Antiseizure Medication Adverse Effects and Interactions
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Direct answer
Antiseizure medications cause dose-related central effects, drug-specific organ toxicities and clinically important interactions, and the choice of drug depends on these profiles. Carbamazepine and oxcarbazepine cause hyponatraemia, valproate causes hepatotoxicity and neural tube defects, and phenytoin and carbamazepine induce hepatic enzymes that lower the efficacy of oral contraceptives, and carbamazepine also lowers warfarin effect.
What you must remember
- Enzyme induction: phenytoin, carbamazepine, phenobarbital and primidone speed the metabolism of contraceptives and many other drugs; carbamazepine lowers warfarin effect, while phenytoin's effect on warfarin is variable.
- Valproate interactions: valproate roughly doubles lamotrigine levels, while carbapenems such as meropenem markedly reduce valproate levels.
- Hyponatraemia: carbamazepine and oxcarbazepine cause hyponatraemia through an SIADH-like effect, so serum sodium needs monitoring.
- Serious rash: lamotrigine, carbamazepine and phenytoin can cause Stevens-Johnson syndrome or toxic epidermal necrolysis; HLA-B*1502 screening is advised before carbamazepine in people of Asian ancestry.
- Hepatotoxicity: valproate is the main culprit, with highest risk in children under two and in patients with POLG mutations; it can also cause pancreatitis.
- Teratogenicity: valproate carries the greatest risk of neural tube defects and reduced IQ in offspring, and phenytoin causes fetal hydantoin syndrome.
- Phenytoin effects: gingival hyperplasia, hirsutism, nystagmus, ataxia and osteomalacia from vitamin D disturbance.
- Topiramate and vigabatrin: topiramate causes kidney stones and metabolic acidosis, while vigabatrin causes irreversible visual field constriction that needs monitoring.
Adverse effects and interactions by drug
Antiseizure drugs are compared on their signature toxicity, their main interaction and their pregnancy concern. The table covers six commonly tested agents.
| Drug | Signature adverse effect | Key interaction | Pregnancy concern |
|---|---|---|---|
| Phenytoin | Gingival hyperplasia, nystagmus, ataxia | Enzyme inducer with zero-order kinetics | Fetal hydantoin syndrome |
| Carbamazepine | Hyponatraemia, diplopia, rash | Enzyme inducer, lowers contraceptive efficacy | Neural tube defects |
| Valproate | Hepatotoxicity, pancreatitis, tremor | Raises lamotrigine levels; carbapenems lower them | Neural tube defects, lower IQ |
| Lamotrigine | Rash, SJS and TEN with fast titration | Valproate doubles levels; inducers lower them | Relatively lower risk, dose changes needed |
| Topiramate | Kidney stones, metabolic acidosis | Reduces contraceptive efficacy at higher doses | Oral clefts |
| Levetiracetam | Irritability, somnolence, low mood | Few pharmacokinetic interactions | Relatively favourable |
The table shows that drug choice in women of childbearing age is shaped as much by teratogenicity and contraceptive interactions as by seizure control. Renal dosing for levetiracetam and dose titration for lamotrigine are also frequently tested. The pattern of toxicity, rather than the seizure type alone, should guide the choice.
Where the exam tries to catch you
Antiseizure questions reward the candidate who links a drug to its specific toxicity or interaction.
- Trap: choosing valproate for a woman of childbearing age with epilepsy Correct: valproate carries the highest risk of neural tube defects and lower IQ, so it is avoided unless no alternative works.
- Trap: assuming an oral contraceptive protects a patient on carbamazepine Correct: enzyme induction lowers hormone levels, so an effective alternative method is advised.
- Trap: attributing new hyponatraemia to other causes without reviewing the drug Correct: carbamazepine and oxcarbazepine commonly cause it, so the drug is the first suspect.
- Trap: adding valproate to lamotrigine without changing the dose Correct: valproate roughly doubles lamotrigine levels, so the lamotrigine dose must be reduced to lower the risk of rash.
Frequently asked questions
Which antiseizure medication is most associated with hepatotoxicity?
Valproate is the antiseizure medication most associated with hepatotoxicity, with highest risk in children under two and in patients with POLG mutations. Liver function should be checked before starting valproate and monitored during treatment.
Why does carbamazepine cause hyponatraemia?
Carbamazepine causes hyponatraemia through an SIADH-like effect that promotes water retention. Oxcarbazepine can cause the same problem, so serum sodium should be checked during treatment, especially in older adults.
Which antiseizure drugs reduce the efficacy of oral contraceptives?
Enzyme inducers such as carbamazepine, phenytoin, phenobarbital and primidone reduce the efficacy of oral contraceptives by speeding hormone metabolism. Topiramate at higher doses also reduces contraceptive efficacy, so an alternative method is advised.
What rash is serious with lamotrigine?
Lamotrigine can cause Stevens-Johnson syndrome and toxic epidermal necrolysis, especially with rapid dose escalation or concurrent valproate. Slow titration and stopping at the first sign of a rash are the key preventive measures.
What is the teratogenic risk of phenytoin?
Phenytoin causes fetal hydantoin syndrome when exposure occurs in pregnancy, with craniofacial abnormalities, digital hypoplasia and developmental delay. Folic acid supplementation is recommended for women with epilepsy who are planning pregnancy.