dose to achieve optimal clinical outcome. table 1: drugs that affect phenytoin concentrations b the induction potency of st. john’s wort may vary widely based on preparation. 7.2 drugs affected by phenytoin table 2 includes commonly occurring drug interactions affected by phenytoin. however, this list is not intended to be inclusive or comprehensive. individual drug package inserts should be consulted. the addition or withdrawal of phenytoin during concomitant therapy with these agents may require adjustment of the dose of these agents to achieve optimal clinical outcome. table 2: drugs affected by phenytoin a the effect of phenytoin on phenobarbital, valproic acid and sodium valproate serum levels is unpredictable 7.3 drug/laboratory test interactions care should be taken when using immunoanalytical methods to measure serum phenytoin concentrations following fosphenytoin administration. interactions 1 interactions 2

city increases with infusion rates above the recommended infusion rate, these events have also been reported at or below the recommended infusion rate. as non-emergency therapy, phenytoin sodium injection should be administered more slowly as either a loading dose or by intermittent infusion. because of the risks of cardiac and local toxicity associated with intravenous phenytoin, oral phenytoin should be used whenever possible. because adverse cardiovascular reactions have occurred during and after infusions, careful cardiac and respiratory monitoring is needed during and after the administration of intravenous phenytoin sodium injection. reduction in rate of administration or discontinuation of dosing may be needed. 5.2 withdrawal precipitated seizure, status epilepticus antiepileptic drugs should not be abruptly discontinued because of the possibility of increased seizure frequency, including status epilepticus. when, in the judgment of the clinician, the need for dosage reduction, discontinuation, or substitution of alternative antiepileptic medication arises, this should be done gradually. however, in the event of an allergic or hypersensitivity reaction, rapid substitution of alternative therapy may be necessary. in this case, alternative therapy should be an antiepileptic drug not belonging to the hydantoin chemical class. 5.3 serious dermatologic reactions serious and sometimes fatal dermatologic reactions, including toxic epidermal necrolysis (ten) and stevens-johnson syndrome (sjs), have been reported with phenytoin treatment. the onset of symptoms is usually within 28 days, but can occur later. phenytoin should be discontinued at the first sign of a rash, unless the rash is clearly not drug-related. if signs or symptoms suggest sjs/ten, use of this drug should not be resumed and alternative therapy should be considered. if a rash occurs, the patient should be evaluated for signs and symptoms of drug reaction with eosinophilia and systemic symptoms (dress) [see warnings and precautions (5.4)]. studies in patients of chinese ancestry have found a strong association between the risk of developing sjs/ten and the presence of hla-b*1502, an inherited allelic variant of the hla b gene, in patients using carbamazepine. limited evidence suggests that hla-b*1502 may be a risk factor for the development of sjs/ten in patients of asian ancestry taking other antiepileptic drugs associated with sjs/ten, including phenytoin. consideration should be given to avoiding phenytoin as an alternative for carbamazepine in patients positive for hla-b*1502. the use of hla-b*1502 genotyping has important limitations and must never substitute for appropriate clinical vigilance and patient management. the role of other possible factors in the development of, and morbidity from, sjs/ten, such as antiepileptic drug (aed) dose, compliance, concomitant medications, comorbidities, and the level of dermatologic monitoring have not been studied. 5.4 drug reaction with eosinophilia and systemic symptoms (dress)/multiorgan hypersensitivity drug reaction with eosinophilia and systemic symptoms (dress), also known as multiorgan hypersensitivity, has been reported in patients taking antiepileptic drugs, including phenytoin. some of these events have been fatal or life-threatening. dress typically, although not exclusively, presents with fever, rash, lymphadenopathy, and/or facial swelling, in association with other organ system involvement, such as hepatitis, nephritis, hematological abnormalities, myocarditis, or myositis sometimes resembling an acute viral infection. eosinophilia is often present. because this disorder is variable in its expression, other organ systems not noted here may be involved. it is important to note that early manifestations of hypersensitivity, such as fever or lymphadenopathy, may be present even though rash is not evident. if such signs or symptoms are present, the patient should be evaluated immediately. phenytoin sodium injection should be discontinued if an alternative etiology for the signs or symptoms cannot be established. 