Tobacco use is known to cause "field cancerization" resulting in a propensity for development of second primary tumors in patients with oral cancer. Case reports have also implicated marijuana smoking as a cause of oral cancer, particularly in younger patients.
Alcohol use is a second independent major risk factor for the development of oral cancer.[11,12,13,14] There is a suggestion that beer and hard liquor confer a greater risk than wine. The risk of oral cancer increases with the number of cigarettes smoked per day and the number of alcoholic drinks consumed per day in a dose-dependent fashion. The combined use of alcohol and tobacco increases the risk for oral cancer far greater than either independently. Alcohol use has been shown to be an independent risk factor for development of oral premalignant lesions (leukoplakia or erythroplakia), which can progress to cancer.
Human papillomavirus (HPV) infection
There is an association between HPV and oral cancer, particularly HPV type 16 as shown in multiple case-control studies.[16,17,18,19,20,21,22] HPV 16 accounts for 90% to 95% of HPV-positive oropharyngeal tumors, but other high-risk subtypes include 18, 31, 33, and 35. The mechanism of HPV in the etiology of oral cancers may be related to its oncoproteins E6 and E7, which bind to and trigger the degradation of the p53 and pRB tumor suppressor proteins, respectively. HPV accounts for a relatively small proportion of oropharyngeal cancers compared to tobacco and alcohol. However, the rates of HPV-associated oropharyngeal cancers appear to be increasing.[20,23]
Carcinoma of the lip, predominantly on the lower lip, occurs in approximately 3,600 persons per year. Epidemiologically, these tumors behave akin to squamous cell carcinoma of the skin, and most are related to sun exposure, although chronic direct exposure to tobacco (i.e., the location where a pipe or cigarette is habitually held) is also associated with an increased risk of carcinoma of the lip.[24,25,26] Men have a higher risk of lip cancer than women. This has been attributed to tobacco use, greater occupational exposure to sunlight among men, and possibly due to the shielding effect of lipstick in women.
Interventions Associated With a Decreased Risk of Oral Cancer
Avoidance and cessation of tobacco use
The cessation of cigarette smoking is associated with a 50% reduction of risk of developing oral cancer within 3 to 5 years  and a return to a normal level of risk for development of oral cancer within 10 years. Dentists and other health professionals can play an integral role in smoking cessation advice and encouragement.
Dentists can also participate in the full scope of pharmacological and behavioral interventions for smoking cessation. A study has shown that only 25% of tobacco users report receiving advice to quit tobacco use from their dentist, a proportion less than that received from their physician. There was a dramatic increase in the use of cigars of about 250% during the period between 1993 and 1998  and heavy cigar use is particularly associated with oral cancer development.