The signal was in men only, and nobody has explained why
In 77,118 adults, men taking over 55 micrograms a day from a standalone supplement for a decade had roughly twice the lung cancer risk of non-users. Women showed nothing.
What the study found
High-dose B12 doubled lung cancer risk in men, and only in men
In 77,118 adults, men taking more than 55 micrograms of B12 a day from a standalone supplement over a decade had roughly twice the lung cancer risk of non-users. Women in the same cohort showed nothing. Neither did 159,232 women followed for 18 years by the same group.
The caveat · Not seen with B12 in multivitamins
It appeared only for standalone high-dose supplements, not for the same vitamins inside a multivitamin, which is hard to explain if the vitamin is doing it. Intake was reported once, at the start. The review clearing B12 includes authors with declared ties to manufacturers.
What we don't know
Whether high-dose B12 supplements raise lung cancer risk is unresolved. One cohort saw about double the risk in men, none in women, a split nobody has explained, and the risk ran higher in smokers, so something besides B12 may explain it.