About Phenytoin


Phenytoin belongs to a class of medications called anticonvulsants. Anticonvulsants are typically used for the treatment of epilepsy and other diseases that cause seizures. Anticonvulsants block abnormal electrical activity in the brain, which helps keep seizure signals from passing through the brain and getting to the muscles.


MentalHealth.com states that phenytoin’s ability to treat epilepsy and seizures depends on its effects on the motor cortex. The motor cortex is part of the brain that controls muscle movement. Phenytoin is thought to change the concentration of sodium ions within the neurons there, which helps keep the neurons from becoming hyperexcitable. Because epilepsy and seizures are caused by abnormal neuron signals, phenytoin is able to inhibit this unusual electrical activity in the brain, suppressing seizures.

Forms and Dosing

According to Drugs.com, phenytoin is typically taken orally, where it can be ingested in tablet or pill form or as a liquid suspension. MentalHealth.com explains that most patients are prescribed a dose of 300 mg per day. This can be done using three separate pills of 100 mg or one extended-release 300 mg tablet.


The body absorbs phenytoin well when it is taken orally. Phenytoin levels reach their peak four to 12 hours after the medications is taken. Phenytoin is primarily excreted from the body via bile, although some of it is also processed and excreted by the kidneys into the urine. Before phenytoin is expelled from the body, the liver metabolizes it; RxList.com notes that because of this, small increases in a patient’s dosing can lead to a large increase in the blood levels of phenytoin.

Side Effects

Phenytoin can cause a mild skin rash as well as gastrointestinal problems (vomiting, nausea and constipation). Patients may also have problems with dizziness or nervousness. Dangerous side effects include confusion, hallucinations, a fever combined with a sore throat, severe skin blistering as well as unquenchable thirst. Patients may also experience liver failure, which can lead to easy bruising, yellowing of the eyes and the skin, and clay-colored stools. Other serious side effects (which require medical attention) include muscle tremors and slurred speech.

Drug Interactions

Drugs.com explains that some sedatives (such as Valium) and selective serotonin reuptake inhibitors can increase the levels of phenytoin in the blood. Medications that reduce stomach acid can also cause phenytoin levels to rise. Certain antibiotics (such as Pactrim) and ADHD medications (methylphenidates) can also slow the metabolism of phenytoin. Patients taking phenytoin should tell their doctor about all medications they are taking (including over-the-counter and herbal supplements) to avoid drug interactions.

About this Author

Adam Cloe has been published in various scientific journals. He is an M.D./Ph.D. student at the University of Chicago. He has a Bachelor of Arts in biochemistry from Boston University, where he won an award for excellence in undergraduate science writing.