16 | Sankaranarayan, 1989[52] | India | 1983–1984 | CCH | M+F | Biopsy proven primary cases | Patients attending teaching hospitals with non-malignant diseases | ST NOS, snuff | Age, religion, bidi smoking, snuff use & alcohol drinking |
| 17 | Jussawalla DJ, 1971[53] | India | 1968 | CCP | M&F | Histopathologically confirmed | City residents from registered voters list | ST NOS, BQ without tobacco | Not mentioned |
| 18 | Wahi, 1965[54] | India | 1950–1962 | CCH | M&F | Confirmed cases of oral cancer | Outpatient department for ailments other than H&N cancer | ST NOS | Not mentioned |
| 19 | Shanta V, 1963[55] | India | Not mentioned | CCP | M+F | Patients with oral cancer | Non-cancerous patients from exhibitions, fairs & general illness clinics | ST NOS | Not mentioned |
| 20 | Shanta V, 1959[56] | India | Not mentioned | CCP | M+F | Patients with cancer | Not mentioned | ST NOS | Not mentioned |
Design- CCH = case-control with hospital controls, CCP = case-control with population controls.
Gender- M+F = separate data for males and females, M&F = gender data combined.
M = exposure data available only for males.
SCC = Squamous cell carcinoma.
H&N = Head and Neck.