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Home > Drug Category > Antiepileptic Medication > Dilantin and Equivalent

Dilantin (Phenytoin) Information

Dilantin (phenytoin) is a foundational anticonvulsant medication belonging to the hydantoin class. It is primarily indicated for the control of Generalized Tonic-Clonic Seizures (grand mal) and Complex Partial Seizures (psychomotor or temporal lobe seizures) in the treatment of epilepsy. It is also used in the prophylaxis and treatment of seizures that may occur during or after neurosurgery. Phenytoin's unique pharmacological properties, including a narrow therapeutic window and nonlinear (zero-order) metabolism at higher concentrations, necessitate highly individualized dosing and careful monitoring through serum level determinations to balance efficacy with the risk of toxicity. Abrupt discontinuation of phenytoin, even in patients who are not epileptic, may precipitate status epilepticus or increase seizure frequency.

The mechanism of action for phenytoin centers on stabilizing neuronal membranes and inhibiting the spread of seizure activity within the motor cortex. It achieves this primarily by modulating voltage-gated sodium channels in the central nervous system. Phenytoin selectively binds to and prolongs the inactive state of these sodium channels, which limits the repetitive firing of high-frequency neurons. By promoting sodium efflux from neurons, phenytoin stabilizes the neuronal threshold against hyperexcitability, thereby preventing the generation and spread of the excessive electrical impulses that characterize a seizure. The therapeutic blood level is typically monitored at 10 to 20 mcg/mL.

Dilantin Side Effects

Phenytoin can cause a range of side effects, many of which are dose-related, particularly those affecting the central nervous system (CNS). Common side effects often associated with the initiation of therapy or doses slightly above the therapeutic range include decreased coordination, nystagmus (uncontrollable eye movements), confusion, slurred speech, and unsteadiness (ataxia). Long-term use is often associated with the unique side effects of gingival hyperplasia (overgrowth of gum tissue), coarsening of facial features, and mild peripheral neuropathy.

However, more serious side effects can occur, although they are rare. If any of the following symptoms are experienced, the patient should stop taking the medication and seek immediate medical attention:

• Blood Dyscrasias: Phenytoin can cause severe, sometimes fatal, blood abnormalities, including aplastic anemia and agranulocytosis (severe decrease in white blood cells).

• Cardiovascular Events (Boxed Warning): Rapid intravenous administration of phenytoin is associated with severe hypotension, heart block, and cardiac arrhythmias, which can lead to cardiac arrest. This necessitates slow IV infusion.

• Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) / Hypersensitivity Syndrome: A severe, multisystem allergic reaction characterized by fever, rash, lymphadenopathy, and possible involvement of internal organs such as the liver or kidneys, typically occurring 1 week to 1 month after therapy initiation.

• Fetal Harm/Teratogenicity (Pregnancy Category D): Use during pregnancy is associated with congenital disabilities, including Fetal Hydantoin Syndrome, characterized by craniofacial and limb anomalies, and neurodevelopmental delay.

• Hepatic Failure: Rare but serious liver injury, which may be fatal.

• Osteoporosis/Osteomalacia: Long-term use of phenytoin is associated with bone density loss, leading to osteopenia or osteoporosis.

• Severe Cutaneous Adverse Reactions (SCARs) (Boxed Warning): Serious and potentially fatal dermatological reactions, including Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN). The risk is higher in patients of Asian descent (due to the HLA-B*15:02 allele) and those carrying the CYP2C9*3 variant.

• Suicidality and Worsening of Depression (Boxed Warning for AEDs): Antiepileptic drugs, including phenytoin, carry an increased risk of suicidal thoughts or behaviour, particularly during the initial phase of treatment.

Dilantin Precautions

A detailed discussion of medical history and concurrent medications is crucial before starting phenytoin. Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzymes CYP2C9 and CYP2C19. Genetic variants in these enzymes, such as CYP2C9 intermediate or poor metabolizers, can lead to higher phenytoin serum concentrations and increased risk of CNS toxicity, potentially requiring lower doses. Phenytoin is contraindicated in patients with a history of hypersensitivity to hydantoin products. Due to the high risk of severe cardiac arrhythmias associated with rapid administration, it is also contraindicated in patients with cardiac conditions like sinus bradycardia, second- and third-degree A-V block, and Adams-Stokes syndrome. Caution is necessary in patients with pre-existing liver or kidney disease. Phenytoin is a potent enzyme inducer and can significantly reduce the efficacy of hormonal contraceptives (birth control pills, patches, injections); therefore, alternative or additional contraception methods are necessary. Phenytoin has numerous drug-drug interactions because it acts as an inducer or inhibitor of various hepatic enzymes. Abrupt cessation of phenytoin must be avoided, as it can precipitate life-threatening seizures.

Do I need a prescription to order Dilantin?

Yes, a prescription is required from your physician to order Dilantin.