Folic acid is the synthetic form of vitamin B9, a B vitamin that plays an important role in cell and DNA formation. It’s found exclusively in vitamins and certain fortified foods.

Conversely, vitamin B9 is called folate when it occurs naturally in foods. Beans, oranges, asparagus, Brussels sprouts, avocados, and leafy greens all contain folate.

The Reference Daily Intake (RDI) for this vitamin is 400 mcg for most adults, though pregnant and breastfeeding women should get 600 and 500 mcg, respectively (1).

Low blood levels of folate have been linked to health issues, such as a higher risk of birth defects, heart disease, stroke, and even certain cancers (2, 3, 4, 5, 6).

However, excess folic acid from supplements may harm your health.

Here are 4 potential side effects of too much folic acid.

Your body breaks down and absorbs folate and folic acid in slightly different ways.

For instance, almost all of the folate you ingest from foods gets broken down and converted into its active form in your gut before being absorbed into your bloodstream (7).

In contrast, a much smaller percentage of the folic acid you get from fortified foods or supplements gets turned into its active form in your gut (7).

The rest requires the help of your liver and other tissues to get converted in via a slow and efficient process (8).

As such, folic acid supplements or fortified foods may cause unmetabolized folic acid (UMFA) to accumulate in your blood — something that doesn’t happen when you eat high folate foods (9, 10).

This is concerning because high levels of UMFA appear to be linked to various health concerns (1, 11, 12, 13, 14, 15, 16, 17).

summary

Your body breaks down and absorbs folate easier than folic acid. Excessive folic acid intake can cause UMFA to build up in your body, which may lead to detrimental health effects.

High folic acid intake may mask a vitamin B12 deficiency.

Your body uses vitamin B12 to make red blood cells and keep your heart, brain, and nervous system functioning optimally (18).

When left untreated, a deficiency in this nutrient can reduce your brain’s ability to function normally and lead to permanent nerve damage. This damage is typically irreversible, which makes a delayed diagnosis of vitamin B12 deficiency particularly worrisome (18).

Your body uses folate and vitamin B12 very similarly, meaning that a deficiency in either can result in similar symptoms.

Some evidence shows that folic acid supplements may mask vitamin-B12-induced megaloblastic anemia, which may cause an underlying vitamin B12 deficiency to go undetected (14, 15).

Therefore, people experiencing symptoms like weakness, fatigue, difficulty concentrating, and shortness of breath may benefit from getting their B12 levels checked.

summary

High intakes of folic acid may mask a vitamin B12 deficiency. In turn, this could increase your risk of brain and nervous system damage.

Excess folic acid intake may speed age-related mental decline, particularly in people with low vitamin B12 levels.

One study in healthy people over the age of 60 linked high folate levels to mental decline in those with low vitamin B12 levels — but not in those with normal B12 levels (15).

The participants with high blood folate levels achieved them through high intake of folic acid in the form of fortified foods and supplements, not through eating naturally folate-rich foods.

Another study suggests that people with high folate but low vitamin B12 levels may be up to 3.5 times likelier to experience a loss of brain function than those with normal blood parameters (19).

The study authors cautioned that supplementing with folic acid may be detrimental to mental health in older adults with low vitamin B12 levels.

Furthermore, other research ties excessive use of folic acid supplements to mental decline (20).

Keep in mind that more studies are needed before strong conclusions can be made.

summary

A high intake of folic acid may hasten mental decline associated with age, particularly in individuals with low vitamin B12 levels. Nonetheless, further research is necessary.

Adequate folate intake during pregnancy is necessary for your baby’s brain development and reduces the risk of malformations (21, 22, 23, 24).

Because many women fail to get the RDI from food alone, women of childbearing age are often encouraged to take folic acid supplements (1).

However, supplementing with too much folic acid may increase insulin resistance and slow brain development in children.

In one study, 4- and 5-year-olds whose mothers supplemented with over 1,000 mcg of folic acid per day while pregnant — more than the Tolerable Upper Intake Level (UL) — scored lower on brain development tests than the children of women who took 400–999 mcg per day (16).

Another study linked higher blood levels of folate during pregnancy to a greater risk of insulin resistance in children ages 9–13 (25).

Though further research is needed, it may be best to avoid taking more than the recommended daily dose of 600 mcg of folic acid supplement during pregnancy unless advised otherwise by a health professional.

summary

Folic acid supplements are a practical way to boost folate levels during pregnancy, but excessive doses may increase insulin resistance and slow brain development in children.

Folic acid’s role in cancer appears to be twofold.

Research suggests that exposing healthy cells to adequate levels of folic acid may protect them from becoming cancerous. However, exposing cancerous cells to the vitamin may help them grow or spread (9, 26, 27).

That said, research is mixed. While a few studies note a small increase in cancer risk in people taking folic acid supplements, most studies report no link (28, 29, 30, 31, 32).

The risk may depend on the type of cancer, as well as your personal history.

For instance, research suggests that people previously diagnosed with prostate or colorectal cancer who supplemented with more than 1,000 mcg of folic acid per day have a 1.7–6.4% higher risk of the cancer recurring (33, 34).

Still, more research is needed.

Keep in mind that eating lots of folate-rich foods does not appear to increase cancer risk — and may even help reduce it (35, 36).

summary

Excessive folic acid supplement intake may increase cancer cells’ ability to grow and spread, though more research is needed. This may be particularly detrimental to people with a history of cancer.

Folic acid is included in most multivitamins, prenatal supplements, and B complex vitamins, but it’s also sold as an individual supplement. In certain countries, some foods are also fortified in this vitamin.

Folic acid supplements are typically used to prevent or treat low blood folate levels. Moreover, pregnant women or those planning to become pregnant often take them to reduce the risk of birth defects (1).

The RDI for folate is 400 mcg per day for most adults, 600 mcg per day during pregnancy, and 500 mcg per day while breastfeeding. Supplement dosages usually range from 400–800 mcg (1).

Folic acid supplements can be purchased without a prescription and are generally considered safe when taken in normal doses (37).

That said, they can interact with some prescription medications, including those used to treat seizures, rheumatoid arthritis, and parasitic infections. Thus, anyone taking medications should consult a health professional before taking folic acid (1).

summary

Folic acid supplements are used to reduce the risk of birth defects, as well as prevent or treat a folate deficiency. They’re generally considered safe but may interact with some prescription drugs.

Folic acid supplements are generally safe and provide a convenient way to maintain adequate folate levels.

That said, excess folic acid supplement intake may cause several side effects, including slower brain development in children and accelerated mental decline in older adults.

While further research is needed, you can work with your healthcare provider to determine your folate levels and see whether a supplement is necessary.