Lamotrigine Unit Converter
Lamotrigine Unit Converter
Convert lamotrigine between µg/mL and µmol/L, with the valproate and enzyme-inducer interactions and the pregnancy-related clearance changes that drive most requests for a level.
Lamotrigine converter
Mass ⇄ molarLamotrigine 6.0 µg/mL
Formula and conversion factor
µg/mL = µmol/L ÷ 3.9049
- 3.9049
- derived from the molecular weight of lamotrigine, 256.09 Da (1000 ÷ 256.09)
- mg/L
- numerically identical to µg/mL
- reference range
- a population range, not a validated therapeutic window — dosing follows clinical effect
Worked example
Lamotrigine 6.0 µg/mL
6.0 × 3.9049 = 23.4 µmol/L
= 6.0 mg/L
Interactions that change the lamotrigine dose
| Interacting drug | Effect on level | Practical consequence |
|---|---|---|
| Sodium valproate | Roughly doubles it | Halve the lamotrigine dose and titrate more slowly |
| Carbamazepine, phenytoin, phenobarbital | Roughly halves it | Higher lamotrigine dose usually needed |
| Combined oral contraceptive | Roughly halves it | Level may fall through the pill-free week, then rise again |
A loose range, and two interactions that matter
Lamotrigine’s reference range is loose, and it is worth being candid about that: unlike some antiepileptics, lamotrigine is dosed to clinical effect — seizure control balanced against side effects — rather than to a specific concentration. A level within the quoted range does not guarantee adequate seizure control, and a level above it does not necessarily mean the dose should fall. Monitoring is most useful for specific questions rather than as routine surveillance: in pregnancy, when an interacting drug is started or stopped, and to check adherence when response is unexpectedly poor.
Two interactions matter enough to change practice directly. Sodium valproate inhibits the glucuronidation pathway that clears lamotrigine and roughly doubles its concentration at a given dose; when the two are combined, the lamotrigine dose must be reduced, typically by around half, and titrated even more slowly than usual. In the opposite direction, enzyme-inducing antiepileptics — carbamazepine, phenytoin, phenobarbital — and combined oral contraceptives roughly halve the concentration, so a dose that worked while taking the pill can become inadequate if it is stopped, or vice versa.
Pregnancy produces a large and progressive fall in lamotrigine clearance through gestation — levels can fall by more than half by the third trimester as oestrogen induces glucuronidation — and then rebound within days of delivery. Monitoring through pregnancy with a planned, proactive postpartum dose reduction is standard practice, since failing to reduce the dose after delivery risks toxicity as clearance returns to its pre-pregnancy baseline.
The slow titration schedule used when starting or restarting lamotrigine exists specifically to reduce the risk of severe cutaneous reactions, including Stevens-Johnson syndrome, and it must not be shortcut — including after any break in treatment of more than a few days, when titration should restart from the beginning rather than resuming the previous dose.
Frequently asked questions
How do I convert lamotrigine from µg/mL to µmol/L?
Multiply by 3.9049. A level of 6.0 µg/mL is 23.4 µmol/L. The factor comes from lamotrigine’s molecular weight of 256.09 Da.
Is there a strict therapeutic range for lamotrigine?
No. The commonly quoted 2.5 – 15 µg/mL range is a population reference rather than a validated target, and lamotrigine is dosed to clinical effect rather than to a level.
Why does valproate change the lamotrigine dose?
Valproate inhibits the glucuronidation pathway that clears lamotrigine, roughly doubling its concentration. The lamotrigine dose is halved and titrated more slowly when the two are combined.
Why does lamotrigine need monitoring in pregnancy?
Clearance rises markedly through pregnancy and levels can fall by more than half by the third trimester, then rebound within days of delivery. Monitoring supports a planned postpartum dose reduction.
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References
- Patsalos PN et al. Antiepileptic drugs — best practice guidelines for therapeutic drug monitoring: ILAE position paper. Epilepsia. 2008;49(7):1239–76.
- Reisinger TL et al. Antiepileptic drug clearance and seizure frequency during pregnancy in women with epilepsy. Epilepsy Behav. 2013;29(1):13–8.