May 28, 2017
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Side Effects


The more important adverse clinical events caused by the IV use of CEREBYX or phenytoin are cardiovascular collapse and/or central nervous system depression. Hypotension can occur when either drug is administered rapidly by the IV route. The rate of administration is very important; for CEREBYX, it should not exceed 150 mg PE/min. The adverse clinical events most commonly observed with the use of CEREBYX in clinical trials were nystagmus, dizziness, pruritus, paresthesia, headache, somnolence, and ataxia. With two exceptions, these events are commonly associated with the administration of IV phenytoin. Paresthesia and pruritus, however, were seen much more often following CEREBYX administration and occurred more often with IV CEREBYX administration than with IM CEREBYX administration. These events were dose and rate related; most alert patients (41 of 64; 64%) administered doses of ≥ 15 mg PE/kg at 150 mg PE/min experienced discomfort of some degree. These sensations, generally described as itching, burning, or tingling, were usually not at the infusion site. The location of the discomfort varied with the groin mentioned most frequently as a site of involvement. The paresthesia and pruritus were transient events that occurred within several minutes of the start of infusion and generally resolved within 10 minutes after completion of CEREBYX infusion. Some patients experienced symptoms for hours. These events did not increase in severity with repeated administration. Concurrent adverse events or clinical laboratory change suggesting an allergic process were not seen (see PRECAUTIONS, Sensory Disturbances). Approximately 2% of the 859 individuals who received CEREBYX in premarketing clinical trials discontinued treatment because of an adverse event. The adverse events most commonly associated with withdrawal were pruritus (0.5%), hypotension (0.3%), and bradycardia (0.2%).

Dose and Rate Dependency of Adverse Events Following IV CEREBYX: The incidence of adverse events tended to increase as both dose and infusion rate increased. In particular, at doses of ≥ 15 mg PE/kg and rates ≥ 150 mg PE/min, transient pruritus, tinnitus, nystagmus, somnolence, and ataxia occurred 2 to 3 times more often than at lower doses or rates.

Incidence In Controlled Clinical Trials

All adverse events were recorded during the trials by the clinical investigators using terminology of their own choosing. Similar types of events were grouped into standardized categories using modified COSTART dictionary terminology. These categories are used in the tables and listings below with the frequencies representing the proportion of individuals exposed to CEREBYX or comparative therapy. The prescriber should be aware that these figures cannot be used to predict the frequency of adverse events in the course of usual medical practice where patient characteristics and other factors may differ from those prevailing during clinical studies. Similarly, the cited frequencies cannot be directly compared with figures obtained from other clinical investigations involving different treatments, uses or investigators. An inspection of these frequencies, however, does provide the prescribing physician with one basis to estimate the relative contribution of drug and nondrug factors to the adverse event incidences in the population studied.

Incidence in Controlled Clinical Trials - IV Administration To Patients With Epilepsy or Neurosurgical Patients: Table 2 lists treatment-emergent adverse events that occurred in at least 2% of patients treated with IV CEREBYX at the maximum dose and rate in a randomized, double-blind, controlled clinical trial where the rates for phenytoin and CEREBYX administration would have resulted in equivalent systemic exposure to phenytoin.

TABLE 2: Treatment-Emergent Adverse Event Incidence Following IV Administration at the Maximum Dose and Rate to Patients With Epilepsy or Neurosurgical Patients (Events in at Least 2% of CEREBYX -Treated Patients)

IV Phenytoin
  Pelvic Pain 4.4 0.0
  Asthenia 2.2 0.0
  Back Pain 2.2 0.0
  Headache 2.2 4.5
  Hypotension 7.7 9.1
  Vasodilatation 5.6 4.5
  Tachycardia 2.2 0.0
  Nausea 8.9 13.6
  Tongue Disorder 4.4 0.0
  Dry Mouth 4.4 4.5
  Vomiting 2.2 9.1
  Nystagmus 44.4 59.1
  Dizziness 31.1 27.3
  Somnolence 20.0 27.3
  Ataxia 11.1 18.2
  Stupor 7.7 4.5
  Incoordination 4.4 4.5
  Paresthesia 4.4 0.0
  Extrapyramidal Syndrome 4.4 0.0
  Tremor 3.3 9.1
  Agitation 3.3 0.0
  Hypesthesia 2.2 9.1
  Dysarthria 2.2 0.0
  Vertigo 2.2 0.0
  Brain Edema 2.2 4.5
  Pruritus 48.9 4.5
  Tinnitus 8.9 9.1
  Diplopia 3.3 0.0
  Taste Perversion 3.3 0.0
  Amblyopia 2.2 9.1
  Deafness 2.2 0.0

Incidence in Controlled Trials - IM Administration to Patients With Epilepsy: Table 3 lists treatment-emergent adverse events that occurred in at least 2% of CEREBYX-treated patients in a double-blind, randomized, controlled clinical trial of adult epilepsy patients receiving either IM CEREBYX substituted for oral Dilantin or continuing oral Dilantin. Both treatments were administered for 5 days.