5.5 hypersensitivity phenytoin and other hydantoins are contraindicated in patients who have experienced phenytoin hypersensitivity [see contraindications 4- (4)]. additionally, consider alternatives to structurally similar drugs such as carboxamides (e.g., carbamazepine), barbiturates, succinimides, and oxazolidinediones (e.g., trimethadione) in these same patients. similarly, if there is a history of hypersensitivity reactions to these structurally similar drugs in the patient or immediate family members, consider alternatives to phenytoin. 5.6 hepatic injury cases of acute hepatotoxicity, including infrequent cases of acute hepatic failure, have been reported with phenytoin. these events may be part of the spectrum of dress or may occur in isolation [see warnings and precautions (5.4)]. other common manifestations include jaundice, hepatomegaly, elevated serum transaminase levels, leukocytosis, and eosinophilia. the clinical course of acute phenytoin hepatotoxicity ranges from prompt recovery to fatal outcomes. in these patients with acute hepatotoxicity, phenytoin should be immediately discontinued and not re-administered. 5.7 hematopoietic complications hematopoietic complications, some fatal, have occasionally been reported in association with administration of phenytoin. these have included thrombocytopenia, leukopenia, granulocytopenia, agranulocytosis, and pancytopenia with or without bone marrow suppression. there have been a number of reports suggesting a relationship between phenytoin and the development of lymphadenopathy (local or generalized) including benign lymph node hyperplasia, pseudolymphoma, lymphoma, and hodgkin's disease. although a cause and effect relationship has not been established, the occurrence of lymphadenopathy indicates the need to differentiate such a condition from other types of lymph node pathology. lymph node involvement may occur with or without symptoms and signs resembling dress [see warnings and precautions (5.4)]. in all cases of lymphadenopathy, follow-up observation for an extended period is indicated and every effort should be made to achieve seizure control using alternative antiepileptic drugs. 5.8 local toxicity (including purple glove syndrome) soft tissue irritation and inflammation has occurred at the site of injection with and without extravasation of intravenous phenytoin. edema, discoloration and pain distal to the site of injection (described as “purple glove syndrome”) have also been reported following peripheral intravenous phenytoin injection. soft tissue irritation may vary from slight tenderness to extensive necrosis, and sloughing. the syndrome may not develop for several days after injection. although resolution of symptoms may be spontaneous, skin necrosis and limb ischemia have occurred and required such interventions as fasciotomies, skin grafting, and, in rare cases, amputation. because of the risk of local toxicity, intravenous phenytoin sodium injection should be administered directly into a large peripheral or central vein through a large-gauge catheter. prior to the administration, the patency of the iv catheter should be tested with a flush of sterile saline. each injection of parenteral phenytoin sodium injection should then be followed by a flush of sterile saline through the same catheter to avoid local venous irritation caused by the alkalinity of the solution. intramuscular phenytoin sodium injection administration may cause pain, necrosis, and abscess formation at the injection site [see dosage and administration 2- (2.3)]. 5.9 renal or hepatic impairment or hypoalbuminemia because the fraction of unbound phenytoin is increased in patients with renal or hepatic disease, or in those with hypoalbuminemia, the monitoring of phenytoin serum levels should be based on the unbound fraction in those patients. 5.10 exacerbation of porphyria in view of isolated reports associating phenytoin with exacerbation of porphyria, caution should be exercised in using this medication in patients suffering from this disease. 5.11 teratogenicity and other harm to the newborn phenytoin sodium injection may cause fetal harm when administered to a pregnant woman. prenatal exposure to phenytoin may increase the risks for congenital malformations and other adverse developmental outcomes [see use in specific populations 8- (8.1)]. increased frequencies of major malformations (such as orofacial clefts and cardiac defects), and abnormalities characteristic of fetal hydantoin syndrome, including dysmorphic skull and facial features, nail and digit hypoplasia, growth abnormalities (including microcephaly), and cognitive deficits, have been reported among children born to epileptic women who took phenytoin alone or in combination with other antiepileptic drugs during pregnancy. there have been several reported cases of malignancies, including neuroblastoma. a potentially life-threatening bleeding disorder related to decreased levels of vitamin k-dependent clotting factors may occur in newborns exposed to phenytoin in utero. this drug-induced condition can be prevented with vitamin k administration to the mother before delivery and to the neonate after birth. 