Introduction
Dilantin Infatabs · Dilantin-125 Dilantin-125 contain phenytoin sodium, an anticonvulsant medication used to manage and prevent certain types of seizures, particularly in epilepsy. Phenytoin is one of the most commonly prescribed antiepileptic drugs (AEDs) and is used in various forms, including extended-release capsules like Dilantin Infatabs · Dilantin-125 Dilantin-125, which provide consistent levels of the medication in the bloodstream.
Uses
Dilantin Infatabs · Dilantin-125 Dilantin-125 are primarily used for the control of generalized tonic-clonic (grand mal) seizures and complex partial seizures. It is also used to prevent seizures following neurosurgery and as a second-line agent in the treatment of status epilepticus. Phenytoin is not effective against absence seizures (petit mal).
Mechanism of Action
Phenytoin works by stabilizing neuronal membranes and decreasing seizure activity. It does this by inhibiting the voltage-dependent sodium channels in neurons, reducing the ability of the neuron to generate repetitive action potentials. This effect diminishes the excitability of neurons and helps prevent the abnormal electrical activity that causes seizures.
How Long Does It Take to Work?
The onset of action of phenytoin is relatively quick when used intravenously in acute situations like status epilepticus, but in oral formulations like Dilantin Infatabs · Dilantin-125 Dilantin-125, it may take a few days of regular dosing to reach therapeutic levels in the bloodstream. Steady-state plasma concentrations are typically achieved within 7 to 10 days with consistent use.
Absorption
Phenytoin is absorbed relatively slowly when taken orally. The absorption rate is influenced by the formulation, with extended-release capsules like Dilantin Infatabs · Dilantin-125 Dilantin-125 designed to maintain steady drug levels. Peak plasma concentrations are typically reached within 4 to 12 hours. Phenytoin follows nonlinear pharmacokinetics, meaning that small changes in dose can result in large changes in plasma levels.
Route of Elimination
Phenytoin is primarily metabolized by the liver via the cytochrome P450 enzyme system, particularly CYP2C9 and CYP2C19. It is eliminated in the urine, mainly as inactive metabolites. Only a small portion of the drug is excreted unchanged.
Dosage
General Adult Dosage:
The recommended starting dose of Dilantin Infatabs · Dilantin-125 Dilantin-125 for adults is 100 mg three times daily. For extended-release formulations, the usual maintenance dose ranges from 300 mg to 400 mg daily, administered in one to two divided doses. Dosage may be adjusted based on therapeutic drug levels and clinical response.
Seizure Control:
- Generalized Tonic-Clonic and Complex Partial Seizures: The usual dose is 300 mg daily in divided doses or as a single daily dose of the extended-release form. Dosage adjustments are made based on serum levels and clinical response, typically targeting serum levels between 10 to 20 mcg/mL.
- Seizure Prevention after Neurosurgery: The typical prophylactic dose is 100 mg three times daily.
Special Populations:
- Elderly: Phenytoin clearance may be reduced in elderly patients, necessitating close monitoring of serum drug levels and dosage adjustments.
- Hepatic/Renal Impairment: Phenytoin metabolism may be impaired in patients with liver disease or renal impairment. Lower doses or increased monitoring may be required.
Administration
Dilantin Infatabs · Dilantin-125 Dilantin-125 should be taken orally, typically once or twice daily, depending on the prescribed dose. They can be taken with or without food, but maintaining consistency with regard to food intake helps stabilize absorption. The extended-release capsules should not be chewed or crushed, as this would compromise the release profile of the medication.
Side Effects
Common side effects include dizziness, drowsiness, ataxia, confusion, nausea, vomiting, and gingival hyperplasia (overgrowth of the gums). Long-term use may lead to complications such as peripheral neuropathy, osteomalacia, and hirsutism (excessive hair growth). Phenytoin may also cause serious side effects, such as Stevens-Johnson syndrome, toxic epidermal necrolysis, and agranulocytosis (a severe reduction in white blood cells).
Toxicity
Phenytoin toxicity occurs when blood levels exceed the therapeutic range. Symptoms of toxicity include nystagmus (involuntary eye movements), ataxia, slurred speech, confusion, and lethargy. Severe toxicity may lead to coma and death. The therapeutic range of phenytoin is 10-20 mcg/mL, with toxicity usually occurring at levels above 30 mcg/mL.
