Well, this is interesting.

Folic Acid and Vitamin B12 Supplementation and the Risk of Cancer: Long-term Follow-up of the B Vitamins for the Prevention of Osteoporotic Fractures (B-PROOF) Trial, Cancer Epidemiology, Biomarkers & Prevention, February 2019

Daily supplementation with folic acid (400 micrograms) and vitamin B12 (500 micrograms) increased the risk of colorectal cancer in this double-blind, placebo-controlled study of older people.

Conclusions:
Folic acid and vitamin B12 supplementation was associated with an increased risk of colorectal cancer.

Some other bits:

The effect on colorectal cancer risk was even more extreme in compliant participants.

Does that mean you still have elevated cancer risk even if you don’t take the vitamins all the time? Seems so.

We cannot yet ascertain whether this is due to an individual effect of folic acid or vitamin B12, or an interactive effect of both folic acid and vitamin B12 combined.

That was my first question! It’s easy not to take folic acid. Vegans get it pretty easily from their diet. Not so with B12. We have to take a supplement. And try to find one that only supplies a few micrograms, not 500 or 1000 mcg.

We only included Caucasian participants aged 65 years and over.

The age range is notable. The study said that increased cancer risk was not found as much in trials with younger populations. It may be that an older person’s immune system isn’t up to the task of wiping out any nascent bowel cancer … to which the B vitamins were contributing.

This trial is one of the few that were done in a population without mandated folic acid fortification and relatively low supplement use. As a result we were able to clearly discern the effect of supplementation in a population with limited intake of folic acid above the tolerable upper intake level.

Americans probably get a healthy dose of folic acid even before they take a supplement. In 1998, the FDA required that folic acid be added to enriched grain products (bread, pasta, rice, cereal).

This will likely reduce the amount of vitamin B12 I take.