Last updated on RxList: 2/25/2021
Cerebyx Side Effects Center

What Is Cerebyx?

Cerebyx (fosphenytoin sodium) Injection is an anti-epileptic drug, also called an anticonvulsant, used to prevent or control seizures. Cerebyx is used only for a short time, such as 5 days, when other forms of phenytoin cannot be given.

What Are Side Effects of Cerebyx?

Common side effects of Cerebyx include:

  • dizziness,
  • drowsiness,
  • headache,
  • nausea,
  • vomiting,
  • constipation,
  • dry mouth,
  • itching,
  • tremor,
  • muscle weakness,
  • loss of coordination,
  • ringing in your ears,
  • pain in your hips or back,
  • flushing (warmth, redness, or tingly feeling under your skin),
  • low blood pressure,
  • fast heart rate,
  • spinning sensation,
  • double vision, and
  • changes in taste.

Dosage for Cerebyx

Cerebyx is only given as needed under a physician's supervision. Dose is determined based on the patient's condition and body weight.

What Drugs, Substances, or Supplements Interact with Cerebyx?

Cerebyx may interact with other medicines such as:

  • disulfiram,
  • birth control pills or hormone replacement therapy,
  • phenothiazines,
  • methylphenidate,
  • barbiturates,
  • stomach acid reducers,
  • sedatives,
  • sulfa drugs,
  • reserpine,
  • carbamazepine,
  • blood thinners,
  • digoxin,
  • furosemide,
  • steroids,
  • theophylline,
  • valproic acid,
  • divalproex sodium,
  • antibiotics, or
  • antidepressants

Tell your doctor all medications and supplements you use.

Cerebyx During Pregnancy and Breastfeeding

During pregnancy, Cerebyx should be used only when prescribed. It may harm a fetus. Since untreated seizures are a serious condition that can harm both a pregnant woman and the fetus, do not stop taking this medication unless directed by your doctor. Hormonal birth control may not work if taken with this medication. Discuss birth control with your doctor. This medication passes into breast milk. Consult your doctor before breastfeeding. Abrupt withdrawal of Cerebyx in epileptic patients may precipitate status epilepticus.

Additional Information

Our Cerebyx (fosphenytoin sodium) Injection Side Effects Drug Center provides a comprehensive view of available drug information on the potential side effects when taking this medication.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.


What Is Epilepsy? Symptoms, Causes, and Treatments See Slideshow
Cerebyx Consumer Information

Get emergency medical help if you have signs of an allergic reaction (hives, difficult breathing, swelling in your face or throat) or a severe skin reaction (fever, sore throat, burning in your eyes, skin pain, red or purple skin rash that spreads and causes blistering and peeling).

Seek medical treatment if you have a serious drug reaction that can affect many parts of your body. Symptoms may include: skin rash, fever, swollen glands, flu-like symptoms, muscle aches, severe weakness, unusual bruising, or yellowing of your skin or eyes. This reaction may occur several weeks after you began using fosphenytoin.

Call your doctor at once if you have:

  • very slow heartbeats, shortness of breath;
  • a light-headed feeling, like you might pass out;
  • confusion, unusual thoughts or behavior;
  • a tingling or burning sensation;
  • easy bruising, unusual bleeding;
  • purple discoloration of your skin around the IV needle;
  • sudden weakness or ill feeling, fever, chills, sore throat, mouth sores, red or swollen gums, trouble swallowing; o
  • low potassium--leg cramps, constipation, irregular heartbeats, fluttering in your chest, increased thirst or urination, numbness or tingling, muscle weakness or limp feeling.

Common side effects may include:

  • dizziness, drowsiness;
  • unusual or involuntary eye movements;
  • vomiting;
  • itching; or
  • problems with balance or muscle movement.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Read the entire detailed patient monograph for Cerebyx (Fosphenytoin Sodium Injection)


If you have had a seizure, it means you have epilepsy. See Answer
Cerebyx Professional Information


The following serious adverse reactions are described elsewhere in the labeling:

  • Cardiovascular Risk Associated with Rapid Infusion [see WARNINGS AND PRECAUTIONS]
  • Withdrawal Precipitated Seizure, Status Epilepticus [see WARNINGS AND PRECAUTIONS]
  • Serious Dermatologic Reactions [see WARNINGS AND PRECAUTIONS]
  • Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)/Multiorgan Hypersensitivity [see WARNINGS AND PRECAUTIONS]
  • Hypersensitivity [see WARNINGS AND PRECAUTIONS]
  • Hepatic Injury [see WARNINGS AND PRECAUTIONS]
  • Hematopoietic Complications [see WARNINGS AND PRECAUTIONS]
  • Sensory Disturbances [see WARNINGS AND PRECAUTIONS]
  • Local Toxicity (Including Purple Glove Syndrome) [see WARNINGS AND PRECAUTIONS]
  • Exacerbation of Porphyria [see WARNINGS AND PRECAUTIONS]
  • Teratogenicity and Other Harm to the Newborn [see WARNINGS AND PRECAUTIONS]
  • Hyperglycemia [see WARNINGS AND PRECAUTIONS]

Clinical Trials Experience

Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in practice.

