Nicotine, the primary psychoactive chemical in cigarettes, is addictive.[96] The resulting use of tobacco is the single greatest cause of preventable death globally. Smoking leads most commonly to diseases affecting the heart, liver and lungs, being a major risk factor for heart attacks, strokes, chronic obstructive pulmonary disease (COPD) (including emphysema and chronic bronchitis), and cancer (particularly lung cancer, cancers of the larynx and mouth, and pancreatic cancer). It also causes peripheral vascular disease and hypertension. On average, each cigarette that is smoked is estimated to shorten life by 11 minutes.[5] Starting smoking earlier in life and smoking cigarettes higher in tar increases the risk of these diseases. The World Health Organization (WHO) estimates that tobacco caused 5.4 million deaths in 2004 and 100 million deaths over the course of the 20th century.
There are, however, some potential health benefits to smoking: According to a review of epidemiological studies "...cigarette smokers are 50% less likely to have PD[Parkinson's disease] or AD[Alzheimer's disease] than are age- and gender-matched nonsmokers." "Ulcerative colitis is a condition of nonsmokers in which nicotine is of therapeutic benefit.