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The addiction of nicotine solidifies the demand for the product.

"In low concentrations (an average cigarette yields about 1 mg of absorbed nicotine), the substance acts as a stimulant in mammals and is one of the main factors responsible for the dependence-forming properties of tobacco smoking. According to the American Heart Association, "Nicotine addiction has historically been one of the hardest addictions to break." The pharmacological and behavioral characteristics that determine tobacco addiction are similar to those that determine addiction to drugs such as heroin and cocaine. Nicotine content in cigarettes has actually slowly increased over the years, and one study found that there was an average increase of 1.6% per year between the years of 1998 and 2005. This was found for all major market categories of cigarettes.

Because of the addictive properties of nicotine, consumers are more apt to continue use, therefore creating loyal patronage."

Source: http://en.wikipedia.org/wiki/Nicotine