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Side Effects & Adverse Reactions
Abrupt withdrawal of phenytoin in epileptic patients may precipitate 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. In the event of an allergic or hypersensitivity reaction, more rapid substitution of alternative therapy may be necessary. In this case, alternative therapy should be an anticonvulsant not belonging to the hydantoin chemical class.
Antiepileptic drugs (AEDs), including phenytoin chewable tablets, increase the risk of suicidal thoughts or behavior in patients taking these drugs for any indication. Patients treated with any AED for any indication should be monitored for the emergence or worsening of depression, suicidal thoughts or behavior, and/or any unusual changes in mood or behavior.
Pooled analyses of 199 placebo-controlled clinical trials (mono- and adjunctive therapy) of 11 different AEDs showed that patients randomized to one of the AEDs had approximately twice the risk (adjusted Relative Risk 1.8, 95% CI:1.2, 2.7) of suicidal thinking or behavior compared to patients randomized to placebo. In these trials, which had a median treatment duration of 12 weeks, the estimated incidence rate of suicidal behavior or ideation among 27,863 AED-treated patients was 0.43%, compared to 0.24% among 16,029 placebo-treated patients, representing an increase of approximately one case of suicidal thinking or behavior for every 530 patients treated. There were four suicides in drug-treated patients in the trials and none in placebo-treated patients, but the number is too small to allow any conclusion about drug effect on suicide.
The increased risk of suicidal thoughts or behavior with AEDs was observed as early as one week after starting drug treatment with AEDs and persisted for the duration of treatment assessed. Because most trials included in the analysis did not extend beyond 24 weeks, the risk of suicidal thoughts or behavior beyond 24 weeks could not be assessed.
The risk of suicidal thoughts or behavior was generally consistent among drugs in the data analyzed. The finding of increased risk with AEDs of varying mechanisms of action and across a range of indications suggests that the risk applies to all AEDs used for any indication. The risk did not vary substantially by age (5 to 100 years) in the clinical trials analyzed.
Table 1 shows absolute and relative risk by indication for all evaluated AEDs.
Indication |
Placebo Patients |
Drug Patients |
Relative Risk: |
Risk Difference: Additional Drug Patients with Events per 1,000 Patients |
Epilepsy |
1 |
3.4 |
3.5 |
2.4 |
Psychiatric |
5.7 |
8.5 |
1.5 |
2.9 |
Other |
1 |
1.8 |
1.9 |
0.9 |
Total |
2.4 |
4.3 |
1.8 |
1.9 |
The relative risk for suicidal thoughts or behavior was higher in clinical trials for epilepsy than in clinical trials for psychiatric or other conditions, but the absolute risk differences were similar for the epilepsy and psychiatric indications.
Anyone considering prescribing phenytoin chewable tablets or any other AED must balance the risk of suicidal thoughts or behavior with the risk of untreated illness. Epilepsy and many other illnesses for which AEDs are prescribed are themselves associated with morbidity and mortality and an increased risk of suicidal thoughts and behavior. Should suicidal thoughts and behavior emerge during treatment, the prescriber needs to consider whether the emergence of these symptoms in any given patient may be related to the illness being treated.
Patients, their caregivers, and families should be informed that AEDs increase the risk of suicidal thoughts and behavior and should be advised of the need to be alert for the emergence or worsening of the signs and symptoms of depression, any unusual changes in mood or behavior, or the emergence of suicidal thoughts, behavior, or thoughts about self-harm. Behaviors of concern should be reported immediately to healthcare providers.
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 (see DRESS/Multiorgan Hypersensitivity below).
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.
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, and/or lymphadenopathy, 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 should be discontinued if an alternative etiology for the signs or symptoms cannot be established.
Phenytoin and other hydantoins are contraindicated in patients who have experienced phenytoin hypersensitivity (see CONTRAINDICATIONS). 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.
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. 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 readministered.
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 of DRESS.
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.
The chronic use of phenytoin in patients with epilepsy has been associated with decreased bone mineral density (osteopenia, osteoporosis, and osteomalacia) and bone fractures. Phenytoin induces hepatic metabolizing enzymes. This may enhance the metabolism of vitamin D and decrease vitamin D levels, which may lead to vitamin D deficiency, hypocalcemia, and hypophosphatemia. Consideration should be given to screening with bone-related laboratory and radiological tests as appropriate and initiating treatment plans according to established guidelines.
Acute alcoholic intake may increase phenytoin serum levels while chronic alcoholic use may decrease serum levels.
