First, the topline numbers: The study was an observational one rather than a randomized control trial, the latter of which is considered to be the “gold standard” of clinical studies. The sample size, however, was huge—it involved more than 70,000 adults aged 50 to 76. More than 800 of these people were found to have invasive lung cancers over the study’s course, and the risk seemed to be especially associated with men who took discrete supplements with vitamin B6 or B12. “[U]se of vitamin B6 and B12 from individual supplement sources, but not from multivitamins, was associated with a 30% to 40% increase in lung cancer risk among men,” wrote the researchers.

Strikingly, this same effect wasn’t seen in women. And even among men, there was one specific group who really drove the trend: Smokers. Smoking is one of the single biggest risk factors for lung cancer, which makes up more than 13% of all new cancer diagnoses and more than one in four cancer deaths in the U.S.

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What the research seems to suggest is that male smokers who take more than the recommended levels of vitamin B6 and B12 seem to have a higher risk for lung cancer, perhaps even considerably so. That’s important because, at the very least, this correlation appears to suggest that taking mega-doses of vitamins to improve general health (including for cancer risk) isn’t an effective strategy and certainly doesn’t reverse the effects of smoking.

Now for some more caveats: The sample, while large, was overwhelmingly made up of white people, and the measure of vitamin intake was based on surveys rather than blood draws (which could potentially lead to some misleading results). Furthermore, some public health experts note it’s possible that cancer itself can change the vitamin levels in the blood.

This essay appears in today’s edition of the Fortune Brainstorm Health Daily. Get it delivered straight to your inbox.