Skip to content
Blood Test Tracker

Lamotrigine

Measures the level of lamotrigine, a seizure and mood medicine, in your blood to check it is being absorbed and cleared as expected.

AI-verified against cited sources. Awaiting expert reviewAlso called LTGLOINC 6948-4

Check your result

Sex (optional)

Type your result to see where it falls.

Nothing you type here leaves your browser.

What it is

Lamotrigine (Lamictal) is a prescription medicine used to treat epilepsy and to help prevent mood episodes in bipolar disorder. This test measures how much is in your blood, usually as a trough just before your next dose. Levels vary a lot from person to person, because other medicines, pregnancy and hormones can change how fast the body clears the drug.

Why it matters

The FDA label says a therapeutic range has not been established, so the test is not used to set your dose by itself. It is most useful when something changes: you start or stop a medicine that interacts with it, you become pregnant, you have breakthrough seizures or side effects, or your team wants to confirm you are taking it as planned. Some clinicians compare new results with a level from a time you were doing well.

If it’s low

A low level can mean the drug is being cleared faster than expected or that doses were missed. It does not always mean treatment has stopped working, since seizure and mood control still depend on how you are doing. Your care team looks at the level together with your symptoms.

Common causes

  • Estrogen-containing birth control, which the FDA label says lowers levels by about 50%
  • Pregnancy, when levels can fall and then recover after delivery
  • Medicines that speed up clearance, such as carbamazepine, phenytoin, phenobarbital, primidone and rifampin, which the label says lower levels by about 40%
  • Some HIV protease inhibitor combinations
  • Missed doses
  • A sample drawn long after the last dose

What can help

  • Tell your care team if you start, stop or change birth control, or if you are pregnant or planning a pregnancy
  • Ask whether a level should be checked during pregnancy and after delivery
  • Check with your care team before changing or skipping any dose

If it’s high

A level above the usual range means more of the drug is in your body than expected. Side effects such as dizziness, sleepiness, nausea and vomiting are more likely at higher levels, and ARUP lists 20 mcg/mL or higher as toxic. Some people feel fine above 15 mcg/mL, so your clinician reads the number together with how you feel.

Common causes

  • Valproate (valproic acid) taken with lamotrigine, which raises lamotrigine levels more than 2-fold according to the FDA label
  • Stopping a medicine that had been lowering the level, such as estrogen-containing birth control, carbamazepine, phenytoin, phenobarbital, primidone or rifampin
  • Reduced kidney function (ARUP notes that levels vary with other medicines and with kidney function)
  • The weeks after delivery, when levels return to their pre-pregnancy values and the dose may need review
  • Taking more than prescribed
  • A sample drawn soon after a dose instead of just before the next one

What can help

  • Tell your care team about every medicine you take and any change to your birth control
  • Ask your care team which dosing schedule is right for you, and keep to it
  • Ask whether your next blood draw should be timed as a trough

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 3–15 mcg/mL. Source: ARUP Laboratories, Drugs in R&D, Drug Design, Development and Therapy, GlaxoSmithKline, via DailyMed (NIH National Library of Medicine), National Institute for Health and Care Excellence (NICE).

What can affect your result

  • Other medicines: valproate raises levels, while carbamazepine, phenytoin, phenobarbital, primidone, rifampin and estrogen-containing birth control lower them
  • Pregnancy and the weeks after delivery
  • Kidney function
  • Time since your last dose
  • How consistently you take your doses
  • ARUP asks that serum or plasma be separated from the blood cells as soon as possible, and within 2 hours of collection.
  • A sample drawn soon after a dose will read higher than a true trough.

A trough, drawn just before your next dose, gives the most consistent comparison from one test to the next. Several days to a couple of weeks to settle at a new level. The FDA label notes that the half-life depends on other medicines taken with it, from about 25 hours alone to about 70 hours with valproate.

When to talk to a clinician

Call your clinician right away if you develop a new rash, especially in the first weeks of treatment or after a dose increase, because lamotrigine can rarely cause serious skin reactions. Also call about new double vision, severe unsteadiness, repeated vomiting, or more seizures than usual. A level of 20 mcg/mL or higher is listed as toxic by ARUP. If you have trouble breathing, swelling of the face or lips, fainting, or a seizure that does not stop, call 911 or your local emergency number.

Questions you could ask

  • What level was I at when I was doing well, so we can compare?
  • Which of my other medicines can change my lamotrigine level?
  • Could one of my other medicines be raising my level?
  • Should we recheck my level, and when?
  • Could my birth control, pregnancy or another medicine explain a low level?
  • Would a level during pregnancy or after delivery be useful?

Questions people ask

Is there a normal lamotrigine level?

Labs often print 3 to 15 mcg/mL. The FDA label says no therapeutic range has been established, so many people do well outside it. Your clinician looks at the level together with your seizures or mood, side effects and other medicines.

Why did my level change after I started or stopped birth control?

Estrogen-containing birth control speeds up how fast the body clears lamotrigine, and the label says it lowers levels by about 50%. Stopping it can let the level rise again. Tell your care team about any change.

Do I need my level checked regularly?

Usually not. Guidelines for bipolar disorder in England advise measuring it only when treatment seems ineffective, adherence is a concern, or toxicity is suspected. Pregnancy and interacting medicines are other common reasons.

Show the science and sources

Units

  • 1 mcg/mL = 1 mcg/mL
  • 1 mcg/mL = 1 mg/L
  • 1 mcg/mL = 3.905 µmol/L

Open the Lamotrigine unit converter

For clinicians and researchers

Lamotrigine is a phenyltriazine that is metabolized mainly by glucuronic acid conjugation, so inducers and inhibitors of glucuronidation change its clearance. In adults the mean half-life in healthy volunteers was about 25 hours at steady state and about 70 hours with valproate (FDA label). Valproate more than doubles concentrations, carbamazepine, phenytoin, phenobarbital, primidone and rifampin lower them by about 40%, and estrogen-containing contraceptives lower them by about 50%. Concentrations fall during pregnancy and return to pre-pregnancy values after delivery. A 2016 review gives a reference range of 2.5 to 15 mg/L, ARUP gives 3 to 15 mcg/mL therapeutic and 20 mcg/mL or higher toxic, and the FDA label states no therapeutic range has been established. A 2026 survey found lamotrigine on under 1% of US hospital critical lists.

LOINC 6948-4 (primary), 25458-1, 74944-0. Specimen: serum-or-plasma.

Sources

  1. Lamotrigine (ARUP test 0090177), ARUP Laboratories
  2. LAMICTAL (lamotrigine) tablets: prescribing information, GlaxoSmithKline, via DailyMed (NIH National Library of Medicine) (2025)
  3. An Updated Overview on Therapeutic Drug Monitoring of Recent Antiepileptic Drugs, Drugs in R&D (2016)
  4. Bipolar disorder: assessment and management (CG185), recommendations, National Institute for Health and Care Excellence (NICE) (2014)
  5. Reference ranges for antiepileptic drugs revisited: a practical approach to establish national guidelines, Drug Design, Development and Therapy (2018)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

Related markers