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* Nicotine is distilled from burning tobacco. It is carried with the inhaled tar droplets into the small airways and alveoli of the lung.1 Nicotine absorption in the mouth depends on pH, while in the lung absorption is less affected by pH. Most cigarette smoke is acidic (pH 5.5–6.5). At this pH, nicotine is primarily ionized and does not cross cell membranes rapidly; therefore, there is very little absorption of nicotine in the mouth. When smoke reaches the alveoli, nicotine is rapidly absorbed, probably as the result of the huge surface area of the alveoli and small airways. Inhalation provides a rapid and efficient route to the brain as it takes less than 10 seconds for nicotine to get from the lungs to the brain.1 Once nicotine enters the bloodstream, some reaches the liver, where it is metabolized mainly into two compounds: about 70–80% is converted to cotinine and 4% to nicotine-N-oxide.2 There is a substantial variability in the rate of metabolism of nicotine in different people and smokers metabolize nicotine more slowly than non-smokers. Some studies have shown ethnic differences in nicotine metabolism. In two studies, African-Americans were shown to have higher cotinine levels than Caucasians suggesting that they may metabolize cotinine more slowly than Caucasians.3,4 Cotinine has a much longer half-life than nicotine and it is used as a marker of nicotine intake. Cotinine is metabolized into trans-3’-hydroxycotinine, the most abundant metabolite in urine.1 The remaining unmetabolized nicotine is the active compound involved in tobacco dependence. The smoke of most cigars has an alkaline pH and, as a result, nicotine contained in the smoke can be readily absorbed across the oral mucosa without inhalation into the lung. Cigarette smokers must inhale to absorb substantial quantities of nicotine, whereas cigar smokers can absorb substantial quantities of nicotine without inhaling. Former cigarette smokers who turn to cigars are likely to inhale cigar smoke.
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