What is the drug phenytoin used for? A comprehensive overview

October 6, 2025 •

4 min read

Phenytoin, first approved by the FDA in 1939, is one of the oldest and most widely used anti-seizure medications. This extensive guide details what is the drug phenytoin used for, exploring its therapeutic applications in controlling and preventing seizures associated with epilepsy and other neurological conditions.

Quick Summary

Phenytoin is an anticonvulsant medication primarily indicated for controlling specific types of seizures, including generalized tonic-clonic and complex partial seizures. It is also used to prevent seizures following neurosurgery and manage status epilepticus.

Key Points

Anticonvulsant Action: Phenytoin controls seizures by blocking voltage-gated sodium channels, which helps stabilize the electrical activity of neurons in the brain.
Primary Seizure Control: It is most commonly used for generalized tonic-clonic and complex partial seizures in patients with epilepsy.
Emergency Use: In acute situations, such as status epilepticus, intravenous phenytoin is a second-line treatment option after initial management with other medications.
Narrow Therapeutic Index: Phenytoin has a narrow therapeutic range, meaning the difference between an effective and a toxic dose is small, requiring regular blood level monitoring.
Common Side Effects: Frequent adverse effects include dizziness, coordination problems, slurred speech, and gingival hyperplasia (gum overgrowth).
Significant Drug Interactions: Phenytoin interacts with many other medications, including oral contraceptives, and can also be affected by diet and alcohol consumption.
Chronic Side Effects: Long-term use of phenytoin is associated with a risk of decreased bone mineral density (osteoporosis).
Not a First-Line Choice for New Epilepsy: Due to its side effect profile and need for monitoring, newer antiepileptic drugs are often preferred for first-line treatment, especially in older adults.

What is Phenytoin?

Phenytoin, commonly known by its brand names such as Dilantin and Phenytek, is a hydantoin derivative that functions as an anticonvulsant, or anti-seizure medication. Its discovery in 1938 marked a significant advancement in epilepsy treatment, offering an effective alternative to sedating drugs like phenobarbital. Today, while newer medications are available, phenytoin remains a crucial and widely prescribed drug, particularly for managing certain acute seizure events and in cases where its benefits outweigh its known side effects.

Mechanism of Action: How Phenytoin Stabilizes the Brain

Phenytoin works by calming the overactive electrical impulses in the brain that cause seizures. Its primary mechanism involves blocking voltage-gated sodium channels on nerve cells. By binding to the sodium channels in their inactive state, phenytoin prolongs their refractory period and reduces their ability to fire repetitively and at high frequency. This use-dependent blockade allows phenytoin to target and inhibit the rapid firing of epileptic neurons while having minimal impact on normal, spontaneous neuronal activity. This stabilization of the neuronal membrane effectively inhibits the spread of seizure activity from its point of origin to other areas of the brain.

Primary Medical Uses for Phenytoin

The primary indications for phenytoin revolve around the management and prevention of seizures.

Epilepsy: Phenytoin is indicated for controlling two main types of epileptic seizures:
- Generalized Tonic-Clonic Seizures: Formerly known as "grand mal" seizures, these involve a loss of consciousness and muscle rigidity followed by convulsive jerking movements.
- Complex Partial Seizures: Also called "psychomotor" or "temporal lobe" seizures, these start in a specific area of the brain and can cause altered awareness or repetitive, non-purposeful behaviors.
Status Epilepticus: This is a life-threatening medical emergency involving continuous or rapidly repeating seizures without recovery between them. While benzodiazepines are the first-line treatment for immediate seizure cessation, intravenous (IV) phenytoin or its prodrug, fosphenytoin, is often used as a second-line agent to maintain seizure control.
Neurosurgery: Phenytoin is prescribed to prevent and treat seizures that may arise during or following surgery on the brain or nervous system.

Other and Off-Label Uses

Beyond its primary uses, phenytoin has been explored for other conditions:

Trigeminal Neuralgia: An off-label use for phenytoin is in treating this severe nerve pain disorder that affects the face. However, carbamazepine is typically considered more effective.
Cardiac Arrhythmias: Phenytoin exhibits antiarrhythmic properties and has been used intravenously for certain ventricular arrhythmias, particularly those associated with digoxin toxicity. However, its use for this indication is limited due to its narrow therapeutic window and multiple drug interactions.
Wound Healing: Research has investigated the use of topical phenytoin for wound healing, with some studies supporting its efficacy in managing certain types of chronic skin wounds.

