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Taking high doses of folic acid supplements may cause side effects, including accelerating mental decline in older adults and increasing the likelihood of cancer recurring.

Folic acid is the synthetic form of vitamin B9, and it’s only found in vitamin supplements and some fortified foods.

When vitamin B9 occurs naturally in foods, it’s called folate. You get folate from beans, oranges, asparagus, Brussels sprouts, avocados, leafy greens, and more.

Regardless of whether it’s in the form of folate or folic acid, vitamin B9 is important for cell and DNA formation in your body (1).

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

On the other hand, high blood levels of folate are not a concern for most healthy adults. Still, consuming excessive amounts of folic acid from supplements can be harmful.

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

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How excess folic acid develops

Your body doesn’t absorb folate as easily as it absorbs folic acid.

It’s estimated that about 85% of folic acid from fortified foods or supplements is absorbed, while only 50% of natural folate from foods is used by your body (1).

After folic acid is absorbed into your bloodstream, it’s broken down into smaller compounds by your liver. However, the liver is only able to process a certain amount of folic acid at a time (7).

As a result, consuming too much folic acid from fortified foods and supplements can cause unmetabolized folic acid (UMFA) to accumulate in your blood. This doesn’t happen when you eat high folate foods (7).

This is concerning because high blood levels of UMFA appear to be linked to various health concerns (1, 8, 9, 10, 11).

Dietary folate equivalents

Because folic acid is more readily absorbed than folate from food, the Food and Nutrition Board (FNB) at the National Academies of Sciences, Engineering, and Medicine have developed dietary folate equivalents (DFEs) to set clearer folate intake recommendations (1).

1 mcg DFEs equals (1):

• 1 mcg of folate from foods
• 0.6 mcg of folic acid from fortified foods or dietary supplements consumed with foods
• 0.5 mcg of folic acid from dietary supplements taken on an empty stomach

There is no upper limit (UL) established for naturally occurring folate from foods.

However, the National Institutes of Health (NIH) recommends that adults over the age of 19 limit their intake of folic acid from fortified foods and supplements to 1,000 mcg per day. The UL for children is even less, ranging from 300–800 mcg depending on age (12)

Keep in mind that most people don’t consume more than 1,000 mcg of folic acid per day unless they’re taking supplements in high doses.

In fact, according to the NIH, it’s estimated that only about 5% of men and women ages 51–70 take more than this amount per day, mostly due to the use of supplements (1).

Your body absorbs folic acid from fortified foods and supplements easier than it absorbs naturally occurring folate from foods. Getting too much folic acid can cause unmetabolized folic acid (UMFA) to build up in your body, which might harm health.

1. Can mask a vitamin B12 deficiency

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 (13).

If left untreated, vitamin B12 deficiency can lead to irreversible nerve damage, which makes a delayed diagnosis particularly worrisome (13).

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

For this reason, folic acid supplements may mask vitamin-B12-induced megaloblastic anemia and cause an underlying vitamin B12 deficiency to go undetected (14, 15, 16).

Megaloblastic anemia is a condition characterized by enlarged red blood cells. It can cause symptoms like weakness, fatigue, difficulty concentrating, and shortness of breath (14, 15).

If you take folic acid supplements and recognize any of these symptoms, consider getting your B12 levels checked.

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

2. May accelerate age-related mental decline

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

One study in people over the age of 60 linked high blood levels of folate or UMFA to mental decline in those with low vitamin B12 levels. This link was not seen in those with normal B12 levels (17).

Another study suggested that people with high folate and low vitamin B12 levels may be up to 3.5 times more likely to experience a loss of brain function than those with normal blood parameters (18).

However, keep in mind that more studies are needed before it can be said with certainty that supplementing with high amounts of folic acid could be detrimental to mental health.

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

3. May slow brain development in children

Adequate folate intake during pregnancy is necessary for your baby’s brain development. It also reduces the risk of birth defects (19, 20, 21, 22).

Because many women don’t meet their folate needs through food sources alone, those of childbearing age are often encouraged to take folic acid supplements (1).

However, supplementing with too much folic acid while pregnant 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 scored lower on brain development tests than the children of women who took 400–999 mcg per day (10).

Another study linked higher blood levels of folate during pregnancy to a greater risk of insulin resistance in children ages 9–13 (11).

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

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

4. May increase the likelihood of cancer recurrence 

Folic acid’s role in cancer development and recurrence 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 high levels of folic acid may help them grow or spread (23, 24).

Still, research is mixed. While a few studies note a small increase in cancer risk in people taking folic acid supplements, most studies haven’t found a link (2, 25, 26, 27).

It’s speculated that the risk may depend on the type of cancer and your health history.

For instance, some older studies suggest that people previously diagnosed with prostate or colorectal cancer who supplemented with more than 1,000 mcg of folic acid per day had a 1.7–6.4% higher risk of cancer recurrence (28, 29).

Still, eating lots of folate-rich foods doesn’t appear to increase cancer risk. It may even reduce it (30, 31, 32).

Ultimately, more research is needed on the potential link between folate, folic acid supplements, and cancer risk and recurrence.

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

Recommended use, dosage, and possible interactions

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

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

The Recommended Dietary Allowance (RDA) for folate is 400 mcg for those over 14. People who are pregnant and breastfeeding should get 600 and 500 mcg, respectively. Supplement doses typically range from 400–800 mcg (1).

You can purchase folic acid supplements without a prescription. They’re generally considered safe when taken in normal doses (33).

That said, they can interact with some prescription medications, including some that are used to treat seizures, rheumatoid arthritis, and parasitic infections. Thus, if you’re taking other medications, it’s best to consult a health professional before using folic acid supplements (1).

Folic acid supplements are used to reduce the risk of birth defects and prevent or treat folate deficiency. They’re generally considered safe if taken in recommended amounts but may interact with some prescription drugs.

The bottom line

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

However, getting too much folic acid 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 professional to determine your folate levels and whether you need to take a supplement.

Just one thing

Try this today: Adding more folate-rich foods to your diet is a great way to increase your intake of folate safely and naturally. Check out this article for a list of the top healthy foods high in folate.