Phenytoin

What is Phenytoin?

Phenytoin is an anticonvulsant medication that belongs to the class of drugs known as hydantoins. It is primarily used to prevent and control seizures in people with epilepsy and can also be used to treat seizures that occur during or after brain surgery. Phenytoin has been a cornerstone of epilepsy treatment for decades and is considered an essential medicine by the World Health Organization.

Uses

Phenytoin is approved for several medical conditions:

• Epilepsy treatment: Controls various types of seizures including tonic-clonic (grand mal) seizures and partial seizures in patients with epilepsy
• Status epilepticus: Used as emergency treatment for prolonged seizures that don’t stop on their own
• Prevention of seizures during neurosurgery: Administered before and after brain surgery to prevent seizures caused by surgical trauma
• Post-traumatic seizures: Helps prevent seizures following severe head injuries or brain trauma
• Trigeminal neuralgia: Sometimes used off-label to treat this severe facial nerve pain condition when other treatments haven’t worked

How it works

Phenytoin works by stabilizing nerve cell membranes in the brain and blocking sodium channels that are responsible for transmitting electrical signals between neurons. By reducing the abnormal electrical activity that causes seizures, phenytoin helps prevent the rapid, uncontrolled firing of nerve cells that characterizes epileptic seizures. This stabilizing effect on brain tissue makes it effective for both preventing seizures and stopping them once they’ve started.

Side effects

Common side effects include:

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• Drowsiness and fatigue
• Dizziness and unsteadiness
• Nausea and vomiting
• Gum overgrowth (gingival hyperplasia)
• Excessive hair growth (hirsutism)
• Skin rash
• Confusion or difficulty concentrating
• Slurred speech

Serious side effects that require immediate medical attention:

• Severe skin reactions including Stevens-Johnson syndrome
• Liver damage or failure
• Blood disorders such as low white blood cell count
• Suicidal thoughts or behavior
• Severe allergic reactions with fever and swollen lymph nodes

Warnings and precautions

• Pregnancy considerations: Phenytoin is classified as pregnancy category D, meaning it may cause birth defects including cleft palate and heart defects. However, the benefits may outweigh risks in pregnant women with epilepsy, as uncontrolled seizures can also harm the fetus.
• Liver function: Patients with liver disease may need dose adjustments, as phenytoin is metabolized in the liver and can cause liver toxicity in some individuals.
• Blood monitoring: Regular blood tests are necessary to monitor drug levels and check for blood disorders, especially during the first few months of treatment.
• Sudden discontinuation: Stopping phenytoin abruptly can trigger severe seizures or status epilepticus, so the dose must be gradually reduced under medical supervision.
• Suicidal risk: Like other anticonvulsants, phenytoin may increase the risk of suicidal thoughts, particularly in the first few months of treatment.

Interactions

Phenytoin has numerous significant drug interactions:

• Warfarin: Phenytoin can increase bleeding risk by affecting warfarin metabolism
• Birth control pills: Reduces effectiveness of hormonal contraceptives, potentially leading to unplanned pregnancy
• Carbamazepine: Can decrease phenytoin levels, requiring dose adjustments
• Fluconazole and other antifungals: Increase phenytoin levels, potentially causing toxicity
• Alcohol: Chronic alcohol use decreases phenytoin effectiveness, while acute intoxication increases it
• Folic acid supplements: Large doses may decrease phenytoin levels and reduce seizure control
• Calcium channel blockers: Some medications like nifedipine may have reduced effectiveness
• Antibiotics: Certain antibiotics like trimethoprim can increase phenytoin levels

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Dosage

Adult dosing varies significantly based on the condition being treated and individual patient factors. For epilepsy maintenance therapy, the typical starting dose is 300-400 mg daily, which may be given as a single dose or divided into multiple doses. For status epilepticus, intravenous loading doses of 15-20 mg per kilogram of body weight are used in hospital settings. Maintenance doses typically range from 200-600 mg daily, with most patients requiring 300-400 mg per day. Blood level monitoring is essential to ensure therapeutic levels are achieved while avoiding toxicity. All dosing decisions should be made by a healthcare provider based on individual patient needs, response to treatment, and blood level results.

Sources: FDA DailyMed drug labels (public domain), BNF, WHO Essential Medicines List. This article is for informational purposes only and does not constitute medical advice. Always consult your healthcare provider before starting or stopping any medication.

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