Can Folic Acid Make You Feel Bad? Understanding Potential Side Effects

October 10, 2025 •

3 min read

While folic acid is widely recognized for its benefits in preventing birth defects and treating anemia, some individuals report feeling unwell after taking it. This is often linked to excessive doses or specific genetic variations that affect metabolism. The potential for negative side effects means it's crucial to understand your body's individual needs and response to this supplement.

Quick Summary

Taking too much folic acid can cause unpleasant side effects, including gastrointestinal and mood issues. Excessive intake can also mask a dangerous vitamin B12 deficiency, leading to irreversible nerve damage. Genetic variations like MTHFR can also cause intolerance. Switching to a different form, like methylfolate, may be an effective solution.

Key Points

High doses cause side effects: Taking high amounts of folic acid (typically over 1mg daily) can cause nausea, bloating, irritability, and sleep problems.
Masks B12 deficiency: Folic acid can correct the anemia of a B12 deficiency but allows irreversible nerve damage to progress, as it does not address the B12 issue.
MTHFR genetic variation impacts metabolism: A genetic mutation, especially in the MTHFR gene, can prevent your body from converting synthetic folic acid properly, leading to a build-up and side effects.
Methylfolate is an alternative: For those with intolerance or MTHFR variants, switching to the active form, methylfolate, can bypass the metabolic block.
Management strategies exist: Taking folic acid with food can alleviate gastrointestinal upset, and a doctor can help adjust dosage or switch supplements.
Always consult a doctor: It's crucial to discuss any side effects with a healthcare provider, especially before stopping supplementation or if you have concerns about B12 deficiency.

The Difference Between Folic Acid and Folate

Before delving into the side effects, it is essential to distinguish between folate and folic acid. Folate is the natural form of vitamin B9 found in foods like leafy green vegetables, citrus fruits, and legumes. In contrast, folic acid is the synthetic, man-made version added to fortified foods and supplements. The body must convert synthetic folic acid into its active, usable form, methylfolate.

How High Doses of Folic Acid Cause Side Effects

Taking folic acid at or below the recommended daily allowance (RDA) is generally safe. However, exceeding 1 mg daily can lead to unpleasant symptoms due to the body's difficulty processing large amounts of the synthetic form.

Common side effects include:

Gastrointestinal Problems: Nausea, stomach cramps, bloating, gas, and a bitter taste.
Neurological and Psychological Symptoms: Irritability, confusion, insomnia, depression, or difficulty concentrating.

The Dangers of Masking a Vitamin B12 Deficiency

A serious risk of high folic acid intake is its potential to mask a vitamin B12 deficiency. Both deficiencies can cause megaloblastic anemia, but high doses of folic acid only address the anemia symptoms without treating the underlying B12 issue. This allows crucial nerve damage due to B12 deficiency to continue undetected, potentially causing irreversible neurological problems like numbness, tingling, and cognitive decline.

MTHFR Genetic Variation and Folic Acid Intolerance

A common genetic variation, particularly in the MTHFR gene, can hinder the body's conversion of folic acid to its active form, methylfolate. This impaired metabolism can lead to unmetabolized folic acid accumulation and adverse reactions even at standard doses, including irritability, anxiety, headaches, and fatigue.

Comparison: Folic Acid vs. Methylfolate

If folic acid causes side effects, methylfolate, the active form of vitamin B9, may be a suitable alternative.


Feature	Folic Acid	L-Methylfolate	Considerations
Form	Synthetic, inactive	Bioactive, requires no conversion	Direct absorption into the body.
Metabolism	Requires MTHFR enzyme conversion	Bypasses MTHFR conversion	Preferred for individuals with MTHFR variants.
Masks B12 Deficiency?	Yes, high doses can mask anemia symptoms.	No, does not interfere with B12 deficiency detection	Safer for those at risk for B12 deficiency.
Side Effects	GI issues, mood changes, insomnia (especially high doses)	Generally well-tolerated, may cause temporary mood changes, anxiety, or headaches in some	Starting with a low dose can help mitigate effects.
Cost	Less expensive	Typically more expensive	Long-term value may outweigh cost for those with intolerance.

Less Common but Serious Side Effects

Rarely, more severe issues can arise from folic acid:

Allergic Reactions: Though uncommon, symptoms like skin rashes, itching, hives, and difficulty breathing require immediate medical attention.
Interactions with Medication: Folic acid can interact with anti-seizure drugs and certain chemotherapy agents, affecting their effectiveness.
Long-Term Concerns: Ongoing research explores potential links between excessive folic acid and other health risks, including impacts on fetal development and natural killer cell activity.

What to Do If You Feel Unwell from Folic Acid

If you suspect folic acid is causing problems, consult a healthcare provider before stopping, especially if pregnant or deficient. They can help identify the cause and recommend solutions:

Adjusting Dosage: Lowering the dose or switching forms may be advised.
Taking with Food: To alleviate GI discomfort, take your supplement with a meal.
Considering Alternatives: Methylfolate may be recommended for those with suspected MTHFR issues or persistent side effects.
Checking B12 Levels: If you've taken high doses or are concerned about deficiency, ask for both folate and B12 level tests.

Conclusion: Balancing Benefits and Risks

Folic acid is a vital nutrient, especially during pregnancy, but it has potential drawbacks. While safe for most at recommended doses, high intake, MTHFR genetic factors, and the interaction with vitamin B12 can cause issues. Awareness of these risks, symptom monitoring, and consulting a healthcare professional are key to obtaining folate's benefits while minimizing adverse effects. Often, simple strategies like taking the supplement with food or switching to methylfolate can resolve problems.

Learn more about folate deficiency symptoms here.

Frequently Asked Questions

Common side effects, which are more likely with high doses, include nausea, abdominal cramps, bloating, and a bitter taste in the mouth. Some individuals may also experience psychological effects such as irritability, confusion, and trouble sleeping.

Yes, high doses of folic acid have been associated with mood changes, including irritability, confusion, and depression. These can also be related to an accumulation of unmetabolized folic acid in individuals with specific genetic variations like MTHFR.

To help prevent or reduce nausea, try taking your folic acid supplement with food or a small snack. Taking it before bed can also be an effective strategy. If issues persist, talk to your doctor about potentially lowering your dose.

The primary danger is that high doses of folic acid can correct the blood-related symptoms of a B12 deficiency, masking the true problem. This can allow irreversible nerve damage, including numbness, tingling, and cognitive decline, to progress undetected.

Individuals who have a genetic variation that affects the MTHFR enzyme, or those who experience side effects from regular folic acid, should consider switching to methylfolate. Pregnant women at risk for neural tube defects or anemia might also benefit from the more readily usable form.

While rare, it is possible to have an allergic reaction to folic acid. Symptoms can include a skin rash, itching, redness, swelling, and difficulty breathing. If you experience these symptoms, seek immediate medical attention.

Yes, some people with genetic variations like the MTHFR polymorphism have difficulty converting synthetic folic acid, even at standard doses. This can lead to a build-up of unmetabolized folic acid and result in adverse side effects.