No adverse effects have been associated with the consumption of excess vitamin B9 (folate) from foods. Concerns regarding safety are limited to synthetic folic acid intake. A review paper from 2016 is discussing excess folate consumption in relation to adverse health outcomes; especially in elderly with B12 deficiency high folate intake could have adverse influence on cognitive function and anemia as well as lowering the natural killer cell activity (63).
One symptom of vitamin B12 deficiency is ‘megaloblastic anemia’, which is indistinguishable from that associated with vitamin B9 (folate) deficiency (see Deficiency Symptoms). Large doses of folic acid given to an individual with an undiagnosed vitamin B12 deficiency could correct megaloblastic anemia without correcting the underlying vitamin B12 deficiency, leaving the individual at risk of developing irreversible neurologic damage. Such cases have been mostly seen at folic acid doses of 5,000 micrograms (5 mg) and above.
Prostate cancer Risk
A follow-up trial of the Aspirin/Folate Polyp Prevention Study (AFPP) (48) indicated a higher incidence of prostate cancer after several years’ intake of vitamin B9 (folic acid) (49). However, experts commented that the study has basic flaws in its design (see also Expert Opinion).
In addition, the results were in clear contradiction to all previously published observations, according to which a good long-term folate status reduces the risk of cancer.
Lung cancer risk
A study than combined data from two randomized placebo-controlled clinical trials suggested an increased risk of lung cancer and mortality in patients with ischemic heart disease treated with folic acid (0.8 mg/day) and vitamin B12 (0.4 mg/day supplements (50). The authors speculated that folate deficiency from food may promote initial stages of cancer development, whereas high doses of folic acid in supplements may enhance growth of cancer cells.
Experts criticized that the study results are inconsistent with current evidence showing that higher average folic acid intake in adults is related to lower cancer incidence and a 50% reduction in mortality from cancer. The short-term findings would nullify the long-term benefits that folic acid fortification may have on population health. Preventive interventions would require long-term evaluation and the time frame for benefit for some preventive interventions may span decades, although smoking cessation may be unique among lifestyle changes that produce a rapid reduction in cancer risk (51).
The European Food Safety Authority has established tolerable upper intake levels (UL) for vitamin B9 (folic acid) intake (52):
|Age (years)||UL (mg/day)|
As no data are available to suggest that other life-stage groups have increased susceptibility to adverse effects of high folic acid intake, the UL is also applicable for pregnant or breast-feeding women.
In order to prevent irreversible damage, a tolerable upper intake level (UL) for supplementary vitamin B9 (folic acid) has been established by the U.S. Food and Nutrition Board (1):
|Infants 0–12 months||Not possible to establish*|
|Children 1–3 years||300|
|Children 4–8 years||400|
|Children 9–13 years||600|
|Adolescents 14–18 years||800|
|Adults 19 years and older||1,000 (= 1 mg)|
*Source of intake should be from food and formula only.
Because of the potential for interactions, dietary supplements should not be taken with medication without first talking to an experienced healthcare provider.
Authored by Dr Peter Engel in 2010, reviewed and revised by Angelika Friedel on 29.06.2017