5.12 slow metabolizers of phenytoin a small percentage of individuals who have been treated with phenytoin have been shown to metabolize the drug slowly. slow metabolism may be caused by limited enzyme availability and lack of induction; it appears to be genetically determined. if early signs of dose-related central nervous system (cns) toxicity develop, serum levels should be checked immediately. 5.13 hyperglycemia hyperglycemia, resulting from the drug's inhibitory effect on insulin release, has been reported. phenytoin may also raise the serum glucose level in diabetic patients. 5.14 serum phenytoin levels above therapeutic range serum levels of phenytoin sustained above the therapeutic range may produce confusional states referred to as "delirium", "psychosis", or "encephalopathy", or rarely irreversible cerebellar dysfunction and/or cerebellar atrophy. accordingly, at the first sign of acute toxicity, serum levels should be immediately checked. dose reduction of phenytoin therapy is indicated if serum levels are excessive; if symptoms persist, termination is recommended.

injection should be administered directly into a large peripheral or central vein through a large-gauge catheter. prior to the administration, the patency of the intravenous (iv) catheter should be tested with a flush of sterile saline. each injection of parenteral phenytoin sodium injection should then be followed by a flush of sterile saline through the same catheter to avoid local venous irritation due to the alkalinity of the solution. phenytoin sodium injection can be given diluted with normal saline. the addition of parenteral phenytoin sodium injection to dextrose and dextrose-containing solutions should be avoided due to lack of solubility and resultant precipitation. treatment with phenytoin sodium injection can be initiated either with a loading dose or an infusion: loading dose: a loading dose of parenteral phenytoin sodium injection should be injected slowly, not exceeding 50 mg per minute in adults and 1 to 3 mg/kg/min (or 50 mg per minute, whichever is slower) in pediatric patients. infusion: for infusion administration, parenteral phenytoin sodium injection should be diluted in normal saline with the final concentration of phenytoin sodium in the solution no less than 5 mg/ml. administration should commence immediately after the mixture has been prepared and must be completed within 1 to 4 hours (the infusion mixture should not be refrigerated). an in-line filter (0.22 to 0.55 microns) should be used. parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution or container permit. the diluted infusion mixture (phenytoin sodium injection plus normal saline) should not be refrigerated. if the undiluted parenteral phenytoin sodium injection is refrigerated or frozen, a precipitate might form: this will dissolve again after the solution is allowed to stand at room temperature. the product is still suitable for use. a faint yellow coloration may develop, however this has no effect on the potency of the solution. for single-dose only. after opening, any unused product should be discarded. monitoring levels: trough levels provide information about clinically effective serum level range and are obtained just prior to the patient’s next scheduled dose. peak levels indicate an individual’s threshold for emergence of dose-related side effects and are obtained at the time of expected peak concentration. therapeutic effect without clinical signs of toxicity occurs more often with serum total concentrations between 10 and 20 mcg/ml (unbound phenytoin concentrations of 1 to 2 mcg/ml), although some mild cases of tonic-clonic (grand mal) epilepsy may be controlled with lower serum levels of phenytoin. in patients with renal or hepatic disease, or in those with hypoalbuminemia, the monitoring of unbound phenytoin concentrations may be more relevant [see dosage and administration (2.3)]. 2.2 status epilepticus in adults, a loading dose of 10 to 15 mg/kg should be administered slowly intravenously, at a rate not exceeding 50 mg per minute (this will require approximately 20 minutes in a 70-kg patient). the loading dose should be followed by maintenance doses of 100 mg orally or intravenously every 6 to 8 hours. in the pediatric population, a loading dose of 15 to 20 mg/kg of phenytoin sodium intravenously will usually produce serum concentrations of phenytoin within the generally accepted serum total concentrations between 10 and 20 mcg/ml (unbound phenytoin concentrations of 1 to 2 mcg/ml). the drug should be injected slowly intravenously at a rate not exceeding 1 to 3 mg/kg/min or 50 mg per minute, whichever is slower. continuous monitoring of the electrocardiogram and blood pressure is essential. the patient should be observed for signs of respiratory depression. determination of phenytoin serum levels is advised when using phenytoin in the management of status epilepticus and in the subsequent establishment of maintenance dosage. other measures, including concomitant administration of an intravenous benzodiazepine such as diazepam, or an intravenous short-acting barbiturate, will usually be necessary for rapid control of seizures because of the required slow rate of administration of phenytoin. if administration of parenteral phenytoin sodium injection does not terminate seizures, the use of other anticonvulsants, intravenous barbiturates, general anesthesia, and other appropriate measures should be considered. intramuscular administration should not be used in the treatment of status epilepticus because the attainment of peak serum levels may require up to 24 hours. 