TABLE 3: Treatment-Emergent Adverse Event Incidence Following Substitution of IM CEREBYX for Oral Dilantin in Patients With Epilepsy (Events in at Least 2% of CEREBYX -Treated Patients)

BODY SYSTEM Adverse Event IM CEREBYX N=179 Oral Dilantin N=61
  Headache 8.9 4.9
  Asthenia 3.9 3.3
  Accidental Injury 3.4 6.6
  Nausea 4.5 0.0
  Vomiting 2.8 0.0
  Ecchymosis 7.3 4.9
  Nystagmus 15.1 8.2
  Tremor 9.5 13.1
  Ataxia 8.4 8.2
  Incoordination 7.8 4.9
  Somnolence 6.7 9.8
  Dizziness 5.0 3.3
  Paresthesia 3.9 3.3
  Reflexes Decreased 2.8 4.9
  Pruritus 2.8 0.0

Adverse Events During All Clinical Trials

CEREBYX has been administered to 859 individuals during all clinical trials. All adverse events seen at least twice are listed in the following, except those already included in previous tables and listings. Events are further classified within body system categories and enumerated in order of decreasing frequency using the following definitions: frequent adverse events are defined as those occurring in greater than 1/100 individuals; infrequent adverse events are those occurring in 1/100 to 1/1000 individuals.

Body as a Whole: Frequent: fever, injection-site reaction, infection, chills, face edema, injection-site pain; Infrequent: sepsis, injection-site inflammation, injection-site edema, injection-site hemorrhage, flu syndrome, malaise, generalized edema, shock, photosensitivity reaction, cachexia, cryptococcosis.

Cardiovascular: Frequent: hypertension; Infrequent: cardiac arrest, migraine, syncope, cerebral hemorrhage, palpitation, sinus bradycardia, atrial flutter, bundle branch block, cardiomegaly, cerebral infarct, postural hypotension, pulmonary embolus, QT interval prolongation, thrombophlebitis, ventricular extrasystoles, congestive heart failure.

Digestive: Frequent: constipation; Infrequent: dyspepsia, diarrhea, anorexia, gastrointestinal hemorrhage, increased salivation, liver function tests abnormal, tenesmus, tongue edema, dysphagia, flatulence, gastritis, ileus.

Endocrine: Infrequent: diabetes insipidus.

Hematologic and Lymphatic: Infrequent: thrombocytopenia, anemia, leukocytosis, cyanosis, hypochromic anemia, leukopenia, lymphadenopathy, petechia.

Metabolic and Nutritional: Frequent: hypokalemia; Infrequent: hyperglycemia, hypophosphatemia, alkalosis, acidosis, dehydration, hyperkalemia, ketosis.

Musculoskeletal: Frequent: myasthenia; Infrequent: myopathy, leg cramps, arthralgia, myalgia.

Nervous: Frequent: reflexes increased, speech disorder, dysarthria, intracranial hypertension, thinking abnormal, nervousness, hypesthesia; Infrequent: confusion, twitching, Babinski sign positive, circumoral paresthesia, hemiplegia, hypotonia, convulsion, extrapyramidal syndrome, insomnia, meningitis, depersonalization, CNS depression, depression, hypokinesia, hyperkinesia, brain edema, paralysis, psychosis, aphasia, emotional lability, coma, hyperesthesia, myoclonus, personality disorder, acute brain syndrome, encephalitis, subdural hematoma, encephalopathy, hostility, akathisia, amnesia, neurosis.

Respiratory: Frequent: pneumonia; Infrequent: pharyngitis, sinusitis, hyperventilation, rhinitis, apnea, aspiration pneumonia, asthma, dyspnea, atelectasis, cough increased, sputum increased, epistaxis, hypoxia, pneumothorax, hemoptysis, bronchitis.

Skin and Appendages: Frequent: rash; Infrequent: maculopapular rash, urticaria, sweating, skin discoloration, contact dermatitis, pustular rash, skin nodule.

Special Senses: Frequent: taste perversion; Infrequent: deafness, visual field defect, eye pain, conjunctivitis, photophobia, hyperacusis, mydriasis, parosmia, ear pain, taste loss.

Urogenital: Infrequent: urinary retention, oliguria, dysuria, vaginitis, albuminuria, genital edema, kidney failure, polyuria, urethral pain, urinary incontinence, vaginal moniliasis.

Post-Marketing Experience

The following adverse reactions have been identified during postapproval use of fosphenytoin. 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.

There have been post-marketing reports of anaphylactoid reaction and anaphylaxis.

