Tobacco is a plant originally indigenous to the Americas which is now grown across the world. Its leaves contain high levels of the addictive chemical nicotine and many cancer-causing chemicals, especially polyaromatic hydrocarbons (PAHs). The leaves may be smoked (in cigarettes, cigars, and pipes), applied to the gums (as dipping and chewing tobacco), or inhaled (as snuff). Nicotine is now being extracted from the leaf to produce novel nicotine products, such as e-cigarettes, but also for Nicotine Replacement Therapies, which are used to treat tobacco addiction. Tobacco use and exposure to secondhand tobacco smoke causes many types of cancer, as well as heart, respiratory, and other diseases.