BACKGROUND: Many studies have examined the association between smokeless tobacco and the risk of oral cancer. In South and Southeastern Asia, the use of smokeless tobacco, which increases the risk of oral cancer, is very common. The aim of this study is to provide a comprehensive review of studies conducted and published in a period of ten years to provide a more accurate assessment of the association between smokeless tobacco and oral cancer.

METHODS: An electronic search in six databases (PubMed, Scopus, Embase, Web of Science, ProQuest, and Cochrane Library) was conducted using keywords equivalent to oral cancer and smokeless tobacco. After selecting the articles according to the inclusion and exclusion criteria, a total of 30 prospective cohort and case-control studies from 2010 to 2020, which investigated the association of smokeless tobacco with oral cancer, were examined. The articles were qualitatively assessed using the Newcastle Ottawa Quality Assessment scale checklist. Then, study design (study type, setting, and duration of data collection), sample population (number, gender, and age), cancer type, smokeless tobacco type, effect size, and confounder adjustment were extracted from the studies.

FINDINGS: Five studies examined smokeless tobacco and chewing tobacco, and 25 studies reported and evaluated the type of smokeless tobacco, most of which were related to betel quid and supari. While the results of 21 studies revealed a positive and significant relationship between oral cancer and smokeless tobacco use (OR: 0.67-149.5), seven studies did not find a significant correlation.

CONCLUSION: This systematic review confirms the positive and significant association between non-smoking tobacco use and the risk of oral cancer.