Vitamin B6 (also known in the bioavailable form of pyridoxal-5-phosphate or P5P) along with methionine was shown to protect against lung cancer when levels were high in the blood. The study that undertook the effort reviewed charts of 400,000 people in 10 European countries over 8 years.
Whether taking vitamin B6 or methionine will increase blood levels and provide the same protection is not yet known. Of course, it does not take a rocket scientist to know that continuing to smoke increases the risk of lung cancer, period. The most effective harm reduction approach is to stop smoking. Short of that, it may be wise to use some B-complex to try to further reduce risk.
B vitamins are very important for a variety of body functions. Other data have shown benefits of B-vitamins such as B1 (thiamine) and B2 (riboflavin) for conditions ranging from lactic acidosis to reducing the risk of premature or low birthweight babies among HIV+ pregnant women.
With regard to methionine, it can be somewhat hard on the liver. However, NAC, which has myriad other potential benefits, may have the more biologically relevant benefits. In addition, S-adenosylmethionine is possibly a more active agent which appears to be good for both the liver and for depression. Methionine converts into SAMe and subsequently cysteine in the body naturally.
Whether taking NAC or B6 can help reduce the risk of lung cancer needs to be evaluated in a clinical trial setting. But again, of course, what we know reduces risk is to quit smoking. Duh.