In one study, nicotine administered to mice with tumors caused increases in tumor size (twofold increase), metastasis (nine-fold increase), and tumor recurrence (threefold increase)"
blahblahblah not a great read but focus on the part I put in bold. Yes it's one study and people can call it inconclusive or whatever but it's still a little scary. I don't ignore results like this as so many others choose to do to justify their continued addiction to nicotine.
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I did focus on the bold part.
The original research was done in 2009 by NCI, National Cancer Institute, source PLOS ONE: Nicotine Promotes Tumor Growth and Metastasis in Mouse Models of Lung Cancer.
And that bold part in your post is from that research, which is also on Wikipedia page about nicotine.
But nobody updated Wikipedia with the latest findings.
And here is the latest press release from the same institute, NCI, about NIH (National Institute of Health) research done in 2011, source Low-dose nicotine does not promote lung tumors in mouse models - National Cancer Institute:
snip...
a limited body of research—including some tissue culture studies conducted by NCI scientists—has suggested that even low levels of nicotine might promote tumor growth. Those earlier findings raised potential concerns about the safety of using NRT. The results of the current study in mice found no evidence to support this concern.
snip...