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.
An increase in seizure frequency may occur during pregnancy because of altered phenytoin pharmacokinetics. Periodic measurement of plasma phenytoin concentrations may be valuable in the management of pregnant women as a guide to appropriate adjustment of dosage (see PRECAUTIONS: Laboratory Tests). However, postpartum restoration of the original dosage will probably be indicated.
If this drug is used during pregnancy, or if the patient becomes pregnant while taking the drug, the patient should be apprised of the potential harm to the fetus.
Prenatal exposure to phenytoin may increase the risks for congenital malformations and other adverse developmental outcomes. Increased frequencies of major malformations (such as orofacial clefts and cardiac defects), minor anomalies (dysmorphic facial features, nail and digit hypoplasia), growth abnormalities (including microcephaly), and mental deficiency have been reported among children born to epileptic women who took phenytoin alone or in combination with other antiepileptic drugs during pregnancy. There have also been several reported cases of malignancies, including neuroblastoma, in children whose mothers received phenytoin during pregnancy. The overall incidence of malformations for children of epileptic women treated with antiepileptic drugs (phenytoin and/or others) during pregnancy is about 10%, or 2- to 3-fold that in the general population. However, the relative contributions of antiepileptic drugs and other factors associated with epilepsy to this increased risk are uncertain and in most cases it has not been possible to attribute specific developmental abnormalities to particular antiepileptic drugs.
Patients should consult with their physicians to weigh the risks and benefits of phenytoin during pregnancy.
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.
Increased resorption and malformation rates have been reported following administration of phenytoin doses of 75 mg/kg or higher (approximately 120% of the maximum human loading dose or higher on a mg/m2 basis) to pregnant rabbits.
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Uses
Phenytoin chewable tablets are indicated for the control of generalized tonic-clonic (grand mal) and complex partial (psychomotor, temporal lobe) seizures and prevention and treatment of seizures occurring during or following neurosurgery. Phenytoin serum level determinations may be necessary for optimal dosage adjustments (see DOSAGE AND ADMINISTRATION and CLINICAL PHARMACOLOGY sections).
History
There is currently no drug history available for this drug.
Other Information
Phenytoin is an antiepileptic drug.
Phenytoin is related to the barbiturates in chemical structure, but has a five-membered ring. The chemical name is 5,5-Diphenylhydantoin, having the following structural formula:
Each phenytoin chewable tablet, USP for oral administration, contains 50 mg of phenytoin, USP. Also contains: colloidal silicon dioxide, compressible sugar, corn starch, D&C Red No. 30 Aluminum Lake, magnesium stearate, peppermint flavor and talc.
Sources
Phenytoin Manufacturers
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Cardinal Health
Phenytoin | Cardinal Health
When given in equal doses, phenytoin chewable tablets yield higher plasma levels than Dilantin Kapseals®*. For this reason serum concentrations should be monitored and care should be taken when switching a patient from the sodium salt to the free acid form.
Dilantin Kapseals®* is formulated with the sodium salt of phenytoin. The free acid form of phenytoin is used in Dilantin-125® Suspensions* and phenytoin chewable tablets. 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.
GeneralNot for once-a-day dosing.
Dosage should be individualized to provide maximum benefit. In some cases, serum blood level determinations may be necessary for optimal dosage adjustments - the clinically effective serum level is usually 10 to 20 mcg/mL. With recommended dosage, a period of 7 to 10 days may be required to achieve steady-state blood levels with phenytoin and changes in dosage (increase or decrease) should not be carried out at intervals shorter than 7 to 10 days.
Phenytoin chewable tablets can be either chewed thoroughly before being swallowed or swallowed whole.
Adult DosagePatients who have received no previous treatment may be started on two phenytoin chewable tablets 3 times daily, and the dose is then adjusted to suit individual requirements. For most adults, the satisfactory maintenance dosage will be six to eight phenytoin chewable tablets daily; an increase to twelve phenytoin chewable tablets daily may be made, if necessary.
Dosing in Special Populations Patients with Renal or Hepatic DiseaseDue to an increased fraction of unbound phenytoin in patients with renal or hepatic disease, or in those with hypoalbuminemia, the interpretation of total phenytoin plasma concentrations should be made with caution. Unbound phenytoin concentrations may be more useful in these patient populations.
Elderly PatientsPhenytoin clearance is decreased slightly in elderly patients and lower or less frequent dosing may be required.
