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Nicotine is an alkaloid in tobacco that is responsible for the addictive nature of cigarette smoking and other tobacco use.  Regular nicotine consumption leads to [[tolerance]]<ref>Porchet, H. C., Benowitz, N. L., & Sheiner, L. B. (1988).  Pharmacodynamic model of tolerance:  Application to nicotine.  ''Journal of Pharmacology & Experimental Therapeutics, 244'', 231-236</ref> (i.e., higher doses are required to achieve the same effect), and [[withdrawal]]<ref>Hughes, J.R., Higgins, S.T., & Bickel, W.K. (1994).  Nicotine withdrawal versus other drug withdrawal syndromes: Similarities and dissimilarities.  ''Addiction, 89'', 1461-1470</ref> symptoms (e.g., irritability, restlessness, nicotine cravings) occur when use is discontinued.  Intake elicits acute increases in heart rate and blood pressure<ref>Benowitz, N.L., Jacob, P., Jones, R.T., & Rosenberg, J. (1982).  Interindividual variability in the metabolism and cardiovascular effects of nicotine in man.  ''Journal of Pharmacology & Experimental Therapeutics, 221'', 368-372.''</ref>, as well as respiratory rate, but also decreases the volume of blood flow by causing the constriction of blood vessels.  Nicotine in cigarette smoke is very easily absorbed through the lungs and buccal tissue in the oral cavity<ref>Miller, M.S., & Cocores, J.A. (1993).  Nicotine dependence: Diagnosis, chemistry and pharmacologic treatments.  ''Pediatrics in Review, 14'', 275-279.</ref>, and reaches the brain about seven seconds after it is inhaled in cigarette smoke<ref>Benowitz, N.L. (1991).  Pharmacodynamics of nicotine: Implications for rational treatment of nicotine addiction.  ''British Journal of Addiction, 86'', 495-499.</ref>.  After reaching the brain, it stimulates the release of a number of [[neurotransmitters]], including [[dopamine]]<ref>Stolerman, I.P., & Shoaib, M. (1991).  The neurobiology of tobacco addiction.  ''Trends in Pharmacological Science, 12'', 467-473.</ref>, [[norepinephrine]]<ref>Summers, K.L., & Giacobini, E. (1995).  Effects of local and repeated systemic administration of (-)nicotine on extracellular levels of acetylcholine, norepinephrine, dopamine and serotonin in rat cortex. ''Neurochemical Research, 20'', 753-759.</ref>, [[serotonin]]<ref>Ribeiro, E.B., Bettiker, R.L., Bogdanov, M., & Wurtman, R.J. (1993).  Effects of systemic nicotine on serotonin release in rat brain. ''Brain Research, 621'', 311-318.</ref>, and [[acetylcholine]]<ref>Summers & Giacobini, 1995.</ref>.  The increased availability of these chemicals in the brain leads to feelings of pleasure and mild euphoria, as well as increased alertness and concentration.  The half-life of nicotine is approximately two hours.

