Over five years the cancer counts came out level
Across 13 trials and 49,621 people taking roughly ten times a fortification dose for an average of 5.2 years, there were 1904 cancers on folic acid and 1809 on placebo.
What the study found
Five years of high-dose folic acid did not raise cancer rates
Across 13 trials and 49,621 people taking roughly ten times a fortification dose for an average of 5.2 years, there were 1904 cancers on folic acid and 1809 on placebo, a difference short of significance. No single site stood out: not bowel, not prostate, not lung, not breast.
The caveat · Middle-aged and up, not young women
Five years cannot rule out a cancer that takes longer to appear, and the authors confine their conclusion to that window. The interval reaches a rate ratio of 1.13, so a modest increase is not excluded, only a large one. Almost everyone enrolled was a middle-aged or older adult in a heart-disease or bowel-polyp trial, so the reassurance reaches women of reproductive age by argument rather than by measurement.