Dilantin: Seizure Control with Established Efficacy
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Synonyms
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Dilantin (phenytoin) is a cornerstone anticonvulsant medication with a long-standing history of clinical use for managing and preventing tonic-clonic (grand mal) and complex partial (psychomotor) seizures. As a hydantoin derivative, it exerts its therapeutic effect by stabilizing neuronal membranes and decreasing seizure activity through voltage-dependent blockade of voltage-gated sodium channels. This action prolongs the inactivated state of the channel, thereby preventing the sustained repetitive firing of action potentials that characterizes a seizure focus. Its well-understood pharmacokinetic profile, including non-linear (saturable) metabolism, makes it a powerful tool in the neurologist’s arsenal, though it requires careful therapeutic drug monitoring (TDM) to maintain concentrations within the narrow therapeutic window of 10–20 mcg/mL for optimal efficacy and minimal toxicity.
Features
- Active Pharmaceutical Ingredient: Phenytoin (as phenytoin sodium).
- Available Formulations: Oral capsules (30 mg and 100 mg [prompt-release]), chewable tablets (50 mg), and an intravenous (IV) solution (50 mg/mL).
- Mechanism of Action: Use-dependent blockade of voltage-gated sodium channels.
- Pharmacokinetics: Exhibits saturable, dose-dependent (Michaelis-Menten) metabolism; highly protein-bound (~90%).
- Therapeutic Drug Monitoring: A narrow therapeutic index necessitates regular serum level monitoring (target range: 10–20 mcg/mL).
Benefits
- Provides effective and reliable control of focal and generalized tonic-clonic seizure activity.
- Offers a rapid onset of action, particularly with IV loading for status epilepticus.
- Features multiple administration routes (oral and IV) for flexibility in both acute and chronic management.
- Has a well-documented efficacy and safety profile based on decades of clinical use and research.
- Supports long-term maintenance therapy for chronic epilepsy management.
- Can be used for the prophylaxis and treatment of seizures following neurosurgery or traumatic brain injury.
Common use
Dilantin is primarily indicated for the management of epileptic seizures. Its most common uses include the prevention and treatment of generalized tonic-clonic seizures and complex partial seizures. It is also a critical agent in the algorithm for terminating active convulsive status epilepticus via intravenous administration. Furthermore, it is frequently employed for seizure prophylaxis in specific clinical scenarios, such as in the immediate period following neurosurgical procedures or significant traumatic brain injury where there is a high risk of seizure development.
Dosage and direction
Dosage is highly individualized and must be titrated based on clinical response and serum phenytoin levels. The non-linear pharmacokinetics mean that small dosage increases can lead to disproportionately large increases in serum concentration.
- Adults: For previously untreated patients, a common starting dose is 100 mg orally three times daily (TID). Dosage adjustment is typically made in 30–100 mg increments at 7–10 day intervals.
- IV Administration: For status epilepticus, a loading dose of 15–20 mg/kg administered at a rate not exceeding 50 mg/minute is standard, followed by maintenance dosing.
- Pediatrics: The initial dose is typically 5 mg/kg/day in 2–3 divided doses, with a maintenance range of 4–8 mg/kg/day. Dosing is based on body weight and must be carefully monitored.
- Administration: Oral capsules should be swallowed whole. The patient must maintain a consistent intake regarding taking it with or without food to avoid fluctuations in absorption. IV administration requires cardiac and blood pressure monitoring due to risks of hypotension and arrhythmia. It is incompatible with many IV solutions; normal saline is the preferred diluent.
Precautions
- Therapeutic Drug Monitoring (TDM): Mandatory. Serum levels should be checked regularly, especially following dosage changes, and during concomitant illness or introduction of other medications.
- Hepatic Impairment: Use with extreme caution; reduced metabolism can lead to rapid accumulation and toxicity. Dose adjustments are necessary.
- Renal Impairment: Caution is advised. Uremia can alter protein binding, leading to an increase in the pharmacologically active free fraction of phenytoin.
- Elderly Patients: Often require lower dosages due to decreased protein binding, reduced hepatic metabolism, and increased sensitivity to central nervous system effects.
- Porphyria: May exacerbate acute intermittent porphyria and should be avoided in patients with this disorder.
- Suicidal Ideation: Antiepileptic drugs, including phenytoin, increase the risk of suicidal thoughts and behavior. Patients should be monitored for the emergence or worsening of depression.