PediatricInitially, 5 mg/kg/day in two or three equally divided doses, with subsequent dosage individualized to a maximum 300 mg daily. A recommended daily maintenance dosage is usually 4 to 8 mg/kg. Children over 6 years old and adolescents may require the minimum adult dose (300 mg/day). If the daily dosage cannot be divided equally, the larger dose should be given before retiring.
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Ncs Healthcare Of Ky, Inc Dba Vangard Labs
Phenytoin | Ncs Healthcare Of Ky, Inc Dba Vangard Labs
When given in equal doses, phenytoin chewable tablets yield higher plasma levels than Dilantin Kapseals®*. For this reason serum concentrations should be monitored and care should be taken when switching a patient from the sodium salt to the free acid form.
Dilantin Kapseals®* is formulated with the sodium salt of phenytoin. The free acid form of phenytoin is used in Dilantin-125® Suspensions* and phenytoin chewable tablets. 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.
GeneralNot for once-a-day dosing.
Dosage should be individualized to provide maximum benefit. In some cases, serum blood level determinations may be necessary for optimal dosage adjustments - the clinically effective serum level is usually 10 to 20 mcg/mL. With recommended dosage, a period of 7 to 10 days may be required to achieve steady-state blood levels with phenytoin and changes in dosage (increase or decrease) should not be carried out at intervals shorter than 7 to 10 days.
Phenytoin chewable tablets can be either chewed thoroughly before being swallowed or swallowed whole.
Adult DosagePatients who have received no previous treatment may be started on two phenytoin chewable tablets 3 times daily, and the dose is then adjusted to suit individual requirements. For most adults, the satisfactory maintenance dosage will be six to eight phenytoin chewable tablets daily; an increase to twelve phenytoin chewable tablets daily may be made, if necessary.
Dosing in Special Populations Patients with Renal or Hepatic DiseaseDue to an increased fraction of unbound phenytoin in patients with renal or hepatic disease, or in those with hypoalbuminemia, the interpretation of total phenytoin plasma concentrations should be made with caution. Unbound phenytoin concentrations may be more useful in these patient populations.
Elderly PatientsPhenytoin clearance is decreased slightly in elderly patients and lower or less frequent dosing may be required.
PediatricInitially, 5 mg/kg/day in two or three equally divided doses, with subsequent dosage individualized to a maximum 300 mg daily. A recommended daily maintenance dosage is usually 4 to 8 mg/kg. Children over 6 years old and adolescents may require the minimum adult dose (300 mg/day). If the daily dosage cannot be divided equally, the larger dose should be given before retiring.
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Mylan Institutional Inc.
Phenytoin | Mylan Institutional Inc.
When given in equal doses, phenytoin chewable tablets yield higher plasma levels than Dilantin Kapseals®*. For this reason serum concentrations should be monitored and care should be taken when switching a patient from the sodium salt to the free acid form.
Dilantin Kapseals®* is formulated with the sodium salt of phenytoin. The free acid form of phenytoin is used in Dilantin-125® Suspensions* and phenytoin chewable tablets. 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.
GeneralNot for once-a-day dosing.
Dosage should be individualized to provide maximum benefit. In some cases, serum blood level determinations may be necessary for optimal dosage adjustments - the clinically effective serum level is usually 10 to 20 mcg/mL. With recommended dosage, a period of 7 to 10 days may be required to achieve steady-state blood levels with phenytoin and changes in dosage (increase or decrease) should not be carried out at intervals shorter than 7 to 10 days.
Phenytoin chewable tablets can be either chewed thoroughly before being swallowed or swallowed whole.
Adult DosagePatients who have received no previous treatment may be started on two phenytoin chewable tablets 3 times daily, and the dose is then adjusted to suit individual requirements. For most adults, the satisfactory maintenance dosage will be six to eight phenytoin chewable tablets daily; an increase to twelve phenytoin chewable tablets daily may be made, if necessary.
Dosing in Special Populations Patients with Renal or Hepatic DiseaseDue to an increased fraction of unbound phenytoin in patients with renal or hepatic disease, or in those with hypoalbuminemia, the interpretation of total phenytoin plasma concentrations should be made with caution. Unbound phenytoin concentrations may be more useful in these patient populations.
Elderly PatientsPhenytoin clearance is decreased slightly in elderly patients and lower or less frequent dosing may be required.