Potential Adverse Effects and Risks

Phenytoin has a narrow therapeutic index, meaning the dose required for treatment is very close to the dose that causes toxicity. This necessitates careful monitoring by a healthcare provider.

Common Side Effects: These can be dose-dependent and include:
- Dizziness, drowsiness, and confusion
- Loss of coordination (ataxia)
- Uncontrollable eye movements (nystagmus)
- Slurred speech
- Nausea and constipation
- Gingival hyperplasia (overgrowth of the gums)
- Hirsutism (unwanted hair growth)
Serious Side Effects: More severe reactions, though less common, require immediate medical attention:
- Severe skin reactions, such as Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN)
- Liver damage and blood disorders
- Suicidal thoughts or behavior
- Cardiovascular issues, especially with rapid intravenous administration

Comparison with Newer Antiepileptic Drugs

Because of its narrow therapeutic window and complex side effect profile, phenytoin is not always the first choice for newly diagnosed epilepsy. Newer antiepileptic drugs (AEDs) are often favored, offering better-tolerated side effects and fewer drug interactions.


Feature	Phenytoin (e.g., Dilantin)	Levetiracetam (e.g., Keppra)
Therapeutic Index	Narrow; requires regular blood level monitoring to prevent toxicity.	Wide; less frequent monitoring required.
Side Effect Profile	More severe and chronic side effects possible, such as gingival hyperplasia, bone density issues, and hirsutism.	Generally well-tolerated with common side effects including mood changes and irritability.
Drug Interactions	Significant interactions with many drugs, including hormonal contraceptives, antibiotics, and other AEDs.	Fewer clinically significant drug interactions.
Dosage Forms	Available in extended-release capsules, chewable tablets, oral suspension, and injectable solution.	Available in tablets, oral solution, and injectable solution.
Usage in the Elderly	Caution needed due to potential for greater sensitivity to side effects and more complex pharmacokinetics.	Often preferred over older AEDs due to a more favorable side effect profile.

Conclusion

Phenytoin has a long and storied history as an effective anticonvulsant and continues to be a valuable tool in medicine for managing specific types of seizures. Its efficacy in treating tonic-clonic and partial seizures, controlling status epilepticus, and preventing neurosurgery-related seizures is well-established. However, its use requires careful management due to its narrow therapeutic index, potential for toxicity, and numerous drug interactions. Patients must work closely with their healthcare providers to monitor blood levels and manage side effects, and newer, safer alternatives are often chosen for long-term therapy or initial treatment. Despite its challenges, phenytoin remains a testament to the evolution of pharmacology and an important medication in certain clinical contexts.

For more information on the management of epilepsy and other seizure disorders, consult resources from authoritative organizations like the Epilepsy Foundation.

Frequently Asked Questions

The primary purpose of phenytoin is to control seizures, specifically generalized tonic-clonic (grand mal) and complex partial seizures associated with epilepsy. It can also be used to prevent seizures during or after neurosurgery.

Phenytoin works by stabilizing nerve cell membranes in the brain. It blocks voltage-gated sodium channels, which reduces the rapid, high-frequency firing of electrical impulses that trigger seizures.

Common side effects include drowsiness, confusion, dizziness, problems with coordination, slurred speech, and uncontrollable eye movements (nystagmus). Long-term use can lead to gum overgrowth (gingival hyperplasia) and unwanted hair growth.

Regular blood tests are essential for patients on phenytoin because it has a narrow therapeutic index. This means the concentration needed for effectiveness is close to the level that can cause toxicity. Monitoring ensures the drug level is in a safe and effective range.

No, you should never stop taking phenytoin abruptly without consulting your healthcare provider. Suddenly stopping the medication can lead to an increase in seizure activity and potentially trigger a life-threatening condition called status epilepticus.

Yes, phenytoin has many significant drug interactions. It can affect the effectiveness of hormonal birth control and interact with other medications like antibiotics, antacids, and blood thinners.

Phenytoin has off-label and historical uses beyond seizure control. These include treating certain types of irregular heartbeats (arrhythmias) and some forms of nerve pain, like trigeminal neuralgia. It has also been studied for wound healing.

Phenytoin is not typically a first-line treatment for newly diagnosed epilepsy due to its complex pharmacokinetics, narrow therapeutic index, and significant side effect profile. Newer, more tolerable medications are often preferred, especially for long-term use.