Globally,
oral cancer is one of the ten common
cancers. In some parts of the world, including the Indian subcontinent,
oral cancer is a major
cancer problem. Tobacco use is the most important risk factor for
oral cancer. The most common form of tobacco use, cigarette smoking, demonstrates a very high relative risk--in a recent cohort study (CPS II), even higher than
lung cancer. In areas where tobacco is used in a
smokeless form,
oral cancer incidence is generally high. In the West, especially in the U.S. and Scandinavia, smokeless tobacco use consists of oral use of snuff. In Central, South, and Southeast Asia smokeless tobacco use encompasses nass, naswar, khaini, mawa, mishri, gudakhu, and betel quid. In India tobacco is smoked in many ways; the most common is bidi, others being chutta, including reverse smoking, hooka, and
clay pipe. A voluminous body of research data implicating most of these forms of tobacco use emanates from the Indian subcontinent. These studies encompass case and case-series reports, and case-control, cohort, and intervention studies. Collectively, the evidence fulfills the epidemiological criteria of causality: strength, consistency, temporality, and coherence. The
biological plausibility is provided by the identification of several
carcinogens in tobacco, the most abundant and strongest being tobacco-specific N-
nitrosamines such as
N-nitrosonornicotine (NNN) and
4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). These are formed by N-nitrosation of
nicotine, the major
alkaloid responsible for addiction to tobacco. The etiological relationship between tobacco use and
oral cancer has provided us with a comprehensive model for understanding
carcinogenesis.