Folic Acid: what it does and what to watch
Also known as: Folate, Vitamin B9, Folacin · Vitamin supplement (B-vitamin) · Updated 2026-08-19 · Educational — not medical advice
In plain language: It supplies folate, a B-vitamin the body needs to build and copy DNA, which is especially important in tissues that renew quickly, such as bone marrow, where red blood cells are made. When folate is scarce, the bone marrow struggles to produce healthy, mature red blood cells, leading to a particular kind of anemia. Supplementing folate gives the bone marrow the raw material it needs to resume normal red blood cell production.
What it's used for
- Folate deficiency
- Supporting healthy red blood cell production
- Reducing the risk of certain birth defects when taken around pregnancy
- Supporting treatment of some types of anemia
Common side effects
- Mild nausea
- Bloating or gas
- Unusual or altered taste (uncommon)
- Sleep disturbances at higher doses (uncommon)
Get medical help if…
Signs of an allergic reaction, such as rash, itching, or swelling (rare)
Ongoing or worsening nerve symptoms such as tingling or numbness in someone with an undiagnosed vitamin B12 deficiency, since folic acid can improve blood counts while masking B12-related nerve damage underneath
New or unusual seizure activity in people with a seizure disorder, since very high folate intake has rarely been linked to reduced effectiveness of some anti-seizure medicines
Lab values it can move
Few guides tell you this part: medicines show up in blood work. If you take Folic Acid and a value below looks off on your report, this may be part of the story — never stop a medicine without your doctor.
Folic acid supplementation directly raises blood folate levels, which is the most direct marker of whether the supplement is being absorbed and used.
Folate (Vitamin B9)When anemia was caused by folate deficiency, hemoglobin typically rises over several weeks as the bone marrow resumes normal red blood cell production.
HemoglobinGood to know
- Can improve blood counts in vitamin B12 deficiency without addressing the nerve-related damage B12 deficiency also causes, so B12 levels are often checked before or alongside folic acid, especially in older adults or strict vegetarians and vegans
- Generally well tolerated at typical supplement amounts
- People with unexplained anemia are usually evaluated for the underlying cause rather than simply starting a supplement
- Some medicines, including certain anti-seizure and diabetes medicines, can interact with folate levels, so existing medicines are worth reviewing with a doctor
Questions people ask
Why does folic acid need to be checked against vitamin B12 levels?
Folate and vitamin B12 both support red blood cell production, and a deficiency in either can look similar on a basic blood count. Taking folic acid can correct the anemia part of a B12 deficiency by supplying what the bone marrow needs, but it does nothing for the separate nerve damage that untreated B12 deficiency can cause, which can then progress quietly. That's why B12 levels are often checked before relying on folic acid alone.
Why is folic acid often recommended around pregnancy?
Folate plays a central role in the rapid cell division and DNA-building needed during early fetal development, particularly in the formation of the neural tube, which becomes the brain and spinal cord. Adequate folate around the time of conception and in early pregnancy is associated with a lower chance of certain neural tube-related birth differences, which is why many guidelines highlight it as part of preconception and early pregnancy care.
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