Raw research findings for this medication. Mostly single studies, not curated or confirmed robust.
Phenytoin is an anticonvulsant: it steadies nerve cells by blocking their sodium channels, so seizures are less likely to start and spread. It is used for epilepsy, including to stop prolonged seizures in hospital. The amount in the blood is often measured, because a small change in dose can make a large change in level. It is sold as Dilantin, among other names.
Two genes have strong evidence for phenytoin. CYP2C9 breaks it down, and people with reduced-function versions (such as *2 and *3) clear it more slowly, so levels can rise high enough to cause side effects like unsteadiness. HLA-B*15:02, an immune-system gene version most common in parts of East and South-East Asia, is linked to rare severe skin reactions. CPIC and DPWG have published guidelines on both genes, and the US drug label mentions them. Studies have also looked at ABCB1, CYP2C19 and sodium channel genes, with weaker evidence.
Clinical guidelines that name a gene for Phenytoin. They sort people into groups by the gene copies they carry.
DPWG guideline: HLA-B. *15:02 positive. 2024.
US FDA: HLA-B. *15:02 positive. Data that Support Therapeutic Management Recommendations.
Every published finding on gene variants and Phenytoin. Upload your DNA file to see which of these rows apply to you.
PharmGKB weighs all studies of a variant and rates how strong the evidence is, from 1A (strongest) to 4.
CYP2C9 CYP2C9*1, CYP2C9*2, CYP2C9*3: Level 1A · replicated. Toxicity: Epilepsy. 21.
CYP2C9 CYP2C9*1, CYP2C9*2, CYP2C9*3, CYP2C9*5, CYP2C9*6, CYP2C9*8, CYP2C9*11, CYP2C9*13, CYP2C9*14, CYP2C9*16, CYP2C9*29, CYP2C9*31, CYP2C9*33, CYP2C9*37, CYP2C9*39, CYP2C9*42, CYP2C9*43, CYP2C9*45, CYP2C9*50, CYP2C9*52, CYP2C9*55: Level 1A · replicated. Metabolism/PK: Epilepsy. 19.
HLA-B HLA-B*15:02: Level 1A · replicated. Toxicity: Drug Reaction with Eosinophilia and Systemic Symptoms, Severe Cutaneous Adverse Reactions, Stevens-Johnson Syndrome, Toxic Epidermal Necrolysis. 18.
HLA-B HLA-B*13:01: Level 3. Toxicity: Drug Reaction with Eosinophilia and Systemic Symptoms, Stevens-Johnson Syndrome, Toxic Epidermal Necrolysis. 3.
Studies of how quickly the body processes this medication.
Studies of how well it worked in people carrying a variant.
Studies of how much of the drug people needed or tolerated.
CYP2C9 CYP2C9*3: CYP2C9 *3 is associated with dose of phenytoin in children with Epilepsy.
Studies of side effects in people carrying a variant.
Study findings and evidence levels: PharmGKB (CC BY-SA 4.0). Guidelines: CPIC and DPWG, as bundled in PharmCAT. Most studies were done mainly in people of European ancestry.
More drugs from the same category
Antiepileptics (class) (Anticonvulsants)
Not a prescribing recommendation. This page is educational information about pharmacogenomic research and clinical guidelines - it is not medical or dosing advice, and not a prediction of how any medication will work for you. Never start, stop, or change a medication or dose based on it; talk to your doctor or pharmacist, who can order clinical-grade testing if appropriate.
Content updated September 2026.
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