phenytoin
6 interactions related to phenytoin
phenytoin + st. john's wort
St. John's Wort activates the pregnane X receptor and induces drug-metabolizing enzymes (CYP3A4, CYP2C9, CYP2C19) and P-glycoprotein. Because phenytoin is cleared mainly by CYP2C9 and CYP2C19, taking St. John's Wort alongside it could speed phenytoin's breakdown and lower its blood levels, raising the theoretical risk of breakthrough seizures. Direct human data for phenytoin specifically are limited, so regulators treat this as a mechanism-based precaution rather than a documented loss of control.
phenytoin + ginkgo
Ginkgo biloba can induce CYP2C19, an enzyme involved in clearing phenytoin, which may lower phenytoin blood levels and raise the risk of breakthrough seizures. A published fatal case report described subtherapeutic phenytoin and valproate levels in a patient who had been self-administering ginkgo. Ginkgo also carries its own seizure-related signal. If you take phenytoin, avoid ginkgo and review any supplement changes with your doctor or pharmacist.
star fruit + phenytoin
Star fruit (Averrhoa carambola) contains caramboxin, a neurotoxin that excites neurons, plus soluble oxalates that can injure the kidneys. In people with reduced kidney function, who cannot clear caramboxin, eating star fruit has triggered intractable seizures and status epilepticus. This is the fruit's own toxicity rather than a chemical reaction with phenytoin, but for someone taking phenytoin to prevent seizures it adds a serious, avoidable risk.
phenytoin + folate
Phenytoin and folate interact in both directions: long-term phenytoin lowers folate through enzyme induction and reduced absorption, while supplemental folate can speed phenytoin clearance and lower its blood level enough to allow seizures to return in some people. The interaction is real but monitorable, so changes should be coordinated with your neurologist rather than avoided.
phenytoin + vitamin d
Phenytoin induces the liver enzymes that break down vitamin D, accelerating clearance of 25-hydroxyvitamin D and lowering circulating levels over time. The downstream result can be reduced calcium absorption, a compensatory rise in parathyroid hormone, and an increased risk of softened bones (osteomalacia) and fractures with long-term use.
phenytoin + calcium
Calcium-containing supplements and antacids can bind phenytoin in the gut and lower how much of the drug is absorbed when the two are taken together, which can reduce phenytoin's blood level. Separately, long-term phenytoin use can reduce calcium absorption by speeding up the breakdown of vitamin D, which is relevant to bone health over time.