What happens when you take phenytoin with ginkgo?
Phenytoin (Dilantin, Phenytek) is a narrow-therapeutic-index anticonvulsant primarily metabolized by CYP2C9 and CYP2C19. Even modest changes in the activity of these enzymes can move phenytoin levels significantly because of phenytoin's saturable, non-linear pharmacokinetics. Push clearance up and levels can fall fast; slow clearance down and levels can climb just as fast.
Ginkgo biloba is one of the best-selling herbal supplements worldwide, taken for memory, cognitive performance, peripheral circulation, and tinnitus. Pharmacology studies have shown that ginkgo extract induces CYP2C19 in humans, with effects on the metabolism of CYP2C19 substrates like omeprazole. Because phenytoin shares this pathway, the predicted effect is reduced phenytoin exposure with chronic ginkgo use.
That prediction is backed up by case report data. The most-cited paper is Kupiec and Raj, published in the Journal of Analytical Toxicology in 2005. They reported a 55-year-old man on phenytoin and valproate who died of a breakthrough seizure. Postmortem analysis showed subtherapeutic levels of both anticonvulsants despite no evidence of nonadherence. The patient had been taking ginkgo biloba and other herbal supplements. The authors proposed CYP2C19 induction by ginkgo as a plausible mechanism for the unexpectedly low drug levels.
There is a second, independent concern: ginkgo seeds contain ginkgotoxin (4'-O-methylpyridoxine), a neurotoxin that can cause seizures, particularly in children or with raw seed consumption. Although ginkgo leaf extracts (used in standardized supplements) contain very little ginkgotoxin compared to seeds, batch-to-batch variability and adulteration have been documented, and seizures have been reported with ginkgo use even in non-epileptic patients.
Why is this important?
For a person with epilepsy on phenytoin, both arms of the ginkgo interaction point the same direction: lower drug exposure, plus an independent pro-convulsant signal from the supplement itself, equals higher seizure risk.
Ginkgo is widely marketed for cognitive concerns, exactly the kind of complaint many patients with epilepsy have, sometimes driven by the seizures themselves, sometimes by their anticonvulsants, and sometimes by ordinary aging. So the at-risk overlap is large. Patients also often consider ginkgo "natural" and do not mention it on medication reconciliation forms, leaving prescribers blind to a real safety hazard.
The interaction is also asymmetric in time. Starting ginkgo lowers phenytoin levels over a couple of weeks. Stopping ginkgo lets enzyme activity regress, which can let phenytoin levels rebound. If the prescriber compensated for low phenytoin levels by increasing the dose during ginkgo use, abrupt herb discontinuation can produce phenytoin toxicity (nystagmus, ataxia, confusion, slurred speech) within a few weeks.
The combined evidence: a documented enzyme-induction mechanism, a fatal case report, an inherent supplement-side seizure signal, and a wide population of likely users. This is enough for major drug information resources to flag the combination as one to avoid.
What should you do?
If you take phenytoin, do not start ginkgo biloba. The risk-benefit is poor: ginkgo's cognitive benefits in healthy adults are modest at best in trials, and the seizure risk in someone with epilepsy is real.
If you have been taking both, do not stop ginkgo abruptly without talking to your prescriber first. Ask for a phenytoin level (free and total) before you stop, plus a follow-up level 2-3 weeks after, and watch for early signs of phenytoin toxicity: side-to-side eye flickering (nystagmus), unsteadiness, slurred speech, double vision, drowsiness, or confusion.
If you are seeking help with memory, mental clarity, or fatigue, talk to your neurologist. Anticonvulsants themselves cause cognitive side effects that may respond to dose adjustment or switching to a less sedating drug. Treatable contributors like depression, sleep apnea, anemia, and thyroid issues are worth a workup before assuming you need a supplement.
If you have already had a breakthrough seizure while on the combination, treat it as a major safety event: do not drive, get a phenytoin level checked, and stop ginkgo under medical supervision rather than continuing.
Which specific products are affected?
On the medication side:
- Dilantin (capsules, chewable tablets, oral suspension)
- Phenytek (extended-release capsules)
- Generic phenytoin sodium
- Fosphenytoin (Cerebyx), a phenytoin prodrug used IV in hospital, is affected by the same metabolic pathway
On the supplement side, all forms of ginkgo biloba are relevant:
- Standardized ginkgo leaf extract capsules and tablets (typically 60-240 mg/day, often labeled "EGb 761" or "24/6" referring to 24% flavone glycosides and 6% terpene lactones)
- Ginkgo teas and tinctures
- Combination "memory" or "brain" supplements that include ginkgo
Even short-term ginkgo use can begin to induce CYP2C19. There is no "safe" daily ginkgo dose in someone on phenytoin.
The bottom line
Ginkgo biloba and phenytoin are a documented bad combination. The herb induces a major phenytoin-metabolizing enzyme, can drop drug levels into the subtherapeutic range, and carries its own seizure risk. A published fatal case report drives the point home. If you take phenytoin, skip ginkgo, and if you are already taking both, do not stop the herb abruptly without arranging for phenytoin level monitoring. Treat any new memory or cognitive complaints as a problem to take to your neurologist rather than the supplement aisle.