Folic acid is an important B vitamin often recommended for pregnant women to support healthy fetal development. However, excessive intake of folic acid can have negative health consequences. In this article, we will explore how folic acid can be harmful, what symptoms may arise from its excess, and how to properly consume this vitamin.
What is Folic Acid?
Key Functions
Folic acid, also known as vitamin B9, plays a key role in DNA synthesis, cell growth, and the formation of red blood cells. It is essential for the proper development of the fetal nervous system. This vitamin helps in the production of new cells, which is especially important during rapid growth, such as in childhood and during pregnancy.
Folic acid also participates in the metabolism of homocysteine, an amino acid that can affect the cardiovascular system. High levels of homocysteine are associated with an increased risk of heart disease, so an adequate level of folic acid can help reduce this risk. Vitamin B9 supports normal brain function, which is important for cognitive processes and mental health.
Sources of Folic Acid
Folic acid can be obtained from foods such as leafy green vegetables (spinach, broccoli), legumes (peas, beans), citrus fruits (oranges, lemons), and fortified grain products (bread, pasta). It is also available in dietary supplement form, which is particularly useful for those with limited access to fresh produce or increased vitamin needs.
Consuming natural sources of folic acid, such as vegetables and fruits, has the advantage of containing other beneficial substances that promote better absorption and utilisation of vitamins in the body. This helps avoid excessive accumulation of folic acid, which can occur with supplement use. Additionally, natural products provide the body with fibre, antioxidants, and other vitamins that support overall health.
Possible Negative Effects of Excess Folic Acid
Unmetabolised Folic Acid
As research shows (Tang et al., Frontiers in Immunology, 2022), an excess of folic acid can lead to the accumulation of unmetabolised forms of the vitamin in the blood, which can cause problems with the immune system. Unmetabolised folic acid can interact with other vitamins and minerals, leading to an imbalance in the body.
This imbalance can affect the body's ability to fight infections and other diseases. The immune system may become less effective, increasing the risk of developing chronic diseases or exacerbating existing ones. Unmetabolised folic acid can also affect metabolism, leading to disruptions in the body's detoxification processes.
Impact on the Nervous System
Research (Sun et al., Behavioural neurology, 2025) indicates that an excess of folic acid may have a toxic effect on the nervous system, causing symptoms such as nervousness, depression, and sleep problems. This may be due to the fact that excess folic acid can affect the levels of other neurotransmitters in the brain, which in turn affects mood and behaviour.
In addition, excessive intake of folic acid can mask a vitamin B12 deficiency, which is critical for the nervous system. A B12 deficiency can lead to serious neurological problems, such as peripheral neuropathy or even dementia. It is important to monitor the levels of both vitamins to avoid such complications.
Other possible consequences
Besides affecting the immune and nervous systems, excessive intake of folic acid can also lead to masking a vitamin B12 deficiency. This can be particularly dangerous for older adults, in whom a B12 deficiency can lead to serious neurological problems. Some studies also indicate a potential increased risk of cancer with prolonged excessive intake of folic acid. This may be due to the fact that folic acid promotes rapid cell growth, including cancer cells if they are already present in the body.
It is important to understand that folic acid can interact with other medications and vitamins. For example, it may reduce the effectiveness of certain drugs used to treat epilepsy. Therefore, it is advisable to consult a doctor before starting any supplements.
Risks for pregnant women
Impact on fetal development
While folic acid is important for pregnant women, excessive intake can have the opposite effect. Excess can lead to neural tube defects in the fetus. This may occur because excessive amounts of folic acid can interfere with the normal metabolism of other important substances in the mother's and fetus's bodies.
It is especially important to monitor folic acid levels during the first trimester of pregnancy when the main organs and systems of the fetus are forming. Excess folic acid can affect this process, potentially leading to congenital anomalies. Additionally, excessive intake can affect the development of the child's brain and nervous system, which may have long-term health implications.
Metabolic disorders
Excessive intake of folic acid can affect the mother's metabolism, which may negatively impact the child's health. This can increase the risk of developing metabolic disorders in the child, such as obesity or diabetes in the future. It is important to consider that metabolic changes can affect the development of the child's cardiovascular system.
Excess folic acid can also affect glucose and lipid metabolism, which may contribute to the development of gestational diabetes in pregnant women. This increases the risk of complications during pregnancy and childbirth. It is important to monitor blood sugar levels and undergo regular medical check-ups to avoid these risks.
Other risks
Some studies also indicate a potential increased risk of developing allergic reactions or asthma in children whose mothers consumed excessive doses of folic acid during pregnancy. This further underscores the importance of adhering to recommended doses. It is also important to consider that excessive intake of folic acid can affect the development of the child's immune system, which may lead to increased susceptibility to infections at an early age.
Moreover, excess folic acid can affect the mother's hormonal balance, which may have consequences for her mental health. It is important to maintain a balance of vitamins and minerals to ensure the health of both the mother and the child.
How to avoid excess folic acid?
Recommended doses
For adults, the recommended daily dose of folic acid is about 400 mcg. Pregnant women are advised to take up to 600 mcg per day. It is important to adhere to these recommendations to avoid negative consequences. If you are taking dietary supplements, be sure to consult a doctor to avoid overdose.
Regular check-ups and blood tests can help monitor folic acid levels in the body. This is especially important for people at risk of vitamin deficiency or excess due to specific medical conditions or diets. It is also important to consider individual body needs, which may change depending on age, gender, physical activity, and overall health.
Nutrition tips
- Include natural sources of folic acid in your diet, such as green vegetables and legumes. This will help ensure an optimal level of the vitamin without the risk of overdose.
- Avoid excessive consumption of fortified foods and supplements without consulting a doctor. Remember that natural sources are always better than synthetic supplements.
- Pay attention to product labels to avoid hidden sources of folic acid that may be present in fortified foods.
It is also important to consider other factors, such as the method of food preparation, as thermal processing can reduce the folic acid content in foods. It is recommended to consume vegetables fresh or lightly processed to preserve the maximum amount of vitamins.
Consultation with a doctor
If you are unsure about your diet or have specific medical needs, consult a doctor or dietitian. They can help you create a nutrition plan that provides all the necessary vitamins and minerals without the risk of overdose.
Consultation with a specialist is especially important for pregnant women, people with chronic diseases, and those taking other vitamin supplements or medications that may interact with folic acid. A doctor may recommend additional tests or examinations to determine the optimal level of vitamins in your body.
Scientific Sources
- MR1-dependence of unmetabolized folic acid side-effects, Frontiers in immunology, 2022
- Toxic Effects of Excess Vitamins A, B6, and Folic Acid on the Nervous System, Behavioural neurology, 2025
Read Also
If you are unsure about your folic acid intake or have other dietary questions, book a consultation to our dietitian for personalised advice.
Frequently Asked Questions
What are the main functions of folic acid?
Folic acid plays a key role in DNA synthesis, cell growth, and the formation of red blood cells, as well as supporting normal brain function.
What are the best sources of folic acid?
The best sources of folic acid are leafy green vegetables, legumes, citrus fruits, and fortified grain products.
What negative effects can occur with excess folic acid?
Excess folic acid can lead to immune system problems, toxic effects on the nervous system, and masking of vitamin B12 deficiency.
How to avoid excess folic acid?
Avoid excessive consumption of fortified foods and supplements, adhere to recommended doses, and consult a doctor.
Article Author: Olga Bezugla — PhD in Medicine, clinical dietitian, and expert in metabolic health and clinical nutrition. Postdoctoral fellowship — DIfE (Deutsches Institut für Ernährungsforschung), Potsdam, Germany. More About the Doctor