The more important adverse clinical reactions caused by the IV use of CEREBYX or phenytoin are cardiovascular collapse and/or CNS 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 [see WARNINGS AND PRECAUTIONS]. The adverse reactions most commonly observed with the use of CEREBYX in clinical trials were nystagmus, dizziness, pruritus, somnolence, and ataxia. With one exception, these reactions are commonly associated with the administration of IV phenytoin. Pruritus, however, was seen much more often following CEREBYX administration and occurred more often with IV CEREBYX administration than with IM CEREBYX administration. These reactions 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 reactions did not increase in severity with repeated administration. Concurrent adverse events or clinical laboratory change suggesting an allergic process were not seen [see WARNINGS AND PRECAUTIONS]. Approximately 2% of the 859 patients 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 Reactions Following IV CEREBYX: The incidence of adverse reactions tended to increase as both dose and infusion rate increased. In particular, at doses of ≥15mg 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.

Incidence In Controlled Clinical Trials -IV Administration To Adult Patients With Epilepsy Or Neurosurgical Patients

Table 1 lists adverse reactions 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 1: Adverse Reaction Incidence Following IV Administration at the Maximum Dose and Rate to Adult Patients with Epilepsy or Neurosurgical Patients (Events in at Least 2% of CEREBYX-Treated Patients)

Adverse Event
IV Phenytoin1
Pelvic Pain 4 0
Asthenia 2 0
Back Pain 2 0
Headache 2 5
Hypotension 8 9
Vasodilatation 6 5
Tachycardia 2 0
Nausea 9 14
Tongue Disorder 4 0
Dry Mouth 4 5
Vomiting 2 9
Nystagmus 44 59
Dizziness 31 27
Somnolence 20 27
Ataxia 11 18
Stupor 8 5
Incoordination 4 5
Paresthesia 4 0
Extrapyramidal Syndrome 4 0
Tremor 3 9
Agitation 3 0
Hypesthesia 2 9
Dysarthria 2 0
Vertigo 2 0
Brain Edema 2 5
Pruritus 49 5
Tinnitus 9 9
Diplopia 3 0
Taste Perversion 3 0
Amblyopia 2 9
Deafness 2 0
1 The study was not designed to assess comparative safety.

Incidence In Clinical Trials -IV Administration To Pediatric Patients With Epilepsy Or Neurosurgical Patients

The overall incidence of adverse reactions and the types of adverse reactions seen were similar among children and adults treated with CEREBYX. In an open-label, safety, tolerability, and pharmacokinetic study of fosphenytoin in pediatric subjects (neonates through age 16), the following adverse reactions occurred at a frequency of at least 5% in 96 subjects treated with intravenous CEREBYX: vomiting (21%), nystagmus (18%), ataxia (10%), fever (8%), nervousness (7%), pruritus (6%), somnolence (6%), hypotension (5%), and rash (5%).

Incidence In Controlled Trials -IM Administration To Adult Patients With Epilepsy

Table 2 lists adverse reactions 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 phenytoin or continuing oral phenytoin. Both treatments were administered for 5 days.

TABLE 2: Adverse Reaction Incidence Following Substitution of IM CEREBYX for Oral Phenytoin in Adult Patients with Epilepsy (Events in at Least 2% of CEREBYX-Treated Patients)

Adverse Event
Oral Phenytoin1
Nausea 5 0
Vomiting 3 0
Ecchymosis 7 5
Nystagmus 15 8
Tremor 10 13
Ataxia 8 8
Incoordination 8 5
Somnolence 7 10
Dizziness 5 3
Paresthesia 4 3
Reflexes Decreased 3 5
Pruritus 3 0
1 The study was not designed to assess comparative safety.

Adverse Events During Clinical Trials In Adult And Pediatric Patients

CEREBYX has been administered to approximately 900 individuals during clinical trials. 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.

Laboratory Test Abnormality: Phenytoin (the active metabolite of CEREBYX) may cause increased serum levels of glucose and alkaline phosphatase.

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; Infrequent: confusion, twitching, Babinski sign positive, circumoral paresthesia, hemiplegia, hypotonia, convulsion, extrapyramidal syndrome, insomnia, meningitis, depersonalization, CNS depression, depression, hypokinesia, hyperkinesia, 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: Infrequent: 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.

Postmarketing Experience

The following adverse reactions have been identified during post-approval 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.

Body as a Whole: Anaphylaxis, angioedema [see WARNINGS AND PRECAUTIONS]

Laboratory Test Abnormality: Phenytoin or CEREBYX may decrease serum concentrations of T4. It may also produce lower than normal values for dexamethasone or metyrapone tests. Phenytoin may also cause increased serum levels of gamma glutamyl transpeptidase (GGT).

Nervous System Disorders: Dyskinesia

Read the entire FDA prescribing information for Cerebyx (Fosphenytoin Sodium Injection)


See Images

© Cerebyx Patient Information is supplied by Cerner Multum, Inc. and Cerebyx Consumer information is supplied by First Databank, Inc., used under license and subject to their respective copyrights.

Health Solutions From Our Sponsors