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According to a new analysis, people taking high doses of the B vitamin folic acid are not at an increased risk of cancer - easing some concern about possible side effects of national fortification programs.

The U.S. and Canada have required flour to be fortified with folic acid since 1998, after deficiencies in pregnant women were tied to brain and spinal cord birth defects in their babies.

However, researchers noted, fortification is not mandatory in Western Europe, for example, in part because of concern that extra folic acid might slightly increase people's risk of cancer due to its role in cell growth. Cells, including cancer cells, need folate - the natural form of folic acid - to grow and divide.

"Overall, this is good news," said nutrition researcher Joshua Miller from Rutgers University in New Brunswick, New Jersey.

However, he noted, there are enough questions about the long-term effects of folic acid that people should still "be a little cautious" about loading up on the vitamin.

For the new analysis, a team of international researchers combined data from 13 separate trials that randomly assigned participants to daily folic acid or a vitamin-free placebo and recorded who went on to develop cancer.

The studies included a total of close to 50,000 volunteers who were followed for just over five years, on average.

During that time, 7.7 percent of people in the folic acid groups and 7.3 percent in the placebo groups were diagnosed with any kind of cancer - a difference that could have been due to chance, the researchers noted in The Lancet.

Likewise, there was no increased risk of individual cancers - including colon, prostate, lung or breast cancer - attributed to folic acid.

Most trials used daily doses of folic acid between 0.5 and 5 milligrams. In the one study that used a much larger dose - 40 mg daily - there was still no difference in cancer diagnoses between people who were and weren't taking the vitamin.

The total daily amount of folic acid delivered through flour fortification is less than 0.5 mg per day for most Americans.

Folic acid is also naturally found in spinach, asparagus, lettuce and other greens. The recommended upper daily limit is 1.0 mg.

"The conclusion you can make from this is that over a relatively short period of time, there was no significant benefit or harm," said Dr. John Baron from the Geisel School of Medicine at Dartmouth in Lebanon, New Hampshire, who worked on the review.

Most cancers take 10 to 20 years to develop, he told Reuters Health. So it's hard to tell from shorter studies if there really is no folic acid-cancer link or if the researchers didn't follow people for long enough to see an association, whether positive or negative.

The possible effect of folic acid on cancer risk has been controversial, in part because of the complex biology behind it, said Miller, who co-wrote a commentary published with the new study.

"It has this dual nature, where extra folic acid could actually prevent cancers from developing in the first place, (but) once cancer is formed, then they're like any other proliferating cells - they need folate to do that," he told Reuters Health.

The researchers agreed this study shouldn't be the last word on the potential side effects of folic acid.

For now, Miller added, people might want to avoid piling supplements on top of multivitamins and fortified food.

"People should realize if they're eating breakfast cereals and bread and pastas, they're getting a good amount of folic acid in food," he said.

"I think they should try not to exceed the upper limit."