UGT1A4 Genotype

Say a standard dose of lamotrigine or clozapine did nothing for you, or hit you harder than it should have. Your genes may be part of the reason. UGT1A4 (UDP-glucuronosyltransferase 1A4) is the gene for an enzyme that clears several seizure and psychiatric medicines, and common variants in it change how that enzyme behaves.

This is still a research-stage marker. No formal dosing rules are built around it the way there are for some other medication genes, so a result is a clue you act on alongside drug blood levels, not a prescription by itself. It's also a clue you can have before you ever start the drug, and it stays true for life.

What the Enzyme Does

Your liver clears a lot of what you swallow by attaching a sugar-based tag to it. The tag makes an oily molecule dissolve in water, so it can leave in urine or bile. Scientists call this glucuronidation, and UGT1A4 is one of the enzymes that does it.

Most enzymes in this family prefer one chemical shape. UGT1A4's specialty is molecules built around nitrogen, plus some steroids. That puts it in the path for clearing lamotrigine, clozapine, olanzapine, and tamoxifen, along with dihydrotestosterone and several cancer-causing industrial chemicals studied in the lab.

It works most strongly in liver cells, with lower and more variable expression reported in the bile duct and parts of the gut. This test reads the instruction for the enzyme, not the enzyme itself. So it predicts your built-in capacity. It doesn't tell you how hard your liver is working today.

The Two Variants Studied Most

Most of what's known centers on two changes. UGT1A42 swaps one protein building block at position 24 (written P24T), and UGT1A43 swaps one at position 48 (L48V). If you carry neither, your report will usually read 1/1, the reference version.

Source: Ehmer et al. (both rows).

Those activity numbers came from enzyme made in cells and tested in a dish, not from people. They show the variants change how the enzyme behaves. They don't tell you which way your medicine levels will move. That depends on the drug, as the next two sections show.

Lamotrigine and Seizure Control

Lamotrigine is where the human evidence is strongest. A 2024 meta-analysis of 11 studies in people taking it for epilepsy found that *3 was linked to how much drug showed up for each milligram of dose and to seizure control. It did not find a clear difference in average blood level by genotype alone.

If you carry *3 and take lamotrigine, your dose may not produce the blood level your prescriber expects. A lamotrigine blood level after you settle on a dose, and after each dose or interacting-medication change, is more useful than guessing from dose alone.

Clozapine and Other Psychiatric Medicines

In a 2026 study using 296 clozapine blood-level measurements from 61 adults, *3 carriers had lower blood levels of clozapine and of its main breakdown product, norclozapine. Here the variant looked like faster clearance, not slower. Valproic acid taken with clozapine lowered norclozapine by about half, with little effect on clozapine itself.

Clozapine is one of the psychiatric drugs for which blood-level monitoring is often used. A genotype that points toward faster clearance can leave a dose that looks right on paper falling short in your blood.

The same gene variant can point in different directions for different molecules. In one dish experiment, *3 lost its grip on dihydrotestosterone. In people taking clozapine, it looked faster. Read this result drug by drug.

Liver Cancer and Gallbladder Disease

Because UGT1A4 clears some cancer-causing chemicals, researchers asked whether 2 or 3 raises cancer risk. In the cohort that first characterized them, neither variant was linked to primary liver cancer.

A larger genetic analysis that combined gene-expression and blood-chemistry data pointed at the wider UGT1A region, not the 2 and 3 variants. In that analysis, a shared genetic signal that raised UGT1A1 expression and lowered UGT1A4 expression was tied to lower bilirubin and lower risk of gallbladder and bile duct disorders. That finding does not carry over directly to your result, because this test centers on protein-coding changes in UGT1A4.

A One-Time Result

Your UGT1A4 sequence was set at conception and won't change. There's no trend to track and no reason to repeat the test unless the lab flags a call as uncertain or low quality.

The value comes from using the result for decades. Keep it in your records and raise it whenever lamotrigine or clozapine comes up. For olanzapine or tamoxifen, treat it as weaker background context unless your medication report gives drug-specific guidance. The test that does need repeating is the drug level itself: once you reach a steady dose, and again after dose or interacting-medication changes.

Why a Clean Result Can Still Mislead

• Panel coverage: targeted panels only look for the variants they were built to find. In 10,030 people tested across many medication genes, sequencing found clinically relevant variants that standard panels missed in 28% of them. That study did not test UGT1A4, but the point applies: a normal result from a narrow panel is only a normal result for the variants listed.
• Look-alike neighbors: UGT1A4 shares most of its sequence with sister genes, and only its first section is unique. A test has to read that stretch carefully to be sure it's looking at UGT1A4 and not a neighbor.
• Ancestry: common variant patterns in this gene region differ between populations, including European, African, and East Asian groups. How often a variant shows up, and how much is known about it, depends on who was studied.
• Unknown variants: sequencing can find rare changes nobody has studied. These may come back as variants of uncertain significance. Treat them as unknown, not as good news or bad news.
• Tumor or consumer reports: tumor-only sequencing can include changes that arose in cancer cells, not the DNA you were born with. Direct-to-consumer raw data can also miss or mislabel medication variants. Use a clinical germline report for medication decisions.

What to Do With Your Result

If you're 1/1 for the variants tested, don't change a drug plan from this result alone. If you carry 2 or 3, what comes next depends on what you take or might take.

• Taking or starting lamotrigine or clozapine: get a blood level of the drug at a steady dose and after every dose or interacting-medication change. Dosing should follow the level, side effects, and the reason you're taking the drug.
• Taking valproic acid with clozapine: valproic acid can lower norclozapine, so blood-level monitoring matters even when the genotype signal is unclear.
• If the call quality is uncertain: confirm the result with a clinical germline test, preferably by a method that can distinguish UGT1A4 from nearby sister genes.
• Building a fuller medication profile: pair this with UGT1A1, the sister enzyme best known for bilirubin and the chemotherapy drug irinotecan, and with CYP2D6 and CYP2C19. Those last two are liver enzymes from a separate family that handle many psychiatric drugs by a different route.
• Getting help reading it: a pharmacist trained in medication genetics or a clinical pharmacologist can map your result onto your actual prescriptions. A genetic counselor matters most if you get an uncertain variant or need help with family questions.

Tell close biological relatives. A child or sibling may share a variant you carry, and at least one of your parents almost always does too. It matters mainly if one of these drugs comes up.