What happens when you take lamotrigine with folate?
Lamotrigine is a widely used anticonvulsant and mood stabilizer. One of its less-publicized properties is that it weakly inhibits dihydrofolate reductase (DHFR), the enzyme that converts dietary folic acid (and dihydrofolate) into the bioactive form your body actually uses, tetrahydrofolate. This was noticed decades ago in animal studies and later confirmed in human biochemistry work.
For most people taking lamotrigine, this matters very little. Standard multivitamin doses of folate (around 400 mcg) and ordinary dietary intake do not appear to cause clinical problems. But the picture changes when high-dose folic acid is added on top of lamotrigine, especially in the setting of bipolar depression.
The pivotal evidence comes from the CEQUEL trial, a randomized controlled trial that tested lamotrigine plus folic acid (500 mcg/day) against lamotrigine plus placebo for bipolar depression. The researchers expected folate to help (since folate has modest antidepressant effects in unipolar depression). Instead, folic acid blocked the antidepressant effect of lamotrigine at 12 weeks. A follow-up biochemical and genetic analysis showed the effect was strongest in patients carrying the COMT Met allele, suggesting a real pharmacodynamic interaction at the folate-methylation-neurotransmitter axis rather than a simple pharmacokinetic one.
Why is this important?
This interaction is unusual because it is not the typical "folate lowers drug levels" or "drug lowers folate levels" story. Lamotrigine plasma concentrations were unchanged by folic acid in the CEQUEL data. The interference happens downstream, likely at the level of one-carbon metabolism, methylation, and monoamine synthesis. That makes it harder to monitor with routine blood tests and easier to miss.
For people with bipolar disorder who rely on lamotrigine to keep depressive episodes at bay, blunting its effect by adding a seemingly benign B-vitamin can mean preventable relapses. For people taking lamotrigine for epilepsy, the bipolar trial does not translate directly, but the underlying DHFR mechanism still raises questions about whether very high doses of folic acid could interfere with seizure control. Real-world reports of seizure breakthrough on folate are rare, but the theoretical concern is enough that most epileptologists do not push high-dose folate on lamotrigine-treated patients without a specific reason.
The other side of the coin: women of reproductive age on lamotrigine still need folate to reduce the risk of neural tube defects in pregnancy. This is a genuine tradeoff, and the right dose for a woman planning pregnancy on lamotrigine is something a neurologist, psychiatrist, and obstetrician should decide together rather than something to figure out from a supplement label.
What should you do?
If you take lamotrigine and you are not pregnant or planning pregnancy, the simplest move is to skip high-dose folic acid supplements (anything in the 500 mcg to 5 mg range) unless your prescriber specifically asked for them. A standard multivitamin with 400 mcg of folate is generally fine.
If you have been told you need higher folate, ask about folinic acid (leucovorin) or methylfolate (5-MTHF). These forms bypass the DHFR step that lamotrigine inhibits, so they are less likely to participate in this interaction. Methylfolate in particular is sometimes preferred in psychiatric practice for exactly this reason.
If you are taking lamotrigine for bipolar depression and feel your medication "stopped working" after starting a B-complex, multivitamin, or prenatal vitamin, mention the timing to your prescriber. It is a fixable problem if caught early.
If you are pregnant or planning pregnancy on lamotrigine, do not stop folate on your own. The risk of neural tube defects without folate supplementation outweighs the theoretical mood concern, and your team will tailor the dose.
Which specific products are affected?
Lamotrigine is sold under the brand name Lamictal and in many generic versions, including extended-release (Lamictal XR) and orally disintegrating (Lamictal ODT) forms. All forms inhibit DHFR to the same extent.
On the folate side, the products most likely to cause concern are:
- Prescription folic acid tablets (1 mg)
- High-dose folic acid supplements (800 mcg to 5 mg)
- Some prenatal vitamins (1 mg folic acid)
- B-complex supplements that include 400-1000 mcg folic acid
- Fortified medical foods marketed for depression that include L-methylfolate at high doses (though methylfolate is mechanistically less risky)
Standard everyday multivitamins with 400 mcg of folic acid, and dietary folate from leafy greens, beans, and fortified grains, are not the concern here. The interaction signal comes from supplemental high-dose folic acid added on top of lamotrigine.
The bottom line
Lamotrigine and high-dose folic acid can clash in a way that is easy to overlook because lamotrigine blood levels stay normal. The cleanest evidence is from bipolar depression, where added folic acid blocked lamotrigine's benefit. If you take lamotrigine, stick to a standard multivitamin level of folate unless your prescriber says otherwise, and ask about folinic acid or methylfolate if you need a higher dose. For epilepsy, the risk is more theoretical, but the same caution applies. Pregnancy planning overrides this concern and should be handled by your medical team.