2.3 non-emergent loading and maintenance dosing because of the risks of cardiac and local toxicity associated with intravenous phenytoin, oral phenytoin should be used whenever possible. in adults, a loading dose of 10 to 15 mg/kg should be administered slowly. the rate of intravenous administration should not exceed 50 mg per minute in adults and 1 to 3 mg/kg/min (or 50 mg per minute, whichever is slower) in pediatric patients. slower administration rates are recommended to minimize the cardiovascular adverse reactions. continuous monitoring of the electrocardiogram, blood pressure, and respiratory function is essential. the loading dose should be followed by maintenance doses of oral or intravenous phenytoin every 6 to 8 hours. ordinarily, phenytoin sodium injection should not be given intramuscularly because of the risk of necrosis, abscess formation, and erratic absorption. if intramuscular administration is required, compensating dosage adjustments are necessary to maintain therapeutic serum levels. an intramuscular dose 50% greater than the oral dose is necessary to maintain these levels. when returned to oral administration, the dose should be reduced by 50% of the original oral dose for one week to prevent excessive serum levels due to sustained release from intramuscular tissue sites. monitoring serum levels would help prevent a fall into the subtherapeutic range. serum blood level determinations are especially helpful when possible drug interactions are suspected. 2.4 parenteral substitution for oral phenytoin therapy when treatment with oral phenytoin is not possible, iv phenytoin can be substituted for oral phenytoin at the same total daily dose. phenytoin capsules are approximately 90% bioavailable by the oral route. phenytoin is 100% bioavailable by the iv route. for this reason, serum phenytoin concentrations may increase modestly when iv phenytoin is substituted for oral phenytoin sodium therapy. the rate of administration for iv phenytoin should be no greater than 50 mg per minute in adults and 1 to 3 mg/kg/min (or 50 mg per minute, whichever is slower) in pediatric patients. when intramuscular administration may be required, a sufficient dose must be administered intramuscularly to maintain the serum level within the therapeutic range. where oral dosage is resumed following intramuscular usage, the oral dose should be properly adjusted to compensate for the slow, continuing im absorption to minimize toxic symptoms [see clinical pharmacology 12- (12.3)]. serum concentrations should be monitored and care should be taken when switching a patient from the sodium salt to the free acid form. phenytoin sodium injection is formulated with the sodium salt of phenytoin. because there is approximately an 8% increase in drug content with the free acid form over that of the sodium salt, dosage adjustments and serum level monitoring may be necessary when switching from a product formulated with the free acid to a product formulated with the sodium salt and vice versa. 2.5 dosing in patients with renal or hepatic impairment or hypoalbuminemia because the fraction of unbound phenytoin is increased in patients with renal or hepatic disease, or in those with hypoalbuminemia, the monitoring of phenytoin serum levels should be based on the unbound fraction in those patients. 2.6 dosing in geriatrics phenytoin clearance is decreased slightly in elderly patients and lower or less frequent dosing may be required [see clinical pharmacology 12- (12.3)]. 2.7 dosing during pregnancy decreased serum concentrations of phenytoin may occur during pregnancy because of altered phenytoin pharmacokinetics. periodic measurement of serum phenytoin concentrations should be performed during pregnancy, and the phenytoin sodium injection dosage should be adjusted as necessary. postpartum restoration of the original dosage will probably be indicated [see use in specific populations 8- (8.1)]. because of potential changes in protein binding during pregnancy, the monitoring of phenytoin serum levels should be based on the unbound fraction. 2.8 dosing in pediatrics a loading dose of 15 to 20 mg/kg of phenytoin sodium injection intravenously will usually produce serum concentrations of phenytoin within the generally accepted serum total concentrations between 10 and 20 mcg/ml (unbound phenytoin concentrations of 1 to 2 mcg/ml). the drug should be injected slowly intravenously at a rate not exceeding 1 to 3 mg/kg/min or 50 mg per minute, whichever is slower.

