Hello,
I have come across to a study that comes up with a result like the title says. According to study, the lung cancer risk in men who are long-term supplemented with high dosed B6 and B12 (individual supplementation, not multivitamins) is 30-40% higher than who don't. The risk is even higher for the smokers.
The mechanism involved is assumed to be related to DNA methylation, in which B vitamins take prominent role.
Why do B vitamins not increase the risk in women? The assumption is that androgens regulate enzymes that take place in one-carbon metabolism.
I want to hear your opinion about this research, since most of the people here supplement with B12 are also using anabolic steroids . The assumption above makes me wonder if the risk is even higher in AAS users, as we have higher amount of androgens circulating in our bodies.

You can find the research on http://ascopubs.org/doi/abs/10.1200/JCO.2017.72.7735
Also a more detailed analysis on the study can be found on https://examine.com/nutrition/vitamin-b-cancer/