I'm aware that ingested (at a dose too high to not be vomited out immediately) nicotine can be toxic, but I certainly have not come across any reputable studies that show nicotine as a carcinogen. Anyone have a link?
http://www.surgeongeneral.gov/library/reports/50-years-of-progress/sgr50-chap-5.pdf
Human Studies
Very little human data are on human cancer risk relating to nicotine. The Lung Health Study is the only study that provides information about long-term users of NRT (Murray et al. 2009). This study was not designed to directly examine nicotine’s potential cancer risk. It was a 5-year randomized trial to assess the effects of smoking cessation and reduction on chronic lung disease and lung function. Among 5,887 persons initially enrolled, the researchers continued to follow them for an additional 7 years (n = 3,220). Study participants were offered NRT without consideration of randomization or study design. Although they were encouraged to use NRT for only 6 months, many continued to use it long term. A total of 75 lung cancers were diagnosed among smokers and quitters of the extended surveillance group,
but the use of NRT was not associated with lung cancer (or other cancers).
Summary
There is insufficient data to conclude that nicotine causes or contributes to cancer in humans, but there is evidence showing possible oral, esophageal, or pancreatic cancer risks.
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The 75 cancers out of 3,220 - were smokers or ex-smokers - but the use of NRT (nicotine only) was
not associated with lung or other cancers. That's a pretty bright differentiation.
In vivo (humans) they were
not 'associated' or 'causative'. Only in vitro (glass) is there an 'association' - as are thousands of substances - considered 'agents', 'tumor promoters' and other associations,
but not carcinogenic.
Wiki: nicotine
In vitro studies have associated it with cancer, but carcinogenicity has not been demonstrated in vivo. There is inadequate research to demonstrate that nicotine is associated with cancer in humans.
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In vitro (within glass - ie test tube) vs. in vivo (within the living). The in vitro 'association' isn't 'causative' but like many substances that aren't carcinogenic, they are 'associated' with other substances that are - either as tumor 'promoters', or simply are substances seen, again without causation, and are not considered at any level of carcinogenicity.