ated with the use of phenytoin sodium injection were identified in clinical studies or postmarketing reports. because these reactions are reported voluntarily from a population of uncertain size, it is not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure. the most notable signs of toxicity associated with the intravenous use of this drug are cardiovascular collapse and/or cns depression. hypotension does occur when the drug is administered rapidly by the intravenous route. the rate of administration is very important; it should not exceed 50 mg per minute in adults, and 1 to 3 mg/kg/min (or 50 mg per minute, whichever is slower) in pediatric patients [see boxed warning, dosage and administration 2- (2.1), and warnings and precautions 5- (5.1)]. body as a whole: allergic reactions in the form of rash and rarely more serious forms (see skin and appendages paragraph below) and dress [see warnings and precautions 5- (5.4)] have been observed. anaphylaxis has also been reported. there have also been reports of coarsening of facial features, systemic lupus erythematosus, periarteritis nodosa, and immunoglobulin abnormalities. cardiovascular: severe cardiovascular events and fatalities have been reported with atrial and ventricular conduction depression and ventricular fibrillation. severe complications are most commonly encountered in elderly or critically ill patients [see boxed warning and warnings and precautions 5- (5.1)] . digestive system: acute hepatic failure [see warnings and precautions 5- (5.6)], toxic hepatitis, liver damage, nausea, vomiting, constipation, enlargement of the lips, and gingival hyperplasia. hematologic and lymphatic system: hematopoietic complications, some fatal, have occasionally been reported in association with administration of phenytoin [see warnings and precautions 5- (5.7)]. these have included thrombocytopenia, leukopenia, granulocytopenia, agranulocytosis, and pancytopenia with or without bone marrow suppression. while macrocytosis and megaloblastic anemia have occurred, these conditions usually respond to folic acid therapy. lymphadenopathy, including benign lymph node hyperplasia, pseudolymphoma, lymphoma, and hodgkin's disease have been reported [see warnings and precautions 5- (5.7)]. laboratory test abnormality: phenytoin may decrease serum concentrations of thyroid hormone (t4 and t3), sometimes with an accompanying increase in thyroid-stimulating hormone (tsh), but usually in the absence of clinical hypothyroidism. phenytoin may also produce lower than normal values for dexamethasone or metyrapone tests. phenytoin may also cause increased serum levels of glucose, alkaline phosphatase, and gamma glutamyl transpeptidase (ggt). nervous system: the most common adverse reactions encountered with phenytoin therapy are nervous system reactions and are usually dose-related. reactions include nystagmus, ataxia, slurred speech, decreased coordination, somnolence, and mental confusion. dizziness, vertigo, insomnia, transient nervousness, motor twitchings, paresthesia, and headaches have also been observed. there have also been rare reports of phenytoin induced dyskinesias, including chorea, dystonia, tremor and asterixis, similar to those induced by phenothiazine and other neuroleptic drugs. cerebellar atrophy has been reported, and appears more likely in settings of elevated phenytoin levels and/or long-term phenytoin use [see warnings and precautions 5- (5.14)]. a predominantly sensory peripheral polyneuropathy has been observed in patients receiving long-term phenytoin therapy. skin and appendages: dermatological manifestations sometimes accompanied by fever have included scarlatiniform or morbilliform rashes. a morbilliform rash (measles-like) is the most common; other types of dermatitis are seen more rarely. other more serious forms which may be fatal have included bullous, exfoliative or purpuric dermatitis, stevens-johnson syndrome, and toxic epidermal necrolysis [see warnings and precautions 5- (5.3)]. there have also been reports of hypertrichosis. local irritation, inflammation, tenderness, necrosis, and sloughing have been reported with or without extravasation of intravenous phenytoin [see warnings and precautions 5- (5.8)]. special senses: altered taste sensation including metallic taste. urogenital: peyronie’s disease