PediatricInitially, 5 mg/kg/day in two or three equally divided doses, with subsequent dosage individualized to a maximum 300 mg daily. A recommended daily maintenance dosage is usually 4 to 8 mg/kg. Children over 6 years old and adolescents may require the minimum adult dose (300 mg/day). If the daily dosage cannot be divided equally, the larger dose should be given before retiring.
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Mylan Pharmaceuticals Inc.
Phenytoin | Mylan Pharmaceuticals Inc.
When given in equal doses, phenytoin chewable tablets yield higher plasma levels than Dilantin Kapseals®*. For this reason serum concentrations should be monitored and care should be taken when switching a patient from the sodium salt to the free acid form.
Dilantin Kapseals®* is formulated with the sodium salt of phenytoin. The free acid form of phenytoin is used in Dilantin-125® Suspensions* and phenytoin chewable tablets. 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.
GeneralNot for once-a-day dosing.
Dosage should be individualized to provide maximum benefit. In some cases, serum blood level determinations may be necessary for optimal dosage adjustments - the clinically effective serum level is usually 10 to 20 mcg/mL. With recommended dosage, a period of 7 to 10 days may be required to achieve steady-state blood levels with phenytoin and changes in dosage (increase or decrease) should not be carried out at intervals shorter than 7 to 10 days.
Phenytoin chewable tablets can be either chewed thoroughly before being swallowed or swallowed whole.
Adult DosagePatients who have received no previous treatment may be started on two phenytoin chewable tablets 3 times daily, and the dose is then adjusted to suit individual requirements. For most adults, the satisfactory maintenance dosage will be six to eight phenytoin chewable tablets daily; an increase to twelve phenytoin chewable tablets daily may be made, if necessary.
Dosing in Special Populations Patients with Renal or Hepatic DiseaseDue to an increased fraction of unbound phenytoin in patients with renal or hepatic disease, or in those with hypoalbuminemia, the interpretation of total phenytoin plasma concentrations should be made with caution. Unbound phenytoin concentrations may be more useful in these patient populations.
Elderly PatientsPhenytoin clearance is decreased slightly in elderly patients and lower or less frequent dosing may be required.
PediatricInitially, 5 mg/kg/day in two or three equally divided doses, with subsequent dosage individualized to a maximum 300 mg daily. A recommended daily maintenance dosage is usually 4 to 8 mg/kg. Children over 6 years old and adolescents may require the minimum adult dose (300 mg/day). If the daily dosage cannot be divided equally, the larger dose should be given before retiring.
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Taro Pharmaceuticals U.s.a., Inc
Phenytoin | West-ward Pharmaceuticals Corp
2.1 General Dosing InformationNicardipine hydrochloride injection is intended for intravenous use. DOSAGE MUST BE INDIVIDUALIZED depending upon the severity of hypertension and the response of the patient during dosing. Blood pressure and heart rate should be monitored both during and after the infusion to avoid tachycardia or too rapid or excessive reduction in either systolic or diastolic blood pressure.
2.2 PreparationWarning: Dilute vials before infusion.
DilutionNicardipine hydrochloride injection is administered by slow continuous infusion at a CONCENTRATION OF 0.1 MG/ML. Each vial (25 mg) should be diluted with 240 mL of compatible intravenous fluid (see below), resulting in 250 mL of solution at a concentration of 0.1 mg/mL.
Nicardipine hydrochloride injection has been found compatible and stable in polyvinyl chloride containers for 24 hours at controlled room temperature with:
Dextrose (5%) Injection, USP
Dextrose (5%) and Sodium Chloride (0.45%) Injection, USP
Dextrose (5%) and Sodium Chloride (0.9%) Injection, USP
Dextrose (5%) with 40 mEq Potassium, USP
Sodium Chloride (0.45%) Injection, USP
Sodium Chloride (0.9%) Injection, USPNicardipine hydrochloride injection is not compatible with Sodium Bicarbonate (5%) Injection, USP or Lactated Ringer’s Injection, USP.
The diluted solution is stable for 24 hours at room temperature.
InspectionVisually inspect nicardipine hydrochloride injection for particulate matter and discoloration prior to administration, whenever solution and container permit. Nicardipine hydrochloride injection is normally light yellow in color.
Do not use the solution if particulate matter, precipitate or crystallization is present, or if the container appears damaged.