Precautions
Patients with hepatic impairment should be closely monitored due to the potential for decreased phenytoin metabolism. Those with a history of hypersensitivity reactions to phenytoin should not use this drug. Caution is advised in patients with pre-existing blood disorders or bone marrow suppression.
Interaction
Phenytoin has significant drug-drug interactions due to its induction of hepatic enzymes. It can lower the plasma levels of many medications, including oral contraceptives, warfarin, and certain antivirals, by increasing their metabolism. Conversely, other drugs like cimetidine and isoniazid can inhibit phenytoin metabolism and increase its plasma levels, leading to toxicity.
Disease Interaction
Phenytoin can exacerbate certain medical conditions, such as liver disease, bone marrow suppression, and certain heart arrhythmias. It may also worsen symptoms in patients with myasthenia gravis or porphyria. Monitoring and dosage adjustments are necessary in patients with these conditions.
Drug Interaction
Phenytoin interacts with a wide range of drugs. Major interactions include:
- Warfarin: Phenytoin can decrease the anticoagulant effect of warfarin by inducing its metabolism.
- Oral Contraceptives: Phenytoin induces hepatic enzymes that can reduce the effectiveness of hormonal contraceptives, potentially leading to contraceptive failure.
- Valproic Acid: Concurrent use may increase phenytoin levels and decrease valproic acid levels, leading to potential toxicity of phenytoin and decreased efficacy of valproic acid.
Food Interactions
Phenytoin absorption can be affected by the intake of certain foods, particularly high-fat meals, which may delay the absorption of the drug. Patients should try to take their medication consistently with regard to meals to ensure stable absorption. Alcohol use should be avoided, as it may alter phenytoin levels.
Pregnancy Use
Phenytoin is classified as Pregnancy Category D due to the potential risk of fetal harm. It has been associated with congenital malformations, including cleft palate, heart defects, and fetal hydantoin syndrome. Pregnant women should only use phenytoin if the potential benefits outweigh the risks, and close monitoring is required.
Lactation Use
Phenytoin is excreted in breast milk in small amounts. Although the effects on a nursing infant are not well-documented, caution is recommended when using phenytoin during breastfeeding. A risk-benefit assessment should be made, and monitoring of the infant for signs of drug exposure is advisable.
Acute Overdose
Symptoms of acute overdose include severe dizziness, ataxia, confusion, lethargy, hypotension, respiratory depression, and coma. In cases of severe overdose, immediate medical intervention is necessary. Treatment is generally supportive and may include activated charcoal administration and symptomatic care. Hemodialysis may be considered in severe cases.
Contraindication
Dilantin Infatabs · Dilantin-125 Dilantin-125 are contraindicated in patients with a history of hypersensitivity to phenytoin or any of the capsule components. It is also contraindicated in patients with sinus bradycardia, sinoatrial block, second- and third-degree atrioventricular block, and those with a history of toxic reactions to hydantoin drugs.
Use Direction
Dilantin Infatabs · Dilantin-125 Dilantin-125 should be taken as prescribed by the healthcare provider. It is important to maintain consistent dosing and not to skip doses. The capsules should be swallowed whole, not chewed or crushed. If a dose is missed, it should be taken as soon as remembered, unless it is almost time for the next dose.
Storage Conditions
Dilantin Infatabs · Dilantin-125 Dilantin-125 should be stored at room temperature, between 20°C to 25°C (68°F to 77°F), away from moisture, heat, and light. Keep the medication in its original container, tightly closed, and out of the reach of children.
Volume of Distribution
Phenytoin has a volume of distribution of approximately 0.6 to 0.8 L/kg, indicating that it is distributed widely throughout body tissues, including the brain, liver, and skeletal muscle.
Half-Life
The half-life of phenytoin varies depending on the dose and plasma concentration due to its nonlinear pharmacokinetics. The half-life can range from 7 to 42 hours, with longer half-lives observed at higher plasma concentrations due to saturation of metabolic pathways.
Clearance
Phenytoin is cleared primarily through hepatic metabolism, with an average clearance rate of 0.02 to 0.03 L/kg/hr. Clearance decreases as phenytoin levels approach the point of metabolic saturation, making dose adjustments challenging at higher concentrations.
