Section 1: Introduction -- The Patient Who Stopped Because They Felt Fine

A 29-year-old man in Kingston has been seizure-free for two years on carbamazepine 400 mg twice daily. He stopped taking it three weeks ago because he felt well and thought he no longer needed it. He presents to the emergency department following a generalised tonic-clonic seizure at the wheel of his car. He is uninjured. The passenger is in critical care.

Anticonvulsant non-adherence is the most common cause of breakthrough seizures in patients with previously controlled epilepsy. The reasons are predictable and preventable: the patient feels well, connects the medication to side effects rather than to seizure prevention, receives no counselling about the consequences of stopping, and does not understand that the absence of seizures is the medication working -- not evidence that they no longer need it.

Anticonvulsant prescribing in the Caribbean has five specific challenges:

Non-adherence and abrupt discontinuation. Anticonvulsants cannot be stopped abruptly. Sudden cessation -- for any drug, at any stage of treatment -- risks status epilepticus. Every patient on an anticonvulsant should receive explicit written and verbal instruction that the drug must never be stopped without medical guidance.

The interaction profile of older anticonvulsants. Phenytoin, carbamazepine, and phenobarbital are the most commonly prescribed anticonvulsants on Caribbean formularies. They are also among the most potent CYP enzyme inducers in clinical use, reducing the plasma levels of dozens of co-prescribed drugs. This interaction profile is discussed in Report 12 (CYP450 Report) but requires a dedicated full-drug-class treatment.

Therapeutic drug monitoring. Phenytoin, carbamazepine, valproate, and phenobarbital have narrow therapeutic indices requiring plasma level monitoring. Interpretation of levels is not straightforward.

Teratogenicity. Valproate is the most teratogenic commonly prescribed drug (Report 5). Carbamazepine, phenytoin, and phenobarbital all carry teratogenic risks. Women of childbearing age on anticonvulsants require specific management that is frequently not provided in primary care.

Driving and employment restrictions. Patients with epilepsy face legal restrictions on driving in most Caribbean territories. These restrictions have clinical relevance -- seizure recurrence has consequences beyond the patient.

Section 2: Anticonvulsant Drug Profiles

2.1 Phenytoin

Phenytoin is one of the oldest anticonvulsants and has one of the most complex pharmacokinetic profiles of any routinely prescribed drug.

Pharmacokinetics: Phenytoin displays zero-order (saturable) kinetics at therapeutic concentrations. This means that small dose increases can produce disproportionately large rises in plasma levels, and that the relationship between dose and level is non-linear. A patient stable at 300 mg/day who is increased to 350 mg/day may go from a sub-therapeutic level to a toxic level.

Therapeutic range: 10-20 mg/L (total phenytoin). Toxicity above 20 mg/L.

Toxicity signs (dose-dependent, related to level): - Nystagmus (first sign; above 20 mg/L) - Ataxia, diplopia (above 30 mg/L) - Dysarthria, confusion (above 40 mg/L) - Seizures, coma (very high levels)

Chronic adverse effects (independent of level): - Gingival hyperplasia (common; requires dental hygiene) - Hirsutism - Coarsening of facial features - Peripheral neuropathy (long-term use) - Folate deficiency (monitor in pregnancy) - Osteoporosis (CYP induction accelerates Vitamin D metabolism)

Drug interactions: Phenytoin is a potent CYP2C9 and CYP3A4 inducer. It also has CYP2C9-inhibiting properties at low concentrations. Key interactions: reduces warfarin, oral contraceptives, corticosteroids, doxycycline, and quetiapine levels. Highly protein-bound (90%); displaced by valproate, NSAIDs, and sulfonamides.

2.2 Carbamazepine

Carbamazepine is discussed in Reports 11 and 12. Key features specific to epilepsy management:

Auto-induction: Carbamazepine induces its own metabolism over the first 2-4 weeks of treatment. A patient stabilised on a starting dose will see levels fall as auto-induction completes -- requiring dose adjustment to maintain therapeutic levels.

Therapeutic range: 4-12 mg/L.

Hyponatraemia: Carbamazepine causes the syndrome of inappropriate antidiuretic hormone secretion (SIADH) in up to 40% of patients, particularly elderly patients. Sodium should be checked at baseline and periodically during treatment. This risk is amplified by concurrent thiazide diuretics.

SJS/TEN risk: Carbamazepine carries a significant risk of Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), particularly in patients of Han Chinese, Thai, and other Asian ancestry who carry the HLA-B*1502 allele. Genetic testing before carbamazepine initiation is recommended in high-risk ethnic groups.