Contraindications
Dilantin is contraindicated in patients with a known hypersensitivity to phenytoin, other hydantoins, or any component of the formulation. It is also contraindicated in patients with sinus bradycardia, sinoatrial block, second- or third-degree atrioventricular (AV) block, or Adams-Stokes syndrome due to its effects on cardiac conduction. Concomitant use with delavirdine is contraindicated due to anticipated loss of virologic response.
Possible side effect
Side effects are often concentration-dependent.
- Common (Often dose-related): Nystagmus, ataxia, slurred speech, dizziness, insomnia, nervousness, tremor, headache, nausea, vomiting, constipation, gingival hyperplasia, coarsening of facial features, hirsutism.
- Serious (Require medical attention): Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)/multiorgan hypersensitivity, Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), hepatic necrosis, blood dyscrasias (e.g., thrombocytopenia, leukopenia, granulocytopenia, agranulocytosis, pancytopenia), lymphadenopathy, peripheral neuropathy, osteomalacia (due to altered vitamin D metabolism), cerebellar atrophy with long-term use.
Drug interaction
Dilantin is a potent inducer of hepatic cytochrome P450 enzymes (e.g., CYP2C9, CYP2C19, CYP3A4) and is also highly protein-bound, leading to a vast number of clinically significant interactions.
- Drugs that increase phenytoin levels: Amiodarone, chloramphenicol, chlordiazepoxide, cimetidine, disulfiram, estrogens, fluconazole, isoniazid, metronidazole, omeprazole, phenothiazines, salicylates (at high doses), sertraline, sulfonamides, ticlopidine, tolbutamide, warfarin.
- Drugs that decrease phenytoin levels: Carbamazepine, chronic alcohol abuse, reserpine, rifampin, sucralfate, theophylline.
- Drugs whose levels are decreased by phenytoin: Oral anticoagulants (e.g., warfarin), corticosteroids, cyclosporine, digoxin, disopyramide, doxycycline, estrogens, furosemide, oral contraceptives, paroxetine, quinidine, rifampin, theophylline, vitamin D, many antiretroviral drugs (e.g., delavirdine, efavirenz, indinavir, nevirapine, ritonavir, saquinavir).
- Protein-Binding Displacement: Drugs like valproic acid and salicylates can displace phenytoin from protein binding sites, increasing the free, active fraction without necessarily changing the total serum level.
Missed dose
If a dose is missed, it should be taken as soon as it is remembered. However, if it is almost time for the next scheduled dose, the missed dose should be skipped. The patient should never take a double dose to make up for a missed one, as this could precipitously raise serum levels into the toxic range due to the drug’s saturable metabolism.
Overdose
Phenytoin overdose is a medical emergency characterized primarily by cerebellar and vestibular effects.
- Symptoms: Nystagmus, ataxia, dysarthria, tremor, lethargy, slurred speech, nausea, vomiting. In severe cases: hypotension, respiratory depression, coma, and potentially death.
- Treatment: Supportive care is mainstay, with attention to respiratory and cardiac function. Gastric lavage may be considered if ingestion was recent. Multiple-dose activated charcoal can enhance elimination, even hours after ingestion, by interrupting enterohepatic recirculation. Hemodialysis is not effective due to high protein binding. Serum level monitoring is crucial to guide management.
Storage
Store at room temperature between 20°C to 25°C (68°F to 77°F), in a tight, light-resistant container as specified in the USP. Keep out of reach of children and pets. Do not use if the formulation shows signs of precipitation or discoloration.
Disclaimer
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or before starting or altering any treatment regimen. Never disregard professional medical advice or delay in seeking it because of something you have read here.
Reviews
“Dilantin remains a first-line option for many of my patients with focal epilepsy. Its efficacy is undeniable, but the necessity for vigilant therapeutic drug monitoring cannot be overstated. Managing the interactions and non-linear kinetics is a commitment, but for the right patient, it provides excellent seizure control.” – Neurologist, 15 years of experience.
“For my son, Dilantin was the drug that finally stopped his tonic-clonic seizures after others failed. We have to be very strict with his blood tests and he has some gum overgrowth, but the trade-off for a seizure-free life is worth it. We work closely with his neurologist and dentist.” – Parent of a patient.
“While newer agents have emerged, Dilantin’s role in status epilepticus protocols is foundational. The ability to rapidly load a patient IV is critical in an emergency setting. We are always mindful of the infusion rate and cardiac monitoring requirements.” – Emergency Medicine Physician.