Folic acid is a synthetic form of folate or vitamin B9, a water-soluble vitamin found in many foods. It is essential for DNA and RNA formation, red blood cell production, and protein metabolism. One of folate's key functions is breaking down an amino acid called homocysteine that becomes toxic at high levels. Despite these very important benefits, folic acid can cause side effects in some instances.


Folic acid toxicity can cause neurological symptoms in rare cases. People may experience dizziness, confusion, poor concentration, impaired motor skills, and difficulty walking. Fortunately, these symptoms are usually reversible. Other temporary symptoms include tingling, numbness, tongue pain, and fatigue. There are currently no confirmed reports of fatal folic acid overdose. Researchers analyzed one suspected case, but they didn't confirm folic acid as the cause of death.

woman at work experiencing numbness in her wrist


Supplement Side Effects

Folic acid, like most supplements, can cause side effects for some people. Most potential side effects are fairly mild, such as a bitter aftertaste, nausea, vomiting, loss of appetite, diarrhea, or stomach gas. These could be due to the body getting used to the supplement. More serious side effects include confusion, irritability, and sleep disruptions. Chest pain, difficulty breathing, or a skin rash may indicate a potentially life-threatening reaction and require immediate medical attention.

middle aged man walking outside grasping chest


Cognitive Decline

High levels of folic acid may contribute to cognitive decline in the elderly. Studies find that a combination of high plasma levels of folic acid and low levels of vitamin B12 increased the rate of cognitive decline. Detrimental effects weren't as pronounced with moderate or high plasma levels of B12. More research is needed to fully understand the impact of folic acid on brain processes among the elderly.

older woman holding her head and looking in pain


Vitamin B12 Deficiency

High amounts of folic acid can disguise symptoms of a vitamin B12 deficiency. B12 and folic acid are both necessary for red blood cell formation, so deficiencies can cause anemia. Large doses of folic acid may relieve symptoms of anemia, at least temporarily. A B12 deficiency could go undetected until the brain and central nervous system sustain enough damage to be noticeable.

person writing Vitamin B12 on a blackboard with chalk


Folic Acid vs. Folate

Surprisingly, too much folic acid can interfere with folate metabolism. The digestive system converts folic acid into folate, which is the usable form of the vitamin. If there is too much folic acid to convert, some of the synthetic version could enter the bloodstream unaltered. Excessive levels of folic acid in the circulatory system may prevent folate from entering cells.

close up of folate on nutritional information list


MTHFR Mutations

The MTHFR gene regulates an enzyme involved in folate metabolism. Some people have mutated variants of the MTHFR gene known as single nucleotide polymorphisms or SNPs. SNP mutations make the conversion of folic acid into folate less effective. A pregnant woman with a MTHFR mutation may take folic acid supplements — as is usually recommended — and develop elevated levels of folic acid in the bloodstream. This circulatory folic acid interferes with homocysteine breakdown. High levels of homocysteine increase the risk of blood clots and potential miscarriage in the first trimester.

digital image of one gene in a DNA strand


Correlation With Cancer

Rates of colorectal cancer started rising during the first few years of widespread folic acid fortification of foods. Average folate blood levels nearly doubled during the same time period. Folate may offer some protection against cancer, but laboratory studies show that folic acid also stimulates cancer cell growth. Researchers are studying the complex functions of folate and any relationship with colorectal cancer.

cancer cells digital 3D rendering


Allergic Reactions

Although it is extremely rare, some people develop an allergy to folic acid. Most allergic reactions are triggered by folic acid supplements, but there are some reported cases of reactions to fortified foods. Allergy symptoms include flushing, hives, itching, vomiting, fever, and anaphylactic shock. People with a folic acid allergy can usually eat foods containing folate without any adverse effects.

young woman scratching her itchy neck


Medication Interactions

Folic acid can interfere with the absorption of some antibiotics, which makes the medications less effective. Ask a doctor if folic acid supplements are safe for use with a prescribed antibiotic. Sometimes taking vitamin supplements at different times can help. Folic acid supplements can also interfere with anti-seizure medications and increase the risk of seizures. Doctors may recommend avoiding folic acid supplements altogether or look for workable solutions if the supplement is necessary.

older woman pouring antibiotics into her hand


Recommended Limits

Toxicity is very rare if naturally occurring vitamins in foods are the only source of folate in a person's diet. Fortified foods and vitamin supplements contain synthetic folic acid. The Institute of Medicine set the upper limit for folic acid intake at 1,000 mcg per day. People taking vitamin supplements may surpass 1,000 mcg daily due to the additional folic acid in fortified foods. Most adults only need approximately 400-500 mcg daily. Although folic acid can cause adverse effects, most people don't experience any side effects at all. Folic acid is water-soluble, so our bodies can excrete most of the excess through urine.

pen hovering over checkboxes on question "are you getting enough folic acid"?


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