2.3 Valproate (sodium valproate / valproic acid)

Therapeutic range: 50-100 mg/L.

Teratogenicity: The most important prescribing concern with valproate. Neural tube defects (1-2% vs 0.06% background), facial abnormalities, cardiac defects, and cognitive impairment in children exposed in utero. Fetal valproate syndrome. The absolute risk of major congenital malformations is approximately 10% -- the highest of any anticonvulsant. The EMA and MHRA have issued strong warnings: valproate must not be prescribed to women of childbearing potential without a pregnancy prevention programme in place and annual review and confirmation of contraception.

Hepatotoxicity: Rare but potentially fatal, particularly in children under 3 years on polytherapy. LFTs at baseline and during initiation.

Pancreatitis: Rare; counsel patients to report severe abdominal pain.

Weight gain: Significant; monitor BMI.

Interactions: - Valproate inhibits the metabolism of lamotrigine -- co-prescription requires lamotrigine dose halving - Valproate displaces phenytoin from protein binding, temporarily increasing free phenytoin (toxicity risk) - Carbamazepine reduces valproate levels (CYP induction)

2.4 Phenobarbital

Phenobarbital is a potent CYP inducer (CYP1A2, CYP2C9, CYP3A4) with a very long half-life (72-120 hours). It is used in some Caribbean territories for epilepsy and neonatal seizures.

Therapeutic range: 15-40 mg/L.

Key concerns: - Sedation and cognitive effects -- particularly in the elderly and in children - Dependence -- abrupt cessation causes severe withdrawal seizures - Teratogenicity (risk profile similar to carbamazepine) - Extensive CYP induction: same interaction profile as carbamazepine, but broader

2.5 Lamotrigine

Lamotrigine is one of the preferred anticonvulsants for women of childbearing age due to its more favourable teratogenicity profile compared with valproate.

Key prescribing points: - Must be titrated very slowly to reduce the risk of Stevens-Johnson syndrome -- rash on rapid titration is a drug emergency - Half-life is dramatically altered by co-prescribed drugs: valproate doubles the half-life; carbamazepine halves it - Oral contraceptive pills induce lamotrigine metabolism, reducing levels by up to 50% -- seizure risk on starting or stopping OCs - Pregnancy itself reduces lamotrigine levels; doses typically need to increase during pregnancy and reduce post-partum

Therapeutic range: 3-15 mg/L (not always required clinically; guide by response and tolerance).

2.6 Levetiracetam

Levetiracetam has a favourable pharmacokinetic profile -- minimal drug interactions, no CYP involvement, renal excretion.

Key points: - No significant drug interactions (not a CYP substrate or inducer/inhibitor) - Renal dosing required in CKD -- reduce dose when CrCl below 80 (Report 2) - Psychiatric adverse effects are the most clinically significant: irritability, aggression, depression, psychosis. These are more common in patients with pre-existing psychiatric conditions - Can be used intravenously for acute seizure management

Section 3: The Interaction Web -- Anticonvulsants and Co-prescribed Drugs

3.1 Anticonvulsants as inducers -- what they reduce

3.2 Drugs that alter anticonvulsant levels

Section 4: Therapeutic Drug Monitoring

Therapeutic drug monitoring (TDM) is essential for phenytoin, carbamazepine, valproate, and phenobarbital. Levels should be checked: - At initiation and after each dose change (at steady state -- after 5 half-lives) - When a new interacting drug is added or stopped - After any suspected adverse effect or toxicity - In pregnancy (levels change significantly) - After a breakthrough seizure

Timing of sample: Trough level (just before the next dose) is standard for all anticonvulsants except phenytoin (where timing matters less due to zero-order kinetics, but consistency is important).

Free phenytoin: In patients with hypoalbuminaemia (malnutrition, renal failure, liver disease), total phenytoin levels underestimate free (active) drug. Free phenytoin level or corrected phenytoin formula should be used.

Section 5: Anticonvulsants in Women of Childbearing Age

5.1 The valproate pregnancy prevention programme

Every woman of childbearing potential on valproate must: - Be informed of the teratogenic risk at initiation and annually - Be using highly effective contraception (not OCP alone -- enzyme-inducing anticonvulsants reduce OCP efficacy) - Have documented informed consent - Have annual review confirming ongoing awareness and contraceptive use - If pregnancy is planned: specialist referral for anticonvulsant switch where possible; folic acid 5 mg/day at least 3 months before conception

5.2 Preferred anticonvulsants in women of childbearing age

5.3 Folic acid

All women on anticonvulsants who could become pregnant should take folic acid 5 mg/day (higher than the standard 400 mcg/day supplement). Anticonvulsants -- particularly enzyme inducers -- deplete folate, increasing neural tube defect risk independently of the drug's direct teratogenic effects.

