Sometimes studies have unexpected results: When researchers were studying the effects of vitamins on cancer, they were surprised to find that long-term use of vitamin B6 or B12 supplements actually raised the risk of lung cancer for some people.

Fortunately, this was only seen among men who smoke. It was not seen in women.

Many men take B complex vitamins that include Vitamin B6 and B12 as a way of raising testosterone levels and building muscle. The lung cancer risk rose among male smokers who took 20 milligrams or more of Vitamin B6 daily for 10 years. They were three times more likely to develop lung cancer than other male smokers.

Male smokers who took 55 micrograms or more of B12 daily for 10 years saw their risk of lung cancer risk rise about four-fold. These vitamins had been thought to reduce cancer risk.

Male smokers who took 20 milligrams or more of Vitamin B6 daily were three times more likely to develop lung cancer than other male smokers.

These doses are higher than those found in a typical daily multivitamin. For comparison, one multivitamin that's heavily marketed to seniors contains 3 milligrams of B6 and 25 micrograms of B12.

The researchers studied 77,000 people from the State of Washington enrolled in the Vitamins and Lifestyle Study, a long-term study designed to examine the effect of vitamin and mineral supplements on cancer risk.

All participants were between 50 and 76 when recruited for the study. Upon enrolling, they provided detailed information about their B vitamin usage over the past 10 years, including the doses that they took. Other information they provided allowed the researchers to adjust for many other factors known to affect cancer risk, including smoking history, age, race, education, body size, alcohol consumption, history of cancer or chronic lung disease, family history of lung cancer and use of anti-inflammatory drugs.

The study can't say why high doses of B6 or B12 might increase lung cancer risk. With over 7,000 chemicals present in cigarette smoke, an interaction with one or several of them is suspected.

In an email, lead author, Theodore Brasky, an Assistant Professor at the Ohio State University College of Medicine and a member of the Cancer Control Team at the OSU Comprehensive Cancer Care Center, offered some context for the study's findings. First, he reminds people that this is just a single correlational study. It can't prove that the vitamins were actually responsible for the increased lung cancer risk, it can only suggest it. Further studies are underway that may shed more light on these findings.

For non-smokers who take B6 or B12 supplements and are now worried about lung cancer, Brasky reminds them that non-smokers have an incredibly low rate of lung cancer to begin with. In fact it's so low that that there were too few cases of lung cancer in the non-smokers in this study to even attempt to evaluate the effect of B6 or B12 supplements on their lung cancer rate.

And for smokers who are taking B6 or B12 supplements and are now concerned about lung cancer, Brasky suggests they would do themselves more good by looking into ways to quit smoking.

Few people in the U.S.A. need B vitamin supplementation. Exceptions include the elderly and some vegetarians, both of whom tend to be deficient in B12. Those people might want to speak to their doctor about whether they have a need for any added B vitamins. For almost everyone else, it is very difficult to be deficient in B vitamins, so Brasky sees no reason for them to take B vitamin supplements.

The study appears in the Journal of Clinical Oncology.