It is not fully understood why mouth cancer develops. However, certain factors make mouth cancer more likely. Smoking of any type, incl bidis or hand-rolled cigarettes with cannabis; chewing tobacco, eg betel quid, gutkha and paan; drinking excessive alcohol(esp at the same time as smoking or chewing tobacco); having already had cancer of the head/neck; exposure to the sun or UV light(this increases the risk of lip cancer) or having a weakened immune system(people with HIV/AIDS, or who are taking medicines that suppress the immune system) puts you at higher risk of mouth cancer. Oral sex is a risk factor for HPV infection, which is associated with oral cancer.
Attribution: BUPA