dose to achieve optimal clinical outcome. table 1: drugs that affect phenytoin concentrations b the induction potency of st. john’s wort may vary widely based on preparation. 7.2 drugs affected by phenytoin table 2 includes commonly occurring drug interactions affected by phenytoin. however, this list is not intended to be inclusive or comprehensive. individual drug package inserts should be consulted. the addition or withdrawal of phenytoin during concomitant therapy with these agents may require adjustment of the dose of these agents to achieve optimal clinical outcome. table 2: drugs affected by phenytoin a the effect of phenytoin on phenobarbital, valproic acid and sodium valproate serum levels is unpredictable 7.3 drug/laboratory test interactions care should be taken when using immunoanalytical methods to measure serum phenytoin concentrations following fosphenytoin administration. interactions 1 interactions 2

l features, nail and digit hypoplasia, growth abnormalities (including microcephaly), and cognitive deficits has been reported among children born to epileptic women who took phenytoin alone or in combination with other antiepileptic drugs during pregnancy [see data]. there have been several reported cases of malignancies, including neuroblastoma, in children whose mothers received phenytoin during pregnancy. administration of phenytoin to pregnant animals resulted in an increased incidence of fetal malformations and other manifestations of developmental toxicity (including embryofetal death, growth impairment, and behavioral abnormalities) in multiple species at clinically relevant doses [see data]. in the u.s. general population, the estimated background risk of major birth defects and of miscarriage in clinically recognized pregnancies is 2 to 4% and 15 to 20%, respectively. the background risk of major birth defects and miscarriage for the indicated population is unknown. clinical considerations disease-associated maternal risk an increase in seizure frequency may occur during pregnancy because of altered phenytoin pharmacokinetics. periodic measurement of serum phenytoin concentrations may be valuable in the management of pregnant women as a guide to appropriate adjustment of dosage [see dosage and administration 2- (2.1, 2.7)]. however, postpartum restoration of the original dosage will probably be indicated. fetal/neonatal adverse reactions a potentially life-threatening bleeding disorder related to decreased levels of vitamin k-dependent clotting factors may occur in newborns exposed to phenytoin in utero. this drug-induced condition can be prevented with vitamin k administration to the mother before delivery and to the neonate after birth. data human data meta-analyses using data from published observational studies and registries have estimated an approximately 2.4-fold increased risk for any major malformation in children with prenatal phenytoin exposure compared to controls. an increased risk of heart defects, facial clefts, and digital hypoplasia has been reported. the fetal hydantion syndrome is a pattern of congenital anomalies including craniofacial anomalies, nail and digital hypoplasia, prenatal-onset growth deficiency, and neurodevelopmental deficiencies. animal data administration of phenytoin to pregnant rats, rabbits, and mice during organogenesis resulted in embryofetal death, fetal malformations, and decreased fetal growth retardation. malformations (including craniofacial, cardiovascular, neural, limb, and digit abnormalities) were observed in rats, rabbits, and mice at doses as low as 100, 75, and 12.5 mg/kg, respectively. 8.2 lactation risk summary phenytoin is secreted in human milk. the developmental and health benefits of breastfeeding should be considered along with the mother’s clinical need for phenytoin sodium injection and any potential adverse effects on the breastfed infant from phenytoin sodium injection or from the underlying maternal condition. 