2.3 Dosage as a Substitute for Oral Nicardipine TherapyThe intravenous infusion rate required to produce an average plasma concentration equivalent to a given oral dose at steady state is shown in the following table:
Oral Nicardipine Dose Equivalent Intravenous Infusion Rate 20 mg q8h 0.5 mg/hr 30 mg q8h 1.2 mg/hr 40 mg q8h 2.2 mg/hr 2.4 Dosage for Initiation of Therapy in a Drug-Free PatientThe time course of blood pressure decrease is dependent on the initial rate of infusion and the frequency of dosage adjustment. Nicardipine hydrochloride injection is administered by slow continuous infusion at a concentration of 0.1 mg/mL. With constant infusion, blood pressure begins to fall within minutes. It reaches about 50% of its ultimate decrease in about 45 minutes.
When treating acute hypertensive episodes in patients with chronic hypertension, discontinuation of infusion is followed by a 50% offset of action in 30 minutes ± 7 minutes but plasma levels of drug and gradually decreasing antihypertensive effects exist for many hours.
TitrationFor a gradual reduction in blood pressure, initiate therapy at a rate of 50 mL/hr (5 mg/hr). If desired blood pressure reduction is not achieved at this dose, increase the infusion rate by 25 mL/hr (2.5 mg/hr) every 15 minutes up to a maximum of 150 mL/hr (15 mg/hr), until desired blood pressure reduction is achieved. For more rapid blood pressure reduction, titrate every 5 minutes.
MaintenanceAdjust the rate of infusion as needed to maintain desired response.
2.5 Conditions Requiring Infusion Adjustment Hypotension or Tachycardia:In case of hypotension or tachycardia, discontinue infusion. When blood pressure and heart rate stabilize, restart infusion at low doses such as 30 mL/hr to 50 mL/hr (3 mg/hr to 5 mg/hr) and titrate to maintain desired blood pressure.
Infusion Site Changes:Change infusion site every 12 hours if administered via peripheral vein.
Impaired Cardiac, Hepatic, or Renal Function:Monitor closely when titrating nicardipine hydrochloride injection in patients with congestive heart failure or impaired hepatic or renal function [see Warnings and Precautions (5.4, 5.5 and 5.6)].
2.6 Transfer to Oral Antihypertensive AgentsIf treatment includes transfer to an oral antihypertensive agent other than nicardipine capsules, initiate oral therapy upon discontinuation of nicardipine hydrochloride injection.
When switching to a TID regimen of nicardipine capsules, administer the first dose 1 hour prior to discontinuation of the infusion.
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American Health Packaging
Phenytoin | American Health Packaging
When given in equal doses, phenytoin chewable tablets, USP yield higher plasma levels than extended phenytoin sodium capsules, USP. For this reason serum concentrations should be monitored and care should be taken when switching a patient from the sodium salt to the free acid form.
Extended phenytoin sodium capsules, USP is formulated with the sodium salt of phenytoin. The free acid form of phenytoin is used in phenytoin oral suspensions and phenytoin chewable tablets, USP. 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.
General:Not for once-a-day dosing.
Dosage should be individualized to provide maximum benefit. In some cases, serum blood level determinations may be necessary for optimal dosage adjustments—the clinically effective serum level is usually 10–20 mcg/mL. With recommended dosage, a period of seven to ten days may be required to achieve steady-state blood levels with phenytoin and changes in dosage (increase or decrease) should not be carried out at intervals shorter than seven to ten days.
Phenytoin chewable tablets, USP can be either chewed thoroughly before being swallowed or swallowed whole.
Adult Dosage:Patients who have received no previous treatment may be started on two chewable tablets three times daily, and the dose is then adjusted to suit individual requirements. For most adults, the satisfactory maintenance dosage will be six to eight chewable tablets daily; an increase to twelve chewable tablets daily may be made, if necessary.
Dosing in Special Populations Patients with Renal or Hepatic Disease:Due to an increased fraction of unbound phenytoin in patients with renal or hepatic disease, or in those with hypoalbuminemia, the interpretation of total phenytoin plasma concentrations should be made with caution.
Unbound phenytoin concentrations may be more useful in these patient populations.
Elderly Patients:Phenytoin clearance is decreased slightly in elderly patients and lower or less frequent dosing may be required.
Pediatric:Initially, 5 mg/kg/day in two or three equally divided doses, with subsequent dosage individualized to a maximum 300 mg daily. A recommended daily maintenance dosage is usually 4 to 8 mg/kg. Children over 6 years old and adolescents may require the minimum adult dose (300 mg/day). If the daily dosage cannot be divided equally, the larger dose should be given before retiring.
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