Supplement-anticonvulsant interactions: what neurologists watch for

Antiepileptic drugs are unusually unforgiving. A 30% drop in serum level can be the difference between seizure control and a breakthrough event; a 30% rise can produce toxicity. Many anticonvulsants are CYP-metabolized, protein-bound, and have therapeutic windows narrow enough that interactions matter clinically even when they would be ignorable for other drug classes. Several common supplements can perturb these levels — sometimes substantially.

Why anticonvulsants are interaction-prone

Most major anticonvulsants are either hepatic enzyme inducers (phenytoin, carbamazepine, phenobarbital, primidone), inhibitors (valproate), or sensitive substrates (lamotrigine via glucuronidation, levetiracetam predominantly renal). Lamotrigine and valproate have especially complex interactions because of competition for glucuronidation and protein binding. The newer agents (lacosamide, eslicarbazepine, perampanel) have somewhat cleaner profiles but are not interaction-free [1].

St John's wort: the largest single risk

St John's wort induces CYP3A4 and CYP2C19. It lowers plasma levels of carbamazepine, oxcarbazepine, perampanel, and several anti-epileptic newcomers. Phenytoin and phenobarbital plasma levels are also reduced, sometimes substantially. Case reports describe loss of seizure control after patients began St John's wort for depression. The interaction is bidirectional in a sense: St John's wort itself has serotonergic effects that may further alter neurotransmitter balance. Neurologists routinely include St John's wort in the do-not-add list for patients on enzyme-substrate anticonvulsants [2].

Ginkgo biloba: pro-convulsant risk

Ginkgo seeds contain 4'-O-methylpyridoxine, a vitamin B6 antagonist that can lower seizure threshold at high exposure. Ginkgo leaf extracts have lower concentrations but case reports describe breakthrough seizures in patients on anticonvulsants who added ginkgo. The American Academy of Neurology's complementary therapy review lists ginkgo as a relative contraindication in epilepsy [3].

Evening primrose, borage, and other GLA sources

Evening primrose oil and borage oil are sometimes used for premenstrual symptoms and dermatologic complaints. Case reports describe lowered seizure threshold, possibly through prostaglandin metabolite effects. The mechanism is debated but the safety signal has appeared in enough case series that anticonvulsant labels in some jurisdictions list these oils as caution items [4].

CBD and cannabidiol products

Prescription-grade CBD (Epidiolex) is approved as adjunctive therapy for Lennox-Gastaut and Dravet syndromes — at therapeutic doses, it raises serum levels of clobazam, valproate, and several other anticonvulsants through CYP and UGT effects. Over-the-counter CBD products are typically lower in dose but variable in purity and can produce similar interaction patterns. The clinically important paired effect is CBD plus valproate, which has shown a hepatotoxicity signal in pediatric trials requiring liver enzyme monitoring [5].

Folic acid: the unrecognized pillar

Many anticonvulsants — particularly the enzyme-inducing older agents — accelerate folate metabolism and produce relative folate deficiency. This is especially clinically important in women of childbearing age because of teratogenicity (neural tube defects), and pre-conception folic acid 4–5 mg/day is recommended for women on antiepileptics planning pregnancy. The interaction is the opposite of harmful — supplementation corrects a drug-induced deficit — but it must be coordinated with the neurologist [6].

Vitamin D and bone health

Long-term enzyme-inducing anticonvulsant therapy accelerates vitamin D 25-hydroxylase metabolism, lowering serum 25-hydroxyvitamin D and contributing to osteoporosis and fracture risk. Vitamin D supplementation (typically 1,000–2,000 IU daily, sometimes higher with monitoring) is a routine adjunct in chronic antiepileptic therapy. This is not an interaction to avoid but one to actively manage [7].

What patients should disclose

Patients with epilepsy should provide their neurologist a complete supplement list at every visit, including herbal teas. New supplement initiation should be discussed before starting. Particular attention is needed for products marketed for "stress," "sleep," "mood," or "energy" — these categories disproportionately include CYP-active herbs (St John's wort, ginkgo, valerian-containing combinations, ginseng products). Therapeutic drug monitoring of serum anticonvulsant levels is the empirical way to detect interactions; patients on tightly controlled regimens may benefit from level checks 2–4 weeks after any supplement change.

The bottom line

Anticonvulsants are a class where supplement-drug interactions can convert a controlled patient into a breakthrough seizure. The highest-risk additions are St John's wort, ginkgo, evening primrose, and over-the-counter CBD products. Folic acid and vitamin D are corrective rather than harmful additions but should be coordinated with the prescribing neurologist. The general rule: epilepsy patients should not self-initiate any new supplement without clinician review, and their neurologist should have an accurate inventory at every visit.