anticonvulsants · Dilantin, Onfi, Briviact
Which Anticonvulsant Is Right for Me?
Seizure type narrows this choice before anything else does. Genetics mostly affects how a drug is dosed, not which one is appropriate.
Anti-seizure medications are among the least interchangeable drugs we cover. A medication that controls one seizure type can worsen another, so the clinical classification of the epilepsy does most of the work in narrowing the options, and it does so before questions of tolerability or metabolism arise.
Within that narrowed field, pharmacogenetics has a genuine but modest role, and it is concentrated on a small number of drugs where getting the dose right matters more than usual. These medications are also used for conditions other than epilepsy, including nerve pain and mood stabilisation, where the same reasoning about dose applies.
Narrow margin phenytoin has a small gap between an effective level and a toxic one, which is why metabolism matters so much for it
How the choice actually gets made
What kind of seizures you have
This is the dominant factor and it has no genetic component. Focal and generalised epilepsies respond to different medications, and some drugs effective for focal seizures can worsen certain generalised seizure types, particularly absence and myoclonic seizures. An accurate classification, usually supported by EEG and imaging, is worth far more than any other input on this page.
Whether you might become pregnant
This changes the calculation substantially and early. Some anticonvulsants carry a significant risk of birth defects and developmental effects, and the relevant decisions are best made well before conception rather than after. It is one of the few considerations that can outrank seizure control in the choice of agent, and it deserves an explicit conversation.
Side effects and how they fit your life
Cognitive slowing, sedation, mood changes, weight change and effects on bone health all differ across the class. For a medication likely to be taken for years, the one you can live with is generally the one that works, because consistency is essential and missed doses have immediate consequences here in a way they do not in most classes.
What else you take
This class is unusually prone to interactions in both directions. Several older anticonvulsants induce liver enzymes strongly, which can reduce the effectiveness of other medications including hormonal contraception and anticoagulants. Others are affected by the drugs you are already taking. Reviewing the whole list is not optional here.
How your body processes the drug
Genetics contributes mainly through two enzymes and mainly to dosing. Phenytoin is the clearest case: it is cleared largely by CYP2C9, its concentration rises disproportionately as the dose increases, and the gap between an effective level and a toxic one is small.[2] Reduced CYP2C9 function means a standard dose can produce toxicity, so a lower starting dose is recommended for poor metabolizers.
Clobazam and brivaracetam are cleared substantially by CYP2C19, where poor metabolizers accumulate higher levels and often need lower doses.[1] Note that fosphenytoin is converted to phenytoin in the body, so the same CYP2C9 reasoning applies to both.
Monitoring, formulation and cost
Some of these drugs require blood level monitoring, which is a practical commitment. Availability of liquid or injectable forms matters for people who cannot swallow tablets reliably. Most older agents are inexpensive; some newer ones are not.
For most of this class, genetics fine-tunes the dose. For phenytoin, where the therapeutic window is narrow, that fine-tuning is worth having up front.
How your genetics can play a role
Two enzymes carry most of the pharmacogenetic guidance in this class, and both act on exposure rather than on which seizure types a drug controls.
| Gene | What it affects |
|---|---|
| CYP2C9 | The main clearance route for phenytoin, and therefore for fosphenytoin, which the body converts to phenytoin.[2] Because phenytoin clearance saturates, small dose increases can produce large jumps in blood level, and reduced CYP2C9 function shifts that whole relationship. Poor metabolizers are advised to start at a reduced dose with careful monitoring. This is one of the situations where knowing the result beforehand is more valuable than discovering it through toxicity. |
| CYP2C19 | A major route for clobazam and a contributor for brivaracetam.[1] Poor metabolizers reach higher concentrations of clobazam's active metabolite and commonly need lower doses to avoid sedation. The effect is real but the consequences are more forgiving than with phenytoin, because the therapeutic window is wider. |
The honest summary for this class is that genetics adjusts doses rather than selecting drugs, because seizure type has already made the selection. That is a smaller role than in classes like statins, where a result points at a specific alternative.