8.4 pediatric use a loading dose of 15 to 20 mg/kg of phenytoin sodium injection intravenously will usually produce serum concentrations of phenytoin within the generally accepted serum total concentrations between 10 and 20 mcg/ml (unbound phenytoin concentrations of 1 to 2 mcg/ml). because of the increased risk of adverse cardiovascular reactions associated with rapid administration phenytoin sodium injection should be injected slowly intravenously at a rate not exceeding 1 to 3 mg/kg/min or 50 mg per minute, whichever is slower [see dosage and administration 2- (2.8) and warnings and precautions 5- (5.1)]. 8.5 geriatric use phenytoin clearance tends to decrease with increasing age [see clinical pharmacology 12- (12.3)]. lower or less frequent dosing may be required [see dosage and administration 2- (2.6)]. 8.6 renal and/or hepatic impairment or hypoalbuminemia the liver is the site of biotransformation. patients with impaired liver function, elderly patients, or those who are gravely ill may show early toxicity. because the fraction of unbound phenytoin is increased in patients with renal or hepatic disease, or in those with hypoalbuminemia, the monitoring of phenytoin serum levels should be based on the unbound fraction in those patients.

)-5-phenylhydantoin (hpph), the principal metabolite, as well as the total amount of drug eventually appearing in the blood. as phenytoin is highly protein bound, free phenytoin levels may be altered in patients whose protein binding characteristics differ from normal. a short-term (one week) study indicates that patients do not experience the expected drop in blood levels when crossed over to the intramuscular route if the phenytoin im dose is increased by 50 percent over the previously established oral dose. to avoid drug accumulation caused by absorption from the muscle depots, it is recommended that for the first week back on oral phenytoin, the dose be reduced to half of the original oral dose (one-third of the im dose). experience for periods greater than one week is lacking and blood level monitoring is recommended. therapeutic effect without clinical signs of toxicity occurs most often with serum total concentrations between 10 and 20 mcg/ml (unbound phenytoin concentrations of 1 to 2 mcg/ml). distribution phenytoin is extensively bound to plasma proteins and is prone to competitive displacement. elimination the serum half-life in man after intravenous administration ranges from 10 to 15 hours. metabolism phenytoin is metabolized by the cytochrome p450 enzymes cyp2c9 and cyp2c19. excretion most of the drug is excreted in the bile as inactive metabolites. urinary excretion of phenytoin and its metabolites occurs partly by glomerular filtration but, more importantly, by tubular secretion. specific populations age: geriatric population: phenytoin clearance tends to decrease with increasing age (20% less in patients over 70 years of age relative to that in patients 20 to 30 years of age). since phenytoin clearance is decreased slightly in elderly patients, lower or less frequent dosing may be required [see dosage and administration 2- (2.6)]. sex/race: gender and race have no significant impact on phenytoin pharmacokinetics. renal or hepatic impairment: increased fraction of unbound phenytoin in patients with renal or hepatic disease, or in those with hypoalbuminemia has been reported. pregnancy: it has been reported in the literature that the plasma clearance of phenytoin generally increased during pregnancy, reached a peak in the third trimester and returned to the level of pre-pregnancy after few weeks or months of delivery. drug interaction studies phenytoin is metabolized by the cytochrome p450 enzymes cyp2c9 and cyp2c19. phenytoin is a potent inducer of hepatic drug-metabolizing enzymes [see drug interactions 7- (7.1, 7.2)].

ames test and in the in vitro clastogenicity assay in chinese hamster ovary (cho) cells. in studies reported in the literature, phenytoin was negative in the in vitro mouse lymphoma assay and the in vivo micronucleus assay in mouse. phenytoin was clastogenic in the in vitro sister chromatid exchange assay in cho cells. fertility phenytoin has not been adequately assessed for effects on male or female fertility.