Phenytoin (Diphenylhydantoin, Phenytoin Sodium)(fen’ i toe in)
Dilantin, Dilantin-125, Dilantin Infatab, Dilantin Injection, Phenytek
PREGNANCY CATEGORY D
Drug Classes
Antiarrhythmic, group 1b
Antiepileptic
Hydantoin
Therapeutic Actions
Has antiepileptic activity without causing general CNS depression; stabilizes neuronal membranes and prevents hyperexcitability caused by excessive stimulation; limits the spread of seizure activity from an active focus; also effective in treating cardiac arrhythmias, especially those induced by cardiac glycosides; antiarrhythmic properties are very similar to those of lidocaine; both are class IB antiarrhythmics.
Indications
Control of generalized tonic-clonic and partial seizures
Prevention and treatment of seizures occurring during or following neurosurgery
Parenteral administration: Control of status epilepticus of the generalized tonic-clonic type
Unlabeled uses: Antiarrhythmic, particularly in arrhythmias induced by cardiac glycosides (IV preparations); treatment of trigeminal neuralgia (tic douloureux); rectal administration
Contraindications and Cautions
Contraindicated with hypersensitivity to hydantoins, sinus bradycardia, sinoatrial block, Stokes-Adams syndrome, pregnancy (data suggest an association between antiepileptic use and an elevated incidence of birth defects; however, do not discontinue antiepileptic therapy in pregnant women who are receiving such therapy to prevent major seizures; stopping drug is likely to precipitate status epilepticus, with attendant hypoxia and risk to both mother and fetus), lactation.
Use cautiously with acute intermittent porphyria, blood dyscrasias, hypotension, severe myocardial insufficiency, diabetes mellitus, hyperglycemia.
Available Forms
Chewable tablets—50 mg; oral suspension—125 mg/5 mL; ER capsules—30, 100, 200, 300 mg; injection—50 mg/mL
Dosages
Adults
Phenytoin sodium, parenteral
Status epilepticus: 10–15 mg/kg by slow IV. For maintenance, 100 mg PO or IV every 6–8 hr. Higher doses may be required. Do not exceed an infusion rate of 50 mg/min. Follow each IV injection with an injection of sterile saline through the same needle or IV catheter to avoid local venous irritation by the alkaline solution. Continuous IV infusion is not recommended.
Neurosurgery (prophylaxis): 100–200 mg IM every 4 hr during surgery and the postoperative period (IM route is not recommended because of erratic absorption, pain, and muscle damage at the injection site).
IM therapy in a patient previously stabilized on oral dosage: Increase dosage by 50% over oral dosage. When returning to oral dosage, decrease dose by 50% of the original oral dose for 1 wk to prevent excessive plasma levels due to continued absorption from IM tissue sites. Avoid IM route of administration if possible due to erratic absorption and pain and muscle damage at injection site.
Phenytoin and phenytoin sodium, oral: Individualize dosage. Determine serum levels for optimal dosage adjustments. The clinically effective serum level is usually between 10 and 20 mcg/mL.
Loading dose (hospitalized patients without renal or liver disease): Initially, 1 g of phenytoin capsules (phenytoin sodium, prompt) is divided into three doses (400 mg, 300 mg, 300 mg) and given PO every 2 hr. Normal maintenance dosage is then
instituted 24 hr after the loading dose with frequent serum determinations.
No previous treatment: Start with 100 mg tid PO. Satisfactory maintenance dosage is usually 300–400 mg/day. An increase to 600 mg/day may be needed.
Single daily dosage (phenytoin sodium, extended): If seizure control is established with divided doses of three 100-mg extended phenytoin sodium capsules PO per day, once-a-day dosage with 300 mg PO may be considered. Do not use prompt phenytoin capsules, suspension, or chewable tablets for once-daily dosing.
Pediatric patients
Phenytoin sodium, parenteral
Status epilepticus: Administer phenytoin IV. Determine dosage according to weight in proportion to dose for a 70-kg adult (see adult dosage earlier on this page; see Appendix I, Calculating pediatric dosages). Pediatric dosage may be calculated on the basis of 250 mg/m2 IV. Dosage for infants and children also may be calculated on the basis of 10–15 mg/kg, given in divided doses of 5–10 mg/kg IV. For neonates, 15–20 mg/kg IV in divided doses of 5–10 mg/kg is recommended.
Phenytoin and phenytoin sodium, oral
Children not previously treated: Initially, 5 mg/kg/day PO in two to three equally divided doses. Subsequent dosage should be individualized to a maximum of 300 mg/day. Daily maintenance dosage is 4–8 mg/kg. Children older than 6 yr may require the minimum adult dose of 300 mg/day PO.
Geriatric patients and patients with hepatic impairment
Use caution and monitor for early signs of toxicity; phenytoin is metabolized in the liver.
Pharmacokinetics
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