There are fresh calls for the government to fortify flour with folic acid in the UK to help protect babies from common birth defects.
A new study concludes higher doses of the B vitamin, which can prevent spina bifida in unborn babies, will not harm the general public.
Folic acid is added to flour in more than 80 countries worldwide, but the UK has previously held off.
Women are advised to take folic acid before pregnancy, but many don't.
Fortification would provide extra cover, say experts.
The idea of mandatory fortification has already been backed by health ministers in Wales, Scotland and Northern Ireland, but not England.
'Out of date'
The latest research, published in Public Health Reviews, claims evidence for limiting intake of folic acid to no more than 1mg a day is out-of-date and flawed.
The new re-analysis of the data finds no relationship between higher levels of folic acid and harm, although some say there is still not enough proof to say fortification is entirely safe.
Research had previously suggested that high doses of folic acid could cause symptoms such as diarrhoea, cramps, sleep disorders, confusion, nausea and seizures.
It can also cover up the symptoms of vitamin B12 deficiency, which is a particular concern in older people.
Prof David Smith, an expert in pharmacology at the University of Oxford, said: "The matter has not been resolved."
However, Prof Tom Sanders, nutrition expert at King's College London, said there was "overwhelming" evidence in support of the case for folic acid fortification to prevent neural tube defects.
A neural tube defect involves abnormal development of the brain, spine or spinal cord. Approximately one in 1,000 pregnancies in the UK is affected.
Researchers say the cost of adding folic acid would be "pennies".
Under the government's 1998 Bread and Flour regulations, white flour is already fortified with iron, calcium and some vitamins - at a cost of just 71p per tonne of flour.
Lead author, Prof Sir Nicholas Wald, of the Wolfson Institute of Preventative Medicine, said: "Fortification would be a major saving.
"Failing to fortify flour with folic acid to prevent neural tube defects is like having a polio vaccine and not using it."
Study co-author Prof Sir Colin Blakemore, from the University of London, said: "If you can dispel the argument for harm... you are left only with the fact that not fortifying flour denies a proven and clear benefit to a significant proportion of the population."
The US has seen a 23% fall in pregnancies with neural tube defects (spina bifida and anencephaly) since the policy was introduced in 1998.
Folic acid occurs naturally in dark green leafy vegetables but research has suggested three-quarters of women across the UK don't consume sufficient levels.
It is voluntarily added to some breakfast cereals and spreads, but it is very difficult for pregnant women to get enough from diet alone.
A Department of Health and Social Care spokesperson said: "We want mums-to-be to have healthy pregnancies, and NHS guidance is that women planning a pregnancy should take a daily supplement of 400 micrograms of folic acid before conception and until the 12th week of pregnancy. We also recommend eating more folate-rich foods to reduce the risk of neural tube defects."
Good sources of folic acid:
- spinach, kale, Brussels sprouts, cabbage, broccoli
- beans and legumes (e.g. peas, blackeye beans)
- yeast and beef extracts
- oranges and orange juice
- wheat bran and other whole grain foods
- poultry, pork, shellfish and liver
- fortified foods (e.g. some brands of breakfast cereals - check the label).
Source: British Dietetic Association