400–800 µg/day; higher doses in depression as an adjunct, under supervision.
Prevents neural tube defects; the active form is useful.
View the study (clinical trial) →⏱ 3 min read
Methylfolate, technically 5-methyltetrahydrofolate or 5-MTHF, is the metabolically active form of vitamin B9, the only form of folate the body can use directly without additional enzymatic steps; classic "folic acid" supplements are a synthetic form that must be converted by the enzyme MTHFR (methylenetetrahydrofolate reductase) before it can be used in the methylation reactions the body needs to synthesize DNA and neurotransmitters and to keep homocysteine levels low.
This difference between folic acid and methylfolate becomes clinically relevant because between 30 and 60 percent of the population carries variants of the MTHFR gene (especially C677T) that reduce enzyme activity by up to 70 percent; in these people, folic acid is not converted efficiently and accumulates as unmetabolized folic acid, whereas direct methylfolate allows them to reach functional plasma and cellular levels that folic acid does not achieve.
The most studied indication of methylfolate as an adjunct to antidepressants is major depression with a partial response to SSRIs: several randomized trials have shown that adding 7.5 to 15 milligrams daily of L-methylfolate for 8-12 weeks to an antidepressant in people with an inadequate response significantly improves depressive symptoms, especially in people with MTHFR variants, elevated inflammation, or overweight, all of which are predictors of a lower antidepressant response.
The most plausible mechanism is that methylfolate is necessary for the synthesis of monoaminergic neurotransmitters (serotonin, dopamine, norepinephrine) and for maintaining the brain methylation processes that are disrupted in many depressive conditions; in people with impaired MTHFR and depression, supplementation with the active form bypasses the enzymatic bottleneck and allows adequate synthesis of the precursors needed for the full antidepressant effect.
High homocysteine levels (hyperhomocysteinemia) are a well-established marker of cardiovascular and cerebrovascular risk, and are especially associated with deficiencies of folate, B12, and B6; supplementation with methylfolate (ideally combined with methylcobalamin B12) lowers homocysteine levels faster and more efficiently than conventional folic acid, especially in people with MTHFR variants who have persistently elevated homocysteine despite taking standard B9.
The clinical debate over whether lowering homocysteine translates into fewer cardiovascular events remains open: some trials have shown a reduced risk of stroke but not of heart attack, which suggests the effect may depend on the cardiovascular subtype and on concomitant risk factors; in any case, normalizing homocysteine when it is high makes physiological sense and is low-cost with methylfolate, without relevant risks.
Folate supplementation before and during the first trimester of pregnancy is one of the public health interventions with the strongest evidence for preventing neural tube defects; mainstream clinical guidelines continue to recommend conventional folic acid (400-800 µg/day) because of its accumulated historical evidence, but in women with impaired MTHFR or a history of a previous child with a neural tube defect, methylfolate (400-800 µg/day) may be a more efficient alternative by bypassing the enzymatic bottleneck.
The doses studied range from 400 µg to 15 milligrams daily depending on the indication: 400-800 µg/day for general preventive use and pregnancy, 1-5 mg/day for hyperhomocysteinemia, 7.5-15 mg/day for refractory depression as an adjunct; ideally combined with methylcobalamin B12 to avoid masking a B12 deficiency that might not become apparent until advanced neurological damage has occurred.
It is generally very well tolerated; adverse effects are rare and mild (irritability, insomnia if taken late at high doses in sensitive people, occasionally digestive discomfort); people with bipolar disorder should be cautious at high doses because of the theoretical risk of inducing mania or hypomania (rarely described with intensive methylfolate supplementation), and it is advisable to ensure adequate B12 levels before starting high-dose methylfolate so as not to mask a B12 deficiency that would require its own independent treatment.
Information, not medical advice. Scientific evidence reflects the quality of the studies.