In this large, four-year study of 27,853 women, aged at least 45, those who got the most B-complex vitamin supplements were 33 percent less likely to develop breast cancer compared to women who got the least B-vitamin supplements.  

For individual B vitamins, women who got the most B6 (pyridoxine) or B1 (thiamine) from supplements were 39 percent less likely than non-users. Women in the bottom half of alcohol consumers had the greatest B-vitamin supplement benefit: 40 to 53 percent lower chances. B vitamins from food were also beneficial, lowering chances by 24 percent for women who got the most B vitamins from diet.

Reference: Nutrients; 2017, Vol. 9, 488.