Other Phenytoin-Associated Adverse Events


Read the Cerebyx (fosphenytoin sodium injection) Side Effects Center for a complete guide to possible side effects


No drugs are known to interfere with the conversion of fosphenytoin to phenytoin. Conversion could be affected by alterations in the level of phosphatase activity, but given the abundance and wide distribution of phosphatases in the body it is unlikely that drugs would affect this activity enough to affect conversion of fosphenytoin to phenytoin. Drugs highly bound to albumin could increase the unbound fraction of fosphenytoin. Although, it is unknown whether this could result in clinically significant effects, caution is advised when administering CEREBYX with other drugs that significantly bind to serum albumin. The pharmacokinetics and protein binding of fosphenytoin, phenytoin, and diazepam were not altered when diazepam and CEREBYX were concurrently administered in single submaximal doses. The most significant drug interactions following administration of CEREBYX are expected to occur with drugs that interact with phenytoin. Phenytoin is extensively bound to serum plasma proteins and is prone to competitive displacement. Phenytoin is metabolized by hepatic cytochrome P450 enzymes CYP2C9 and CYP2C19 and is particularly susceptible to inhibitory drug interactions because it is subject to saturable metabolism. Inhibition of metabolism may produce significant increases in circulating phenytoin concentrations and enhance the risk of drug toxicity. Phenytoin is a potent inducer of hepatic drug-metabolizing enzymes.

The most commonly occurring drug interactions are listed below:

Note: The list is not intended to be inclusive or comprehensive. Individual drug package inserts should be consulted.

Drugs That Affect Phenytoin Concentrations

  • Drugs that may increase plasma phenytoin concentrations include: acute alcohol intake, amiodarone, anti-epileptic agents (ethosuximide, felbamate, oxcarbazepine, methsuximide, topiramate), azoles (fluconazole, ketoconazole, itraconazole, miconazole, voriconazole), capecitabine, chloramphenicol, chlordiazepoxide, disulfiram, estrogens, fluorouracil, fluoxetine, fluvastatin, fluvoxamine, H2-antagonists (e.g., cimetidine), halothane, isoniazid, methylphenidate, omeprazole, phenothiazines, salicylates, sertraline, succinimides, sulfonamides (e.g., sulfamethizole, sulfaphenazole, sulfadiazine, sulfamethoxazole-trimethoprim), ticlopidine, tolbutamide, trazodone, and warfarin.
  • Drugs that may decrease plasma phenytoin concentrations include: anticancer drugs usually in combination (e.g., bleomycin, carboplatin, cisplatin, doxorubicin, methotrexate), carbamazepine, chronic alcohol abuse, diazepam, diazoxide, folic acid, fosamprenavir, nelfinavir, reserpine, rifampin, ritonavir, St. John's Wort, theophylline, and vigabatrin.
  • Drugs that may either increase or decrease plasma phenytoin concentrations include: phenobarbital, valproic acid, and sodium valproate. Similarly, the effects of phenytoin on phenobarbital, valproic acid and sodium plasma valproate concentrations are unpredictable.
  • The addition or withdrawal of these agents in patients on phenytoin therapy may require an adjustment of the phenytoin dose to achieve optimal clinical outcome.

Drugs Affected by Phenytoin

  • Drugs that should not be coadministered with phenytoin: Delavirdine (see CONTRAINDICATIONS).
  • Drugs whose efficacy is impaired by phenytoin include: azoles (fluconazole, ketoconazole, itraconazole, voriconazole, posaconazole), corticosteroids, doxycycline, estrogens, furosemide, irinotecan, oral contraceptives, paclitaxel, paroxetine, quinidine, rifampin, sertraline, teniposide, theophylline, and vitamin D.
  • Increased and decreased PT/INR responses have been reported when phenytoin is coadministered with warfarin.
  • Phenytoin decreases plasma concentrations of active metabolites of albendazole, certain HIV antivirals (efavirenz, lopinavir/ritonavir, indinavir, nelfinavir, ritonavir, saquinavir), antiepileptic agents (carbamazepine, felbamate, lamotrigine, topiramate, oxcarbazepine, quetiapine), atorvastatin, chlorpropamide, clozapine, cyclosporine, digoxin, fluvastatin, folic acid, methadone, mexiletine, nifedipine, nimodipine, nisoldipine, praziquantel, simvastatin and verapamil.
  • Phenytoin when given with fosamprenavir alone may decrease the concentration of amprenavir, the active metabolite. Phenytoin when given with the combination of fosamprenavir and ritonavir may increase the concentration of amprenavir.
  • Resistance to the neuromuscular blocking action of the nondepolarizing neuromuscular blocking agents pancuronium, vecuronium, rocuronium, and cisatracurium has occurred in patients chronically administered phenytoin. Whether or not phenytoin has the same effect on other nondepolarizing agents is unknown. Patients should be monitored closely for more rapid recovery from neuromuscular blockade than expected, and infusion rate requirements may be higher.
  • 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.

Monitoring of plasma phenytoin concentrations may be helpful when possible drug interactions are suspected (see Laboratory Tests).

Drug/Laboratory Test Interactions

Phenytoin may decrease serum concentrations of T4. It may also produce artifactually low results in dexamethasone or metyrapone tests. Phenytoin may also cause increased serum concentrations of glucose, alkaline phosphatase, and gamma glutamyl transpeptidase (GGT). Care should be taken when using immunoanalytical methods to measure plasma phenytoin concentrations following CEREBYX administration (see Laboratory Tests).

Read the Cerebyx Drug Interactions Center for a complete guide to possible interactions

This monograph has been modified to include the generic and brand name in many instances.

Last reviewed on RxList: 11/17/2016

Side Effects

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