Nicotine is an alkaloid in tobacco that is responsible for the addictive nature of cigarette smoking and other tobacco use.  Regular nicotine consumption leads to [[tolerance]]<ref>Porchet, H. C., Benowitz, N. L., & Sheiner, L. B. (1988).  Pharmacodynamic model of tolerance:  Application to nicotine.  ''Journal of Pharmacology & Experimental Therapeutics, 244'', 231-236</ref> (i.e., higher doses are required to achieve the same effect), and [[withdrawal]]<ref>Hughes, J.R., Higgins, S.T., & Bickel, W.K. (1994).  Nicotine withdrawal versus other drug withdrawal syndromes: Similarities and dissimilarities.  ''Addiction, 89'', 1461-1470</ref> symptoms (e.g., irritability, restlessness, nicotine cravings) occur when use is discontinued.  Intake elicits acute increases in heart rate and blood pressure<ref>Benowitz, N.L., Jacob, P., Jones, R.T., & Rosenberg, J. (1982).  Interindividual variability in the metabolism and cardiovascular effects of nicotine in man.  ''Journal of Pharmacology & Experimental Therapeutics, 221'', 368-372.''</ref>, as well as respiratory rate, but also decreases the volume of blood flow by causing the constriction of blood vessels.  Nicotine in cigarette smoke is very easily absorbed through the lungs and buccal tissue in the oral cavity<ref>Miller, M.S., & Cocores, J.A. (1993).  Nicotine dependence: Diagnosis, chemistry and pharmacologic treatments.  ''Pediatrics in Review, 14'', 275-279.</ref>, and reaches the brain about seven seconds after it is inhaled in cigarette smoke<ref>Benowitz, N.L. (1991).  Pharmacodynamics of nicotine: Implications for rational treatment of nicotine addiction.  ''British Journal of Addiction, 86'', 495-499.</ref>.  After reaching the brain, it stimulates the release of a number of [[neurotransmitters]], including [[dopamine]]<ref>Stolerman, I.P., & Shoaib, M. (1991).  The neurobiology of tobacco addiction.  ''Trends in Pharmacological Science, 12'', 467-473.</ref>, [[norepinephrine]]<ref>Summers, K.L., & Giacobini, E. (1995).  Effects of local and repeated systemic administration of (-)nicotine on extracellular levels of acetylcholine, norepinephrine, dopamine and serotonin in rat cortex. ''Neurochemical Research, 20'', 753-759.</ref>, [[serotonin]]<ref>Ribeiro, E.B., Bettiker, R.L., Bogdanov, M., & Wurtman, R.J. (1993).  Effects of systemic nicotine on serotonin release in rat brain. ''Brain Research, 621'', 311-318.</ref>, and [[acetylcholine]]<ref>Summers & Giacobini, 1995.</ref>.  The increased availability of these chemicals in the brain leads to feelings of pleasure and mild euphoria, as well as increased alertness and concentration.  The half-life of nicotine is approximately two hours.

Medications containing nicotine are also used to help smokers who are trying to quit.  These are referred to as nicotine replacement therapy (NRT).  The idea behind NRT is that it allows smokers to break the behavioral component of their smoking habit (e.g., becoming used to not smoking while driving) while gradually reducing the amount of nicotine in their bloodstream, thus minimizing the effects of physiological nicotine withdrawal.  Several different forms of NRT are available, including a transdermal patch, chewing gum, and nasal spray.  Nicotine has also been found to be a useful treatment for certain physical conditions, though the mechanisms are not well understood.  For example, smoking appears to reduce the risk of Kaposi's sarcoma, as well as breast cancer among women at high risk for it due to the presence of the BRCA gene.  Nicotine also appears to have therapeutic benefit for individuals with ulcerative colitis and one form of epilepsy.   

Medications containing nicotine are also used to help smokers who are trying to quit.  These are referred to as nicotine replacement therapy (NRT).  The idea behind NRT is that it allows smokers to break the behavioral component of their smoking habit (e.g., becoming used to not smoking while driving) while gradually reducing the amount of nicotine in their bloodstream, thus minimizing the effects of physiological nicotine withdrawal.  Several different forms of NRT are available, including a transdermal patch, chewing gum, and nasal spray.  Nicotine has also been found to be a useful treatment for certain physical conditions, though the mechanisms are not well understood.  Smoking has been associated with a lower risk of Kaposi's sarcoma<ref>http://dceg.cancer.gov/pdfs/goedert942002.pdf</ref>, as well as breast cancer among women at high risk for it due to the presence of the BRCA gene.  Nicotine also appears to have therapeutic benefit for individuals with ulcerative colitis and one form of epilepsy.   

Nicotine has been shown in some medical studies to be more addictive than cocaine and even heroin. Tobacco often occurs along with cocaine use because the nicotine in tobacco greatly enhances the euphoria one receives from cocaine.  

Nicotine has been shown in some medical studies to be more addictive than cocaine and even heroin. Tobacco often occurs along with cocaine use because the nicotine in tobacco greatly enhances the euphoria one receives from cocaine.