Folic acid is the synthetic form of folate, a B vitamin naturally present in many foods. It is widely used in supplements and added to fortified foods like bread and cereals. This nutrient is recognized for its role in preventing neural tube defects in developing fetuses. While beneficial, consuming folic acid in high amounts through supplements can lead to several potential health issues.
Common Side Effects from High Doses
When folic acid intake from supplements and fortified foods exceeds the established Tolerable Upper Intake Level (UL) of 1,000 micrograms per day for adults, a range of side effects can emerge. The most frequently reported issues are gastrointestinal, including nausea, gas, bloating, stomach pain, and loss of appetite. These digestive disturbances are generally mild and may subside as the body adjusts.
Beyond digestive complaints, excessive folic acid can lead to other systemic effects. Some individuals report experiencing a distinct bitter or metallic taste in the mouth. Neurological and mood-related changes have also been noted, such as difficulty sleeping, irritability, a sense of overexcitement, or even confusion. These symptoms are more likely to occur with very high doses and tend to manifest after a period of consistent use.
It’s important to differentiate between the recommended daily intake and the upper limit. For instance, the recommended daily amount for pregnant women is 600 mcg to prevent birth defects, which is well below the 1,000 mcg upper limit. The side effects are linked to unmetabolized folic acid in the bloodstream, which occurs when the liver’s capacity to convert folic acid into its active form is overwhelmed by high supplemental doses.
Masking Vitamin B12 Deficiency
A primary concern with high folic acid intake is its ability to hide a vitamin B12 deficiency. Both vitamins are needed for the production of healthy red blood cells, and a deficiency in either can lead to megaloblastic anemia, a condition with large, inefficient red blood cells. High doses of folic acid can correct this specific type of anemia, making blood tests appear normal and resolving symptoms like fatigue and weakness.
While the anemia is resolved, the B12 deficiency continues to cause harm, particularly to the nervous system. Vitamin B12 is necessary for maintaining the protective sheath that surrounds nerves. Without sufficient B12, neurological damage can progress silently, leading to symptoms like peripheral neuropathy, memory loss, and cognitive decline.
This masking effect is the primary reason health authorities established a tolerable upper intake level for folic acid. If the B12 deficiency is not diagnosed and treated, the neurological damage can become irreversible. This risk is especially pronounced in older adults, who have a higher prevalence of B12 absorption problems. Therefore, it is standard practice to check vitamin B12 levels before beginning high-dose folic acid therapy to prevent this masking effect.
Potential Drug Interactions
Folic acid can interact with certain medications. These interactions can alter the effectiveness of a drug or change how the body processes folic acid. For example, taking folic acid can decrease the concentration of certain anti-seizure medications, such as phenytoin, fosphenytoin, and phenobarbital, potentially leading to a loss of seizure control.
Another interaction is with methotrexate, a drug used to treat some cancers and autoimmune diseases like rheumatoid arthritis. Folic acid can interfere with how methotrexate works. Doctors sometimes prescribe folic acid to patients on methotrexate specifically to reduce the drug’s side effects, but the timing and dosage must be carefully managed to avoid compromising the treatment’s efficacy.
Other substances can also interfere with folic acid absorption. Antacids that contain aluminum or magnesium can prevent folic acid from being properly absorbed if taken too close together. It is recommended to wait at least two hours between taking these types of antacids and a folic acid supplement. Anyone taking prescription medications should discuss their folic acid intake with a healthcare provider to manage any potential interactions.
Allergic Reactions
An allergic reaction to folic acid is a rare but potentially severe event. While side effects are dose-dependent reactions, an allergic reaction is an immune system response to the substance itself, regardless of the dose. These signs require immediate action.
Symptoms of an allergic reaction to folic acid can include skin issues like a rash, hives, redness, or intense itching. A more serious reaction known as anaphylaxis may also occur. Signs of anaphylaxis include swelling of the face, lips, tongue, or throat, which can lead to wheezing and difficulty breathing.
If any of these symptoms appear after taking a folic acid supplement, treat it as a medical emergency. Discontinue the supplement immediately and seek medical attention. A healthcare provider can confirm the allergy through testing and provide guidance on how to avoid folic acid in supplements and fortified foods in the future.