One important gap: the most consequential genetic test in epilepsy care is HLA-B*15:02 screening before carbamazepine in people of certain Asian ancestries, because of the risk of severe skin reactions. That is an immune-system gene rather than a metabolising enzyme and is not part of standard metabolism-focused panels, including ours.
Want to know how you process phenytoin?
A Gene2Rx report reads your own DNA to show how it may affect your response to Phenytoin and your other medications.
Find out todayWhen to consider pharmacogenetic testing
The strongest case is before or shortly after starting phenytoin, where a CYP2C9 result informs the starting dose in a drug that is genuinely unforgiving about dosing. It is also useful when side effects such as unsteadiness, drowsiness or confusion appear at ordinary doses, since those are consistent with reduced clearance.
For clobazam, a CYP2C19 result helps explain excessive sedation. Beyond these, the yield in this class is modest, and no metabolism panel substitutes for the HLA testing recommended before carbamazepine.
What you can do next
- Make sure the seizure classification is clear and current, since it determines which medications are appropriate far more than any other factor.
- If pregnancy is possible now or in future, raise it early with your neurologist rather than waiting, because it can change which medication is preferred.
- Bring a complete list of other medications, including hormonal contraception, since several anticonvulsants reduce its effectiveness.
- If you have taken phenytoin and had unsteadiness, drowsiness or confusion at a normal dose, mention it specifically and ask your prescriber whether a CYP2C9 result would explain it.
- Ask whether blood level monitoring is needed and how often, so that the practical burden is clear before you commit to a medication.
Pick a set of results to see how the same 3 medications sort differently.
Your genetics don't flag any of these 3 - none is genetically better or worse for you.
Your genetics separate these 3 into 2 groups.
Your genetics separate these 3 into 2 groups.
A simplified example using hypothetical results, drawn from the same published guidelines as your report. Bands describe how a medication is likely to be dosed or tolerated, not how well it will work for you, and none of this replaces a conversation with your prescriber.
Related medications
Related guides
- Phenytoin (Dilantin) Side Effects and Toxicity: The CYP2C9 Link
- 23andMe Drug Response: What You'll Actually See in a Report From Your Data
- 23andMe Pharmacogenetics: How to Get a Drug Response Report From Your Existing Data
- AncestryDNA for Drug Testing: Get Pharmacogenetics From Your Ancestry Data
- Can Two Safe Medications Be Dangerous Together? How Metabolism Interactions Work
- Dante Labs Pharmacogenetics: Turn Your Dante WGS Data Into a PGx Report
Frequently asked questions
Can genetics tell me which seizure medication will work?
No. Which medication controls your seizures is determined mainly by seizure type, and no genetic test predicts that. What testing can do is inform the dose for a few specific drugs, most usefully phenytoin, where the margin between an effective and a toxic level is narrow.
Why is phenytoin dosing so difficult?
Its clearance saturates, meaning that beyond a certain point a small dose increase produces a disproportionately large rise in blood level. Reduced CYP2C9 function shifts where that point sits, which is why poor metabolizers are advised to start lower and why levels are monitored.
Does your test include the HLA gene checked before carbamazepine?
No. HLA-B*15:02 screening, recommended before carbamazepine in people of certain Asian ancestries because of the risk of severe skin reactions, involves an immune-system gene rather than a drug-metabolising enzyme and is not part of our panel. If carbamazepine is being considered, ask your doctor about that test specifically.
I feel drowsy and unsteady on my medication. Is that genetic?
It can be. Those effects are dose-related for several anticonvulsants, and reduced CYP2C9 or CYP2C19 function raises drug levels at a standard dose. They also have many other causes. Report the symptoms to your prescriber rather than adjusting the dose yourself, because abrupt changes can trigger seizures.
References
- Clinical Pharmacogenetics Implementation Consortium (CPIC). CPIC Guidelines. cpicpgx.org
- U.S. Food and Drug Administration. Table of Pharmacogenomic Biomarkers in Drug Labeling (2024). fda.gov
- PharmGKB / Stanford University. PharmGKB: The Pharmacogenomics Knowledge Base. pharmgkb.org
Disclaimer: This content is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your healthcare provider before making changes to your medication. Never stop or change a medication without medical supervision.