5.4 Lamotrigine in pregnancy

Lamotrigine levels fall progressively during pregnancy due to increased renal clearance and altered protein binding. Seizure breakthrough in pregnancy may reflect falling lamotrigine levels rather than disease progression. Monitor levels monthly from the second trimester. Dose increases are frequently required. Levels typically rise sharply in the first weeks after delivery -- reduce dose promptly to avoid toxicity.

Section 6: Never Stop Abruptly -- The Clinical Rule That Saves Lives

All anticonvulsants can cause status epilepticus on abrupt withdrawal. This applies regardless of the drug, the dose, or the duration of treatment. Status epilepticus carries a mortality of approximately 20%.

Practical instructions for every anticonvulsant patient:

• Never stop the drug suddenly, even if side effects develop acutely -- contact a clinician first
• If a dose is missed, take it as soon as remembered unless the next dose is due -- do not double up
• If vomiting prevents oral intake, attend for clinical assessment -- anticonvulsants cannot be withheld for a full day without risk
• Driving cessation following any breakthrough seizure is legally required in most Caribbean territories (duration varies by jurisdiction -- typically 6-12 months seizure-free)

Planned discontinuation: If anticonvulsants are to be stopped (after 2+ seizure-free years, by specialist decision), the taper must be slow -- typically reducing by no more than 10-25% every 2-4 weeks, with the patient warned of the seizure risk during the taper period.

Section 7: The Epilepsy Prescribing Checklist

Section 8: About ElesRx

ElesRx flags anticonvulsant drug interactions across the full range of CYP-mediated induction effects -- identifying co-prescribed oral contraceptives, warfarin, antipsychotics, and immunosuppressants whose levels will fall when carbamazepine, phenytoin, or phenobarbital are added. The system also flags valproate teratogenicity in patients of childbearing age, the valproate-lamotrigine interaction requiring dose halving, and the carbamazepine-clarithromycin toxicity risk.

The tool is available at elesrx.com. ElesRx is a product of PIPPS Smart Apps, a division of J.C. Epiphany Limited (Jamaica, est. 1998).

Section 9: Methodology and References

9.1 Data sources

Anticonvulsant data is drawn from the ElesRx clinical database, DailyMed, the European Medicines Agency, Health Canada, the Epilepsy Society (UK) guidelines, the International League Against Epilepsy (ILAE) guidelines, and published clinical pharmacokinetic references.

9.2 Limitations

Epilepsy surgery, vagal nerve stimulation, ketogenic diet therapy, and specialist-level management of refractory epilepsy are beyond this report's scope. Neonatal seizure management and paediatric anticonvulsant dosing are covered only at the level relevant to general clinical awareness.

9.3 Author and conflict of interest disclosure

This report was authored by Juliet Duncan, BPharm, founder of J.C. Epiphany Limited and developer of ElesRx. The author has a commercial interest in ElesRx. This report is published without an access gate as a contribution to Caribbean clinical education. No external funding was received.

9.4 Citation

Duncan J. The Epilepsy Report: Anticonvulsant Prescribing, Interactions, and the Drug That Cannot Be Stopped. ElesRx Clinical Reports, Report 20. Prepared June 2026. Published 2027 at elesrx.com/reports/epilepsy-report/. J.C. Epiphany Limited, Jamaica.

References

1. Patsalos PN, Berry DJ, Bourgeois BFD, et al. Antiepileptic drugs -- best practice guidelines for therapeutic drug monitoring. Epilepsia. 2008;49(7):1239-1276. doi:10.1111/j.1528-1167.2008.01561.x

2. Bromley RL, Baker GA. Fetal antiepileptic drug exposure and cognitive outcomes. Seizure. 2017;44:225-231. doi:10.1016/j.seizure.2016.10.006

3. Tomson T, Battino D, Bonizzoni E, et al. Comparative risk of major congenital malformations with eight different antiepileptic drugs: a prospective cohort study of the EURAP registry. Lancet Neurol. 2018;17(6):530-538. doi:10.1016/S1474-4422(18)30107-8

4. European Medicines Agency. Valproate and related substances: measures to prevent exposure in pregnancy. EMA/369850/2018. 2018.

5. Kwan P, Brodie MJ. Phenobarbital for the treatment of epilepsy in the 21st century: a critical review. Epilepsia. 2004;45(9):1141-1149. doi:10.1111/j.